Applications of Droplet Behaviors

## **Chapter 5**

## Bioinspired Smart Surfaces and Droplet Dynamics-A Brief Review

*Raza Gulfam*

## **Abstract**

Mimicking the topographic structures and designs of living surfaces (e.g., lotus leaf, pitcher plant and beetle) onto the non-living surfaces (e.g., metallic plates, glass wafers, wood and fabrics) is known as bioinspiration. Consequently, the pristine topography of the non-living surfaces is robustly modified, known as bioinspired smart surfaces, providing novel surface regimes, i.e., wetting regimes and droplet dynamic regimes. Herein, factors affecting the droplet dynamics and its applications in bioinspired smart surfaces are presented. The droplet dynamics is a complicated phenomenon being affected by the various factors, encompassing the surface roughness, axial structural interspacing (ASI), structural apex layer (SAL), surface positioning, structural alignment, liquid droplet-surface interaction (LD-SI), and various stimuli, etc. Further, the droplet dynamics can be seen many applications, such as droplet manipulation, self-cleaning effect, design of controllable chemical reactors and electric circuits, water harvesting and condensation heat transfer, and oil/water separation, amongst others. The chapter has been mainly divided in three sections enclosed between the introduction and conclusion, comprehensively elaborating the classification of surface regimes, factors affecting the droplet dynamics and the applications at lab and industrial scales. In all, the contents are expected to serve as the guideline to accelerate advancement in the surface science.

**Keywords:** biomimetic, Superhydrophobic, slippery surfaces, wetting, droplets

## **1. Introduction**

Surface science has revolutionized the modern technology based on bioinspired alternatives and novel applications, attempting to control and resolve academic, industrial and societal challenges across the globe. It is important herein to explicitly define the surface so as to remove the literature anomaly because it is elusive whether to consider the top-most region as a surface or the whole of it. On the one hand, the entity under study should be called the substrate, and its top-most region should be named the surface. On the other hand, if the entity under study is named as surface (as we see in the literature), its top-most region should better be termed as the topography, meaning that the region lying under the topography is the surface. Therefore, we are also likely to adopt and utilize the latter terminologies henceforward, i.e., the topography is the part of a surface where the water droplet interacts. Altogether, surface science is based on the fundamental surface models (Young's model, Wenzel

model and Cassie-Baxter model) that are employed to understand the wettability/ wetting, droplet dynamics and the involved surface engineering processes [1–3].

Surfaces can be categorized as living and non-living surfaces.

Living surfaces include birds, insects, plants and animals. Upon careful observation, it has been found that the living surfaces behave very uniquely and differently with the water droplets. For example, the rainwater cannot wet the wings of the butterfly even during rain. Water striders can move on the water surface with great ease. Water can flow on the rice leaves very fast. Water droplets can dance on the lotus leaf and finally rolls off by taking the dust with them. Water droplets can stay on the rose petals even if it is vertically standing. Fish can travel through water very efficiently, and so on. When the microscopic examination of various living surfaces is carried out, the topographic morphologies have been found different in each case, consisting of micro/nano-structures of a wide variety of geometries, arrangements and interspacing. In addition, a particular kind of layer surrounding the micro/nanoscaffolds has been discovered, which is named the epicuticular wax [4]. Therefore, a constructive conclusion can be drawn that the topography of various surfaces is the main driver that affects the water droplets. In other words, living surfaces provide different levels of adhesions and slipperiness based on the epicuticular wax.

Non-living surfaces encompass metals, wood, polymer, glass, fabric and paper, etc. Until recently, rigorous efforts have been underway, aiming to modify the topographies of the non-living surfaces with help of bioinspiration. The bioinspiration is a broad technological and scientific concept in which the micro/nano-structures of living surfaces are studied, and then similar structures are created onto the topographies of non-living surfaces, which are termed the bioinspired smart surfaces. With the bioinspired conversion of wettability (i.e., bioinspired mimicking) as depicted in **Figure 1(a–c)**, the newly-born topographic structures include extruding topographic structures (ETS) (**Figure 1b**) and intruding topographic structures (ITS) (**Figure 1c**). The major influencers for bioinspired wettability are the axial structural interspacing (ASI) and the structural apex layer (SAL). Based on the ETS/ ITS and ASI, the newly-born area on the topography is called the projected area over which the contact angle is known as the apparent contact angle *θ*a (°). By dividing *θ*<sup>a</sup> (°) by the equilibrium contact angle *θ*e (°) of the pristine surface, the roughness *R* can be obtained. The SAL can be defined as the sites existing on the top of ETS/ITS responsible for introducing the same properties as the epicuticular wax does in the living surfaces. Therefore, SAL can be obtained by coating certain materials having either high surface energy (providing hydrophilic or superhydrophilic characteristics such as silicon dioxide [5]) or low surface energy (providing hydrophobic or superhydrophobic characteristics such as silanes [6, 7]). An experimental case study of bioinspired smart surface, known as the slippery liquid-infused porous surface (SLIPSs), can be seen in **Figure 1(d–f )**. SLIPSs are prepared by getting inspiration from the pitcher plants.

To create ITS/ETS, there are many traditional and advanced surface engineering methods [10–12], such as chemical oxidation, chemical etching, reactive ion etching, grafting, dipping, spinning, photo-lithography, electron beam-lithography, electrodeposition, imprinting, templating, hot embossing, plasma treatment, vaporization, selective tunneling, anodizing, laser ablation, etc. Thus, by selecting the most suitable method or combination of several methods, the ETS and ITS having a wide variety of geometries can be created, for example: plate-like, wire-shaped, whisker, conelike, square-arrays, fibre-like, vertical pillars, spikes, honeycomb-shaped, grooves, regular deep-pores, holes, channels, trenches, voids, etc., and the common feature of

*Bioinspired Smart Surfaces and Droplet Dynamics-A Brief Review DOI: http://dx.doi.org/10.5772/intechopen.104540*

#### **Figure 1.**

*Bioinspired mimicking depicting (a) pristine copper surface and topography, growth of (b) ETS and (c) ITS [8]. An experimental case study demonstrating the (d) porous scaffold of pitcher plant [9], and (e) fabrication of bioinspired SLIPSs consisting of (f) porous scaffold alike pitcher plant. [Note: Microscopic images in (a), (b) and (f), as well as the contact angles and their images, belong to the author(s)].*

all geometries is their size which tends to exist at micro, nano or micro-nano mixed (hierarchical) scale [12]. Hence, droplet wetting and droplet dynamic regimes of the pristine non-living surfaces can be completely tailored. Droplet dynamics of the bioinspired smart surfaces greatly rely on the SAL and ASI of the ITS/ETS.

In brief, the bioinspired smart surfaces are more efficient and capable of controlling industrial challenges. Consequently, a wide variety of tangible applications and salient surface characteristics has been unveiled, for example, dust-free solar cells [13], nonwetting leathers [14], efficient and durable oil/water separators [15], anti-biofouling surfaces [16], anti-reflective surfaces [17], pump-free microfluidic and lab-on-chip devices [18], nanogenerators for energy harvesting [19], efficient water vapor condensation and

enhanced heat transfer [20], as well as the stimuli-responsive surfaces [21], gas sensors [22], smart gating-based valves [23], and functionalized immunoassays [24].

This chapter shed a light on the bioinspired smart surfaces, and comprehensively elaborates the droplet dynamics. In between Section 1 (Introduction) and Section 5 (Conclusion), the main chapter breakdown provides three sections. By taking the examples of living surfaces, the classification of surface regimes, liquid dropletsurface interactions and the quantification criteria of wetting and dynamic regimes are included in Section 2. The major factors affecting the droplet dynamics have been unveiled in Section 3. The importance and role of droplet dynamics in emerging applications have been summarized in Section 4.

## **2. Classification of surface regimes and droplet-surface interactions**

The surface regimes can be majorly categorized into wetting and dynamic regimes with reference to the interacting liquid droplets. In surface science, the living surfaces can provide four kinds of wetting regimes against water droplets, namely, hydrophilic, super hydrophilic, hydrophobic and superhydrophobic, as demonstrated in **Figure 2**. The droplet dynamics can be typically categorized into two main branches, namely, sticky and slippery regimes (**Figure 2**). The droplet dynamics is defined as the study of droplet growth, droplet mobility, droplet speed, droplet transport range and the underlying forces.

The droplet wetting regimes are conventionally quantified via equilibrium contact angle *θ*e (°) that a liquid droplet makes with the surface, varying in a range of values (**Figure 1a**). It should be noted that, in several other studies, the inherent wettability of the pristine surfaces is denoted by the equilibrium contact angle *θ*e (°), while the artificial wettability of the smart surfaces is denoted by the apparent contact angle *θ*a (°). It means that the way to differentiate the pristine and smart surfaces can also be understood by the notation of contact angles. Guided by Young's model, *θ*e (°) defines the static interaction of a stationary droplet with the flat topography of a horizontallypositioned solid substrate. Apparently, the solid substrate surfaces have a compact topography, but microscopically, the voids exist consisting of certain interspacing that affects the wetting interaction and create the philic (referring to hydrophilic and superhydrophilic) and phobic (referring to hydrophobic and superhydrophobic) regimes based on the extent of droplet penetration. For the philic regime, the microinterspacing is supposed to exist into which droplet penetrates more intensely, and the opposite holds true for the phobic regime, i.e., nano-interspacing prohibits the droplet penetration. Thus, the surface topography affects the liquid droplet-surface interaction (LD-SI) that plays a decisive role regarding the droplet shape, i.e., with dominant micro-interspacing, the extent of droplet penetration is high, so the LD-SI area tends to be large with hemi to quarter-spherical droplet shape, specifying the philic regime in range of 0° ≤ *θ*<sup>e</sup> ≤ 90° [25–27]. While with dominant nano-interspacing, the LD-SI area is deemed to be small with hemi to full-spherical shape, which is called the phobic regime in the range of 90° ≤ *θ*<sup>e</sup> ≤ 180° [25–27]. The LD-SI can further be divided into two classes liquid droplet-dry surface interaction (LD-DSI) and liquid-droplet wet surface interaction (LD-WSI), as depicted in **Figure 2**. Depending on the LD-DSI and LD-WSI, the mechanism of droplet dynamics can be entirely changed as discussed below.

The droplet dynamic regimes can be quantified by the rolling/sliding angle α (°), which defines the dynamic interaction of a mobile droplet with the underlying tilted *Bioinspired Smart Surfaces and Droplet Dynamics-A Brief Review DOI: http://dx.doi.org/10.5772/intechopen.104540*

**Figure 2.**

*Bioinspired classification of droplet wetting and droplet dynamic regimes. Liquid droplet-dry surface interaction (LD-DSI) and liquid droplet-wet surface interaction (LD-WSI) are depicted depending on the topographic states.*

substrate. It should be noted that the rolling and sliding angles are specified just to distinguish the different surfaces due to dissimilar dynamic mechanisms depending on the droplet-surface interactions, e.g., the droplet is driven by the rolling mechanism on superhydrophobic surfaces due to LD-DSI (case of lotus leaf-inspired surfaces), while that on the slippery liquid-infused porous surfaces (SLIPSs), the sliding mechanism occurs due to LD-WSI (case of pitcher plant-inspired SLIPSs). But in both cases, α (°) is measured by the same method, i.e., the droplet is inserted onto the surface after which it is gradually tilted, and the angle is measured when the droplet starts moving. α is mainly helpful to demarcate the slippery and sticky regimes, regardless of the dynamic mechanisms of the involved surfaces types (superhydrophobic surfaces or SLIPSs).

The slippery dynamic regime, where virtually no droplet adhesion/pinning is considered, allows the ease of droplet mobility with value of α varying in range of 0° ≤ α ≤ 5° [28]; while the sticky dynamic regime, where the droplet adhesion/ pinning is variably considered, may provide different mobility behaviors with a range of 5° ≤ α ≤ 90°. A low adhesion Wenzel state or slippery Wenzel state can be conceived with a range of 5° ≤ α ≤ 50° where the droplet pinning is relatively stronger compared with that of slippery regime [29]. A high adhesion Wenzel state corresponds to the range of 50° ≤ α ≤ 90° [30], inducing the highest pinning against the droplet. A full adhesion Wenzel state occurs when α = 90° where the droplet does not move [31]. Therefore, the traditional norm defining that the Wenzel state induces the complete sticky regime (i.e., the droplet does not move at all), can be negated henceforth based on the above categorization and experimental proofs.

## **3. Factors affecting the droplet dynamics**

Droplet dynamics is a complicated phenomenon, which is drastically affected by several salient features such as surface roughness, ASI, SAL, surface position (horizontal and inclined), topographic structural alignment (isotropic, anisotropic and gradient), LD-SI and droplet shape, and stimuli. They are elaborated as follows:

#### **3.1 Roughness, ASI and SAL**

The droplet dynamics is a very complicated phenomenon that simultaneously takes the topographic structural geometry, structural arrangement, structural direction, size scale, wetting regime, ASI (axial structural interspacing) and SAL (structural apex layer) into account.

Based on the values of *θ*e/*θ*a (°) and α (°), the wetting and dynamics can be interlinked in the above-categorized regimes (philic, phobic, slippery and sticky), but this link is supposed to be weak, especially when studying the droplet dynamics. Tuning the structural geometry with size from micro-scale to nano-scale (roughness) [31] can result in the philic to phobic regime, meaning that *θ*a (°) can be symmetrically increased from 0° to 180°, but *α* (°) cannot be obtained in this order, i.e., it may either increase or decrease instead.

With a hierarchical structural arrangement even in the phobic regime, the droplet may lie in sticky regime where the adhesion can cause partial or even no mobility. The first example in such a case is the rose petal effect where the wetting lies in the extreme superhydrophobic regime but droplet does not move even at the tilt angle of 90°, evidencing the full adhesion Wenzel state [32]. The second example includes the butterfly wings and rice leaves with a superhydrophobic wetting regime where the structural direction simultaneously builds the sticky (full adhesion Wenzel state) and slippery regimes [33]. In addition, the efficient droplet dynamics do not completely rely on the wetting regime (phobic or philic), i.e., rose petal and lotus leave both have a hierarchical structural arrangement and lie in an extreme superhydrophobic regime [34], but the former does not support droplet mobility, while the latter is the most effective for droplet mobility. Indeed, the axial structural interspacing (ASI) is a prevalently crucial parameter [32] and with ASI as small as possible, the droplet dynamics can be made efficient. In general, the nano-scaled ASI [35] (producing the slippery regime with efficient droplet dynamics) is relatively encouraged compared with microscopic ASI; nonetheless, this principle does not always hold true, because now it is likely to convert the sticky regime even with high micro-scaled ASI into the perfect slippery regime with help of artificial SAL as discussed above. Thus, we conclude with reasoning that ASI and SAL are the main drivers together with entrapped-air as the secondary driver in all kinds of structural voids, establishing the exclusive droplet wetting and dynamic regimes.

#### **3.2 Horizontal surface position**

Horizontal surface position refers to the droplet lying in the static state. Indeed, the contact line [36] is the key factor influencing the extent of droplet adhesion, especially in the horizontal surface position. Herein, the droplet dynamics can be inferred by slowly evaporating the droplet resting on the surface, which can also be named evaporative droplet dynamics, as shown in **Figure 3**. It follows the simple rule, that is, the contact line immobility implies the sticky regime that works under the constant contact radius (CCR) mode [37], while the contact line mobility (normally inward)

*Bioinspired Smart Surfaces and Droplet Dynamics-A Brief Review DOI: http://dx.doi.org/10.5772/intechopen.104540*

#### **Figure 3.**

*Study of droplet dynamics under evaporation in horizontal surface position, depicting the (a) contact line (CL) pinning in CCR mode, and (b) depinning in CCA mode.*

confirms the slippery regime working under the constant contact angle (CCA) mode [38]. In special cases, CCR and CCA mixed modes can also occur [39]. Quantitatively, the contact line is extracted in terms of the base diameter through experiments, and the depinning force *F*d [40] can be measured.

In horizontal surface positions, the growth of droplets can also be carried out and controlled. For instance, the droplets tend to undergo frosting/icing/evaporation [41], and in such cases, the target is to delay or enhance the droplet growth rate.

In horizontal surface position, following factors also affect the droplet dynamics:

#### *3.2.1 Wettability gradient*

The specific arrangement of wetting regimes spanning from superhydrophobic to superhydrophilic can create the difference in wettability, known as the wettability gradient [42]. It plays an important role, providing the droplet mobility from a low wettability regime to a high wettability regime even when the surface is in horizontal position. This leads to self-propelled droplet mobility, i.e., no external energy in the form of gravity or stimulus is required [43]. Generally, with a high wettability gradient, the droplet dynamics are deemed to be efficient, and vice versa. An example of a wettability gradient can be seen in the beetle surface where the droplet dynamics play an important role in the water transport toward mouth [44].

#### *3.2.2 Droplet impact*

On the horizontal surface, the droplet impact can be obtained in two ways, i.e., by dropping the droplet from a certain height [45] or by coalescence of two or more droplets [46]. The droplet impact may exhibit a very unique phenomenon that strongly depends on the wetting regimes. While dropping the droplet, if the droplet rebounds several times, the slippery dynamics in superhydrophobic regime can be inferred; which is also known as droplet jumping, enabled by the conversion of energies [45, 46]. If the droplets do not rebound or show coalescence, the sticky dynamics are exhibited, which may lie in any wetting regime.

### **3.3 Inclined surface position**

Depending on the wetting regime and droplet adhesion, the inclined mode allows the droplet movement under the effect of gravity, i.e., the gravitational force overcomes the under-lying resistive forces, which can be quantified by measuring the α (°). The droplet does not move in the sticky regime, while it moves along the incline in the slippery regime, as exhibited in **Figure 4**. The droplet pinning/depinning is exhibited at the contact line (defined as the interface between the droplet and the surface topography) across the front/rear ridges. The contact angle at the front ridge is called the advancing angle *θ*a, while at the rear ridge, it is known as the receding angle *θ*r. The difference between the advancing angle and receding angle is called the contact angle hysteresis [31], i.e., the interfacial resistive forces attempting to prohibit the droplet mobility. In general, the smaller the CAH, the greater the droplet mobility.

The contact line plays the main role to overcome the interfacial resistive forces encountered by the droplets. The contact line tends to be discontinuous if the surface is in the slippery regime [33], requiring a smaller α that simply demonstrates the small interfacial resistive forces. Conversely, the contact line becomes continuous and stable (Wenzel state) in the sticky regime [33] due to which a larger α is indispensable.

In inclined surface position, further factors are important as discussed below:

## *3.3.1 Isotropic alignment*

Isotropic is the surface feature in which the topographic structures are aligned equally, enabling droplet mobility in every direction. An example of such a case is the movement of a rain-drop on slippery surfaces such as taro leaf and lotus leaf [47].

## *3.3.2 Anisotropic alignment*

Anisotropic is the surface feature in which the topographic structures are directionally-aligned, meaning that the droplet can only move along certain directions. For example, the rain droplet can move on the butterfly wings and rice leaves in only one direction [48].

## *3.3.3 Liquid transport modes*

There are two most common liquid transport modes during the condensation process (conversion of the gaseous phase into the liquid phase), naming the filmwise

**Figure 4.** *Study of droplet dynamics in the inclined surface position.*

### *Bioinspired Smart Surfaces and Droplet Dynamics-A Brief Review DOI: http://dx.doi.org/10.5772/intechopen.104540*

mode and dropwise mode. They are studied normally on inclined surfaces. Each mode provides different droplet dynamics depending on the wetting regimes. In filmwise mode, droplets come into existence, soon after which the droplet coalescence (merging of small neighboring droplets to form a big droplet is called coalescence) starts occurring. Consequently, a thin liquid film is developed underneath that remains affixed to the surface topography, providing the under-layered path over which the bulk of liquid keeps on transporting. This happens due to the high surface energy that prompts the sticky regime where the filmwise mode occurs. Comparatively, the slippery regime, empowers the dropwise mode depending on the surface type underneath (i.e., either dry or wet). In the dropwise mode, the drops come into existence and move as soon as they attain the critical droplet size. The dry slippery hydrophobic/superhydrophobic surfaces give rise to the dropwise mode under the influence of rolling/jumping mechanisms [49] where the liquid drops tend to be transported either in the discrete pattern or coalesced pattern. While the wet slippery hydrophobic/hydrophilic surfaces support the dropwise mode enabled by the sliding mechanism [50] of droplets which can also either be in discrete or coalesced patterns.

## **3.4 LD-SI and droplet shape**

The combined effect of LD-SI (droplet-surface interaction) and droplet shape may be considered a factor, however, droplet dynamics may not be fully predicted based on their combined effect. For example, the droplet usually adopts a full spherical shape (e.g., superhydrophobic lotus leaf and rose petals), hemispherical shape (e.g., hydrophobic cloverleaf) or quarter spherical shape (e.g., and hydrophilic herb) on LD-DSI, but some of them provide slippery dynamics, while some provide sticky dynamics. The droplet shape always tends to be a hemispherical shape (e.g., hydrophobic pitcher plant) or quarter spherical shape (e.g., hydrophilic beetle or fish surface) on LD-WSI, and they can all support slippery dynamics.

In the slippery regime, both LD-DSI and LD-WSI play a pivotal role in eventually deciding the two dynamic mechanisms or patterns of a mobile droplet (i.e., on the inclined surface). Owing to the underlying dry surface, the droplet tends to roll off, and the rolling mechanism is established as can be seen in the case of lotus leaf (superhydrophobic regime) and cloverleaf (hydrophobic regime). In contrast, because of the underlying wet surface, the droplet tends to slide-off over the liquid interface, and the sliding mechanism is produced as can be seen in case of pitcher plant and beetle surface.

## **3.5 Surface durability**

Surface durability is of practical importance that can drastically affect the droplet dynamic regimes. The surface durability can be defined as the withstanding capability of bio-inspired surfaces deciding the active lifespan, and it can be realized with reference to the impact of several parameters and phenomena, including the solvents (i.e., neutral, acids, alkalis), weather conditions (i.e., temperature, pressure, airflow), operational conditions (i.e., condensation, evaporation, shearing, friction), etc. Depending on the applications and the types of involved surfaces, different parameters and phenomena can affect the surface durability up to different extents. In general, the low surface durability can be caused by many factors, naming a few, the poor growth of ETS/ITS, the poor attachment of ETS/ITS with the parent substrate (pristine topography), the poor adherence/infusion of SAL with the ETS/ITS, the poor chemical compatibility, amongst

others. For example, the SLIPSs have shown practical challenges and low durability due to oil depletion through cloaking (the encapsulation of water droplets by the oil), evaporation (oil is evaporated at ambient or at high temperature) or physical shearing (the mobile droplets continuously induce friction at the droplet-surface interface and keeps on removing the oil with the passage of time) [30], finally resulting in low durability.

### **3.6 Stimuli**

In practice, there are three well-defined major stimuli that affect droplet dynamics. First is the gravitational force (external stimulus) that comes into action when the surface is in an inclined position; Second is the active stimulus (external stimulus) provided by several sources including temperature, photon (light), pH, stress, mechanical vibration, gas, reactive solvents, magnetic field and electric potential [51]. Active stimulus can enable tunable droplet dynamics. For example, tuning the movement of a droplet on the paraffin-infused surfaces by changing the electric potential [52] or temperature [30]. It should however be noted that the tunable droplet dynamics are hard to be found on living surfaces. Third is the self-stimulus that is enabled by the wettability gradients. The various stimuli of droplet dynamics can co-exist depending on the wetting regimes and surface positions.

## **4. Droplet dynamics-lab scale and industrial applications**

Based on the intended applications of non-living surfaces, the droplet dynamic regimes (whether sticky or slippery) are vital, e.g., the corrosion-combating surfaces [53] or anti-icing surfaces [54] are intensively required where the main target is to prohibit the contact between the water and surface [55], thus the sticky or moderately slippery dynamics are favorable. While in other cases such as water vapor condensation [56] and vehicle transportation through the water [57], the main target is the quick transport of water droplets on the surface and the drag reduction on the object, respectively, both necessitating the slippery dynamic regime. Therefore, droplet dynamic regimes are mainly decided by the applications. Generally, there are many applications, but a few emerging applications of droplet dynamics are presented underneath.

### **4.1 Droplet manipulation**

Droplet manipulation can be defined as the control of droplets through various design strategies of under-lying surfaces as well as droplets, such as rendering the droplet mobile, immobile, directional movements, static growth, etc. A few case studies are listed underneath.

#### *4.1.1 Droplet guided-tracks*

Guided tracks are the directional pathways where the droplet can move with great ease, demonstrating the efficient droplet dynamics. Following the bioinspired mimicking, guided tracks can be made in various shapes, such as straight, inclined, s-shape, and zig-zag amongst others. The example is given in **Figure 5** [58], where the s-shaped track is depicted. The under-lying surface owns a slippery superhydrophobic regime, which has been precisely carved. Most importantly, the

*Bioinspired Smart Surfaces and Droplet Dynamics-A Brief Review DOI: http://dx.doi.org/10.5772/intechopen.104540*

**Figure 5.** *Droplet transport along the guiding track [58].*

carved region and the droplet volume need to be effectively controlled so that the droplet cannot stay out of the track.

## *4.1.2 Droplet segmentation/merging*

Droplet segmentation/merging can be carried out in several ways, i.e., by making the segmented pathways over which the droplets can move or by merged pathways helping droplets come together and coalesced. This can be controlled by the bioinspired surface modifications as well as by the geometry modifications. For example, by modifying the surface of a common steel-made blade [58], the superhydrophobic regime can be obtained, which is then helpful to segment a big-sized droplet into two smaller droplets, as depicted in **Figure 6**.

## *4.1.3 Stimuli-responsive manipulation*

As described above, various stimuli can affect the droplet mobility and droplet transport range. The droplets can be either oil or water. On the inclined phase change slippery liquid-infused porous surface (PC-SLIPSs) [30], the droplet mobility has been achieved in solid, mush and liquid phases under the temperature-stimulus which is also known as thermo-responsiveness, as shown in **Figure 7a**. The wetting regime is hydrophobic, while the dynamic regimes are influenced by the phases and temperature. The solid phase provides the low adhesion Wenzel (LAW) state. Particularly, at the melting temperature of PC-SLIPSs, the droplet mobility suddenly changes from high adhesion Wenzel (HAW) state into a slippery state. The oil-droplet mobility has been realized underwater by using the electric stimulus [59], depicting the controllable dynamics, as depicted in **Figure 7b**.

### *4.1.4 Self-propelled droplet manipulation*

The droplet manipulation can be achieved without any external energy drivers or stimuli. The wettability gradient is the main driver, stimulating the self-mobility from low wettability toward high wettability. The example of steam condensation [60] is demonstrated (**Figure 8a**) where the liquid droplets are generated and move from left to right along the horizontal wires, leading to the droplet coalescence along the way. Eventually, droplet shedding occurs when the critical size of droplet has been achieved.

By means of the wettability gradient, the uphill droplet manipulation (**Figure 8b**) can also be achieved [43]. For example, the droplet can move upward on different

#### **Figure 6.**

*Droplet segmentation via superhydrophobic blade [58].*

#### **Figure 7.**

*(a) Thermo-responsive water droplet manipulation [30], (b) electro-responsive underwater oil droplet mobility [59].*

#### **Figure 8.**

*Wettability gradient-induced self-propelled (a) droplet merging and shedding [60], as well as droplet uphill movement [43].*

tracks, including circular geometry, straight geometry and s-shaped geometry. The droplet shape changes from the start until the end while covering the different transport ranges, depicting the various wetting regimes which are the characteristics of the wettability gradient.

## **4.2 Self-cleaning**

When the mobile droplets move on the surfaces, they can carry the dust and dirt particles with them, which is named the self-cleaning effect. Therefore, the slippery dynamic regime is the key, that can be executed either on dry or wet slippery surfaces. For example, the droplet moves on the phase change material-based superhydrophobic surface, carrying the dust away as can be seen in **Figure 9** [61].

## **4.3 Chemical reactor and circuit controller**

The mixed slippery and sticky regimes of the droplet dynamics can help build chemical reactors and electric circuit controllers. A dual stimuli-responsive SLIPSs have been presented [62], influenced by the temperature and force. A droplet (8 μL) of sodium hydroxide (NaOH) can be made mobile under the optimum effect of temperature and strain-induced force, letting it mix with the phenolphthalein droplet where the chemical reaction takes place, as exhibited in **Figure 10a**. Likewise, under the optimum effect of temperature and strain-induced force, the sliding and pinning of NaOH droplet can switch off and on the lamp, exhibiting the feasible electric controller (**Figure 10b**).

## **4.4 Condensation-water harvesting and heat transfer**

The droplet dynamics play the most influential role in the condensation, where the filmwise mode and dropwise mode are considered. By means of condensation, water harvesting and heat transfer applications can be realized. In both of them, the dropwise mode is of great interest, providing the high droplet nucleation and ease of droplet transport which can be achieved through slippery dynamic regimes. For example, the dropwise mode has been demonstrated in the superhydrophobic regime and hydrophilic regime, enabling the fast droplet transport suitable for the efficient water harvesting (**Figure 11(a** and **b)**) [63]. Likewise, the heat transfer (**Figure 11c**)

**Figure 9.** *Self-cleaning via mobile droplets [61].*

**Figure 10.**

*Droplet sliding/pinning helping build (a) chemical reactor [62] and (b) electric circuit controller [62].*

has been obtained higher in the slippery hydrophobic regime of phase change slippery liquid-infused porous surfaces (PC-SLIPSs) and slippery superhydrophobic regime compared to that in the sticky hydrophilic regime of pristine copper plate [20]. It should be particularly noted that efficient droplet dynamics are necessary during condensation, i.e., the dropwise mode may also underperform if the droplet shedding is slow as concluded in Ref. [20].

## **4.5 Oil/water separation**

The oil/water separators are prepared by developing smart coatings on the porous networks. Depending on the wetting regimes of the separator, one phase is blocked, while the other is permitted through the porous network. Sticky and slippery regimes of droplet dynamics are crucial for oil/water separators; however, it should be particularly noted that these regimes may be insignificant during the separation mechanisms of vertically-aligned separators. It is because the blocking of one phase is not driven by the sticky dynamic regime, as well as the permitting of the other phase is not driven by the slippery dynamics. Instead, the separation mechanism is driven by the positive and negative capillary effects of two phases, inducing physical absorption *Bioinspired Smart Surfaces and Droplet Dynamics-A Brief Review DOI: http://dx.doi.org/10.5772/intechopen.104540*

#### **Figure 11.**

*Water harvesting under dropwise mode in (a) superhydrophobic (SHPo) regime and (b) hydrophilic (HPi) regime [63], (c) steam condensation heat transfer on pristine copper plate, superhydrophobic copper plate and PC-SLIPSs [20]. Scale bar, 5 mm.*

and physical adhesion, as shown in **Figure 12a** [15]. Thus, in vertically-aligned separators, the sticky and slippery dynamics put forward a positive contribution in other ways. For example, the low sticky regime or moderately-high sticky regime is relatively effective, which can create easiness in the recovery process of separation medium after the oil/water separation. The self-recovery is important because the smart coatings can retain either oil or water during the separation process. With low sticky dynamics, the retained oil or water can easily evaporate, providing efficient self-recovery, as depicted in **Figure 12a**. The slippery dynamics can also play a vital role before or after the separation process, for example, a superhydrophobic-oleophilic copper mesh has been presented which is an effective oil/water separator. The slippery dynamics are of great significance to resolve the cleaning challenges of the separators (**Figure 12b**) [64].

#### **4.6 Spraying**

The spraying of different liquids is encountered almost everywhere, for example, spray of water, fuels, solvents, paints and perfumes, amongst others. In the spraying process, a multitude of droplets, consisting of various sizes, are generated. The droplet dynamics in the spray process can be affected by many factors, naming a few, the droplet type (i.e., oil, fuel, water, acids, alkalis, etc.), the droplet traveling speed, atomizer distance, and the surfaces position (horizontal, vertical or inclined). Upon impact with the surfaces, different droplets can behave differently depending on the wetting regimes and surface roughness. The impact of fuel droplets and surface roughness have been studied during the fuel spray in combustion engines [65]. It has been concluded that the various wetting regimes (depicted by the two values of surface roughness of *R*a ~ 2.5 and 7.7) can produce various levels of fuel film thickness, depicting the unique film development mechanism (**Figure 13**). It shows that the droplet dynamics and the wetting regimes together play a pivotal role during the spraying, urging deeper studies in this field especially addressing the contact angles, sliding angles and contact angle hysteresis, etc.

**Figure 12.**

*Oil/water separators. (a) Low sticky dynamics depict the ease of self-recovery [15], (b) self-cleaning of separator [64].*

#### **Figure 13.**

*Spray process of fuel droplets onto the horizontally-positioned glass slides having different roughness (Ra) values [65].*

## **5. Conclusion**

Bioinspired smart surfaces have been reported, with a special focus on the droplet dynamics. The surface regimes have been fundamentally classified into four droplet wetting and two droplet dynamic regimes. The droplet dynamics is defined as the study of the droplet growth, droplet mobility, droplet speed, droplet transport range and the underlying forces. The droplet dynamics is of great interest to a wide variety of scientific areas. With help of bioinspired smart surfaces, the droplet dynamics, either in sticky or slippery regimes, have been greatly improved. There are many factors that affect the droplet dynamics, that is, it is hard to rely on a single factor. In particular, the specific dynamic regime and the influential factors need to be modified depending on the applications, i.e., some applications may need the sticky regime, some may need the slippery regime, and some may need the co-existence of both regimes.

This chapter implies that the droplet dynamics are potentially significant, however further efforts are necessary to sustain the efficient droplet dynamics. It indeed depends on the chemical and mechanical strength of the droplet wetting regimes of the under-lying bioinspired surfaces, i.e., the surface durability. All bioinspired surfaces (superhydrophobic, SLIPSs, etc.), as presented so far, suffer from the bottlenecks corresponding to the durability, rendering the droplet dynamics inefficient up to various levels. It is therefore recommended to use alternative materials (e.g., solid slippery materials such as waxy and non-waxy phase change materials) and advanced surface engineering approaches to enhance the durability of the bioinspired surfaces that can sustain the droplet dynamics during real-time applications. Briefly, the efficient droplet dynamics can potentially enhance the performance of those systems intending to minimize the global energy crisis (e.g., less drag-inducing pipes can help save energy during the pumping process, slippery surfaces of photovoltaics can prohibit/postpone the ice accretion as well as avoid the formation of dust layers), save the operational time (e.g., condensers can provide high output in less time based on the high droplet transport) and provide the high throughput (e.g., microfluidic devices).

## **Acknowledgements**

The support provided by the IntechOpen, as well as by the Postdoctoral Program of Southeast University are highly acknowledged.

## **Author details**

Raza Gulfam School of Energy and Environment, Southeast University, Nanjing, China

\*Address all correspondence to: gulfamrazahaidery@hotmail.com; gulfam@seu.edu.cn

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## **Chapter 6**

## Internal Flow and Spray Dynamics of Multi-Hole Nozzle

*PengBo Dong*

## **Abstract**

Faced with dual challenges of "carbon neutral" and emission control, fossil fuel-based internal combustion engines need to explore new ways and technical paths to reduce harmful emissions and Carbon dioxide emissions simultaneously. Fuel injection process is playing a significant role not only in traditional engines but also in new low/zero carbon engines. Multi-hole nozzles have a wide range of applications in the fuel supply system. While the accepted spray study work and jet break-up models are usually developed under the quasi-steady-state of fuel injection by a single-hole nozzle. There are rare models that can describe the whole break-up processes of multihole nozzle spray, including complex internal flow factors, plume interaction, and the effect of start/end of injection. In this chapter, characteristics of spray morphology, evolution processes, and evaporation characteristics, emerging from the practical diesel multi-hole nozzles, were discussed and analyzed during the transient injection processes in detail. Moreover, the relationship between multi-hole nozzle internal flow properties and the corresponding spray behaviors was investigated by numerical simulation method systematically. Therefore, multi-hole spray modeling processes under engine operating conditions and the optimized design of diesel multi-hole nozzles are expected to get some benefits and clues from the current results.

**Keywords:** fuel injection, nozzle internal flow, cavitation, near-field spray, multi-hole nozzle, spray diagnostics, diesel engine

## **1. Introduction**

The research on fuel sprays of Internal Combustion Engines has been drawing attention for last century. As for Diesel Engines, spray evolution, which includes the jet breakup, fuel atomization, air entrainment, and mixture formation processes, is regarded as one of the determinants of engine performance and emission formation. Furthermore, nozzle geometry can directly affect the characteristics of the complicated internal flow patterns inside the nozzles, such as the cavitated turbulence. As a result, in order to improve the quality of the atomization and mixture homogeneity, researchers have spared no effort to investigate the internal flow [1–3] and spray behaviors [4–6], and many fundamental and classic theories were formed over the past decades. Originally, due to the simple structure and the easiness of applying diagnostic techniques and arranging instruments surrounding the nozzle and the spray plume, single-hole nozzle was widely used in fundamental research.

Consequently, spray and combustion models, which are adopted in the numerical simulation study are usually developed from the experimental results of the singlehole nozzle spray.

On the other hand, it is known that multi-hole nozzles, which can emerge several spray plumes simultaneously, are generally applied in the engineering field of practical Diesel Engines. In this case, the orifices are normally aligned around the sac symmetrically and located off-axis to the nozzle axis. As a result, the optical access to the complex configuration and the mutual interference between the spray plumes create a difficulty for scholars to explore the internal flow and spray characteristics of multi-hole nozzles.

In recent years, there is a trend that considerable efforts have been exerted to conduct an experimental and computational study about the internal flow and sprays of the multi-hole nozzles. Different kinds of verisimilar scaled-up or real-size multihole nozzle models were designed to reveal the flow pattern properties inside the nozzles and the initial emerging spray dynamics [7–12]. The application of the synchrotron x-ray sources with high energy pulses has also been extended to the study of fuel sprays during the last decade [13, 14]. At the same time, a series of studies using the three-dimensional computational fluid dynamics simulations were also conducted to link the experimental results to the numerical calculation [15–18]. Based on all the approaches mentioned above, the vortex flow and string-type cavitation inside the sac, the counter-rotating vortices, film-type and string-type cavitation inside the hole volume, the needle lateral oscillation effect, and the unstable spray behaviors were found in succession.

Moreover, smaller hole diameter and more holes, accompanied by higher injection pressure are becoming prevalent in dealing with the strict emission regulations and economic demand [19, 20], because it is proven to be able to generate better fuel atomization and more homogeneous fuel/air mixture [21, 22]. In the conventional study, hole diameter is usually larger than 0.10 mm. Even though a few researchers applied the micro-hole nozzle to conduct the fundamental experimental study about the mixture formation and combustion processes, mainly single-hole nozzles were used in their experiments. As a result, the limited nozzle types and experimental conditions hindered a more satisfactory situation in understanding the flow dynamics of the real multi-hole nozzles under practical operating conditions thoroughly. There is rare information about the spray properties of multi-hole nozzles with micro orifices in the current archives, and it is worthwhile to carry out a monographic study on this issue.

A comparison between the spray properties of the nozzles with different hole diameter is made in the present chapter to provide deep insights into the multi-hole nozzles sprays, which are formed in the real scenario of Diesel Engines, and sufficient analyses about the effect of the hole diameter were conducted systematically as well. In addition, the CFD simulation results for these different nozzle configurations were also presented in this chapter with the aim of correlating the observed spray behaviors to the internal flow properties inside the nozzles.

## **2. Methodology**

#### **2.1 Experimental apparatus, procedures, and conditions for spray observation**

High-speed video camera observations based on the Mie scattering were made for the sprays injected by the multi-hole injectors, and the specific experimental

## *Internal Flow and Spray Dynamics of Multi-Hole Nozzle DOI: http://dx.doi.org/10.5772/intechopen.104601*

apparatus is shown in **Figure 1**. **Figure 1(a)** presents the global experimental arrangement, including the high-pressure chamber, fuel injection system, and optical system. A common rail injection system could generate the injection pressure up to 220 MPa, and high-pressure constant volume vessels with optically accessible quartz windows were employed to create the high-pressure ambient environment of Diesel fuel injection. A delay pulse generator (Stanford Inc., DG535) and an electronic control unit (ECU) were applied to control the image timing, injection time, and injection quantity. The optical path layout is shown in **Figure 1(a)**. A xenon lamp (USHIO Corp., SX-UID501XAMQ) and two high-speed video cameras (Nac MEMRECAM HX-3, Photron FASTCAM-APX RS) were used to record the fuel injection processes.

When it comes to the installation of the multi-hole injectors, the detailed information about the specially designed chamber head is shown in **Figure 1(b)**. It is well known that the observation of the multi-hole nozzle sprays is difficult due to the conical structure formed by the plumes. In this study, the multi-hole injector was installed into this specialized chamber cover obliquely, as shown in **Figure 1(c)**, to prevent spray interference. An appropriate angle between the axis of the multi-hole nozzle and the horizontal plane was designed to maintain that one of the spray plumes could be observed as vertically as possible.

**Figure 1.** *Experimental apparatus.*

The experimental apparatus for the injection rate measurement are presented in **Figure 2**.

In this study, the Bocsh rate of injection meter [23] was applied. The experimental conditions corresponded to the spray observation experiments, which will be introduced in the upcoming sections. The timing of the start of injection was determined by comparing the injection pause signal with the injection rate curve signal recorded in the oscilloscope.

**Figure 3** shows the schematics of the nozzles applied in this study, the two multihole nozzles (10 holes) have the same configuration except for the hole diameter (D = 0.10 and 0.07 mm). The experimental conditions, which are shown in detail in **Table 1**, were determined in consideration of the real operation conditions of small Diesel engines.

The setting of cameras is also presented in **Table 1**, and the experimental measurement was conducted at least 10 times for each condition. The test fuel was the JIS #2 diesel. The injection quantity was 2 mm<sup>3</sup> /hole. In order to keep the ambient gas density similar to that of the combustion conditions, for fundamental spray research of non-evaporation conditions, the ambient temperature and pressure were 300 K and 1.5 MPa, respectively.

## **2.2 Methods of image processing**

The typical image processing processes and the definitions of spray properties in the current study are shown in **Figure 4**. The same processing method was applied to the spray images of different nozzles. The spray image taken under the baseline condition is shown here as an example. The central spray of each frame was characterized as the spray tip penetration (i.e., the maximum penetration distance of the spray, *S*), the corresponding angle of 100 times hole diameter length (i.e., the spray cone angle, *θc*), and the corresponding angle at the half point of *S* (i.e., the spray angle, *θs*). The parameters are plotted as a function of the time after start of injection and nozzles with different hole diameters.

The spray images were processed to calculate their properties by the following steps. First, each frame was converted to an effective image by subtracting the

**Figure 2.**

*Schematic of injection rate measurement experiment apparatus.*

*Internal Flow and Spray Dynamics of Multi-Hole Nozzle DOI: http://dx.doi.org/10.5772/intechopen.104601*

**Figure 3.** *Schematic of different nozzles applied in experiments.*

background images taken without the spray injection. After that, the spray edge could be detected by using the binarization image, which is converted by a threshold algorithm. The colored edges of different recording times show that the spray contour and the temporal variations can be well captured spatially. The spray tip penetration of the central spray plume is determined by scanning the contours from the corresponding orifice point. In this way, the morphological algorithm could also be used to extract the parameters of spray cone angle and the spray angle. The near-field spray properties around the nozzle exit region were also captured by the same method introduced above.

One of the most important factors that can introduce uncertainty into the measurement results is the selection of the threshold. Compared with the single-hole nozzle, the hole-to-hole variation and the cycle-to-cycle variation of multi-hole nozzle sprays cause the image processing to be more sensitive to the algorithm. Hence, the assessment of the threshold value is necessary for the current study. Principles that are suitable for the threshold selection are summarized as (1) The threshold should ensure that the spray profiles are as similar to the raw images as possible; (2) The threshold should remove the background noises in the images; (3) The error should be within the cycle-to-cycle injection variations. As a result, the intensity threshold of 5 (the maximum intensity: 255) was selected in this study after a series of statistics.

## **2.3 Computational setting for internal flow study**

In the current study, to aid the interpretation of the experimental results, the influences of the micro-hole diameter on the internal flow and cavitation characteristics have been numerically investigated by the commercial CFD Code FIRE Version


#### **Table 1.**

*Experimental conditions.*

2017 (AVL). **Figure 5** shows the computational meshes of the multi-hole nozzle with ten holes, and only one-tenth of the entire volumetric domain was selected considering the geometric periodicity, symmetry, and calculation timing.

The specific settings for this computation have been listed in **Table 2**. The same setting was applied to the two different nozzle configurations to make comparisons between the internal flow patterns. The Reynolds Averaged Navier–Stokes Simulation (RANS) model and a four-equation *k-ζ-f* model developed from the standard k-ε model were adopted to simulate the turbulent flow. The *k-ζ-f* model introduces new transportation equations to describe the variable *ζ* which has a relationship with the turbulence viscosity. As a result, the property of anisotropic turbulence can be taken into the consideration. A multi-phase flow model was selected to approximate the fluid conditions inside the nozzles. Furthermore, a Linearized Rayleigh model [24] was used to express the cavitation bubble behaviors within the nozzle.

Validation was conducted by taking the experimental results published by Blessing et al. [25] as the criteria, in which the characteristics of nozzles and boundary conditions covered many features of the current study, and the turbulence and cavitation models were proved reasonable before further computational studies. Moreover, the effect of mesh size was also taken into the consideration by making comparison between the different computational results from the meshes with different mesh

*Internal Flow and Spray Dynamics of Multi-Hole Nozzle DOI: http://dx.doi.org/10.5772/intechopen.104601*

#### **Figure 4.**

*Image processing for spray properties measurement.*

**Figure 5.** *Computational meshes.*

sizes (7.769, 4.413, and 2.896 μm). During all the verification processes, it was proven that the distribution and occurrence of the cavitation could be predicted accurately relatively by this simulation.

Furthermore, aiming to make comparisons, the same transient needle-lift curve, as shown in **Figure 6**, measured from a similar type of multi-hole injector was applied to the two nozzle meshes.


**Table 2.** *Computational setting.*

**Figure 6.** *Needle lift curve.*

## **3. Results and discussion**

## **3.1 Injection processes and spray characteristics**

**Figure 7** shows the injection rate results of the two different nozzles under the conditions of Qinj = 2.0 mm<sup>3</sup> /hole. These two curves present apparent distinctions. It can be seen that the micro-hole conspicuously changes the previous regularity of the fuel injection. The injection rate of the micro-hole (0.07 mm) nozzle is much lower than that of the nozzle with 0.10 mm hole diameter when maintaining the same injection quantity per hole. While the injection duration of the micro-hole nozzle is prolonged a lot. Moreover, the initial stage of the injection duration attracts attention *Internal Flow and Spray Dynamics of Multi-Hole Nozzle DOI: http://dx.doi.org/10.5772/intechopen.104601*

**Figure 7.** *The injection rate of different injectors.*

to analysis in detail, which is emphasized and enlarged in **Figure 7**. The injection rate of the nozzle with a micro-orifice is a little higher than that of the other one with larger orifices in the initial stage of injection. In fact, it is known that the fuel injection velocity and the effective flow area alter the injection rate simultaneously. Furthermore, the fuel injection velocity mainly depends on the upstream pressure in the sac, and the effective flow area is affected by the hole numbers, hole diameter, and the discharge coefficient. The theoretical flow area of the micro-hole nozzle, whose diameter is 0.07 mm, is much smaller than that of the normal one (0.10 mm), and its sac pressure discharge rate should also be much lower than that of the nozzle with normal holes. As a result, all the phenomena described above reveal that the effect of micro-holes plays entirely different roles in the injection rate at different injection stages. Specifically, the higher sac pressure is mainly caused by the relatively lower sac pressure discharge rate at the initial stage of injection, and the injection rate of this stage is dominated by the consequent higher flow velocity inside the micro-holes. However, at the middle and post-stage injection, the relatively larger effective flow area and the consequent higher mass flow rate inside the nozzle with 0.10 mm hole diameter mainly dominate the injection rate of the nozzles.

Typical false-colored and temporal spray images of different nozzles are shown in **Figure 8**. According to the theoretical foundation of Mie scattering, the scattered light intensity is a symbolic characteristic of the droplet size and fuel concentration. The spray contours can help elucidate the effects of ambient gas entrainment and interactions between spray plumes [26].

As analyzed before, even the total injection mass per hole is held constant, the fuel injection quantity of the 0.10 mm hole is larger than that from the micro-hole at the same timing ASOI. Based on the Mie-scattering principle, the intensity is in proportion to the droplet size and concentration. Hence, the global intensity of the central sprays of the multi-hole nozzle with normal hole diameter, which are shown in **Figure 8(a)**, is much higher than that of the micro one, particularly at the beginning stage of fuel injection. The high-intensity area can even extend to the downstream region of the central spray. It follows that the fuel concentration of the micro-hole condition is leaner, and the atomization effect is better [27].

**Figure 8.**

*False-colored spray images of different injectors.*

As for the spray morphology, the edges of the spray of 0.10 mm holes are irregular compared with the micro-hole one, and the hole spray plumes are flanked by evident wavelike contours. In the case of micro holes, the edges of the spray upstream regions are neater and more orderly than those of nozzles with larger hole diameters.

Moreover, central sprays show a large eccentricity in the spray tip around the end of injection (0.8 and 1.0 ms ASOI) under the larger-hole nozzle condition, and the adjacent two plumes also present asymmetrical morphology. The low-pressure regime between the sprays generated by the air entrainment can enhance the sprawling diffusion of the multi-hole nozzle sprays. The Coanda effect [28] plays significant role in these phenomena. However, under the 0.07 mm hole diameter condition, the sprays have well symmetry. Therefore, it is safe to say that decreasing the hole diameter can supposedly reduce the uncontrollability and instability of the sprays emerging from multi-hole nozzles. A more specific investigation and discussion about this phenomenon will be introduced in the upcoming sections.

It is known that the spray propagation distance is governed by the upstream pressure and the ambient conditions. The calculated sac pressure and the measured spray tip penetration variation of different nozzles is shown in **Figure 9**. The Bernoulli equation is used to calculate the corresponding averaged sac pressure variations based on the injection rate measurement results in **Figure 7**. The equation is written as below, where *Qf* is the injection rate, α is the average discharge coefficient, A represents the theoretical flow area, and Ps is the sac pressure.

$$Q\_f = aA\sqrt{2(P\_s - P\_d)/\rho\_f} \tag{1}$$

As for *α*, it has much relationship with the local flow area, which is a direct reflection of the cavitation intensity of nozzle orifices. Nurick [29] and Payri et al. [30] conducted their experiments under quasi-steady conditions and concluded that the flow discharge coefficient of the nozzle hole mainly depended on its cavitation number under the cavitated conditions.

$$\mathbf{C}\_{N} = \left(P\_{\text{inj}} - P\_{V}\right) / \left(P\_{\text{inj}} - P\_{a}\right) \tag{2}$$

#### **Figure 9.**

*Calculated sac pressure and spray pemetration variation of different injectors.*

However, it is still difficult to get accurate quantitative results from the internal flow of a practical Diesel multi-hole nozzle, which is usually high pressure, high velocity, turbulent, and micro size. The theory developed under the quasi-steady condition was attempted to be expanded to the transient condition in the current study, and a comparative analysis was made between the different nozzles qualitatively to provide a reference for explaining different spray behaviors.

Throughout the injection duration, the pressure in the sac of the nozzle with micro-holes is all higher than that in the nozzle with normal holes. The different effective flow area, caused by the different hole diameter plays a significant role in this issue. As for the corresponding penetration result, it is in accordance with the injection rate variations analyzed previously. As the color arrows emphasize, the micro-hole nozzle spray tip penetration is longer at the initial stage. As the time elapses, the penetration of 0.10 mm hole nozzle passes over the micro-hole nozzle spray tip penetration. In different injection stages, it is the different factors (effective flow area or sac pressure) that dominate the spray propagation of nozzles with different holes. In the calibration processes of combustion system of Diesel engines, the fuel injection quantity, injection timing, and injection times/cycle are usually adjusted as the fuel supply strategy. The diffusion and deceleration of the multi-hole nozzle spray are usually associated with the transfer of the spray momentum to the turbulence energy [31]. Consequently, when concerning the optimization of the Diesel engine preference, effects of the micro-hole on fuel injection of multi-hole nozzles should be given attention emphatically.

**Figure 10** shows the temporal variation of the spray angle and spray cone angle. It is evident that the spray angle is wider under the 0.1 mm hole condition. The maximum deviation appears at 0.1 ms ASOI, up to 11°. Furthermore, it seems that the micro-hole can exert more influence on the spray angle reduction compared with that on the spray tip penetration. On the other hand, different from the spray tip penetration results, no overlap happens among the two spray angle curves of nozzles with different holes. The simulation results in the upcoming section can be used to explain this phenomenon in depth.

The difference between the spray cone angles of the nozzles with different holes is also evident. It should be noted that the penetration of the larger-hole nozzle is shorter than 100 times the hole diameter at 0.1 ms ASOI; hence, only the spray angle can be measured, as shown in the figure. Because the position (10 mm) of 100 times of hole diameter is around the spray tip area of the 0.10 mm hole, it results in a small value of spray cone angle, and the spray cone angle difference between the two nozzles is not too much at 0.2 ms ASOI. After that, when the spray penetrates long enough, attributed to the more completed internal flow inside the larger holes [32], the 0.10 mm hole nozzle spray cone angle increases a great deal suddenly.

When it comes to the micro-hole condition, the value of the spray cone angle is much smaller and the variation is steadier. This can be explained like that since the cavitation collapse and turbulence flow inside different nozzles are two of the major mechanisms of the spray primary break-up [31, 33], the fuel jet enters the chamber

**Figure 10.** *Spray angle and cone angle variation of different injectors.*

with a less cavitation level, reduced mass flow rate, momentum, and less turbulence caused by the increased ratio of nozzle hole length to diameter, which can result in the narrower spray cone angle.

According to the above discussion, because of the unique geometric structure, the micro-hole nozzle has a lower injection rate, higher sac pressure, and wider spray angle and spray cone angle compared to those of the nozzle with normal holes. Combing these phenomena with the interlaced relationship between the trends of the spray tip penetration of different nozzles, it is concluded that the effect of the microhole on different spray properties is discrepant.

#### **3.2 Characteristics of the near-field spray**

In order to investigate the spray behaviors near the nozzle tip region in detail, high-speed imaging of 100,000 fps was applied to take a close-up view of this regime. The Higher spatial and temporal resolutions allowed a more detailed observation of the very emergence of the fuel from the nozzle orifice. **Figure 11** shows the close-up gradient spray images of different nozzles, respectively.

According to the images of typical timings, the normal-hole nozzle sprays pulsate out from the nozzle tip to the radial direction, and the edges of them fluctuate seriously. The perturbation of the spray is marked and emphasized by different color arrows in the figure. On the other hand, since the injection duration is longer under the micro orifices condition when maintaining the same injection quantity, the selection for the typical timing of the images is a little different. With the same results under the imaging rate of 10,000 fps, the spray illumination intensity becomes weaker under the micro-hole condition. Of interest is that the spray pulsating phenomenon almost disappears, and the profiles of the sprays become much neater and more stable.

The angle, determined by 10 times the hole diameter away from the nozzle tip, is defined as the spray dispersion angle. The average spray dispersion angle and the single-shot results are shown in **Figure 12**. Generally, the spray dispersion angle under the hole diameter of 0.10 mm condition is much wider than that of the micro-hole one. As for the single-shot result, corresponding to the pulsating phenomenon of the near-field spray, as the capital letters and color arrows indicate in the figures, the spray dispersion angle curve waves and fluctuates strongly, especially in the initial stage of the injection (0.1–0.25 ms ASOI). The integrated speculations and reasonable explanations for this phenomenon can be excavated by linking the previous results [34] and the current study.

Different from the single-hole nozzle, due to the off-axis arrangement of the orifices, there is usually vortex flow inside the sac of multi-hole nozzles, especially under the condition of low needle lift. Moreover, with the needle moving, the location variation of the unstable vortex core results in the unstable spiraling flow pattern emerging through the hole with the vortex. During these processes, there is usually the generation of string-type cavitation in the sac and hole flow field. All the properties, only belonging to the internal flow of multi-hole nozzles, play significant roles in the phenomenon of near filed spray pulsating [7, 35]. When the hole diameter is reduced to 0.07 mm, the fluctuation of the spray dispersion angle curve decreases dramatically, and the angle becomes much narrower than that of the normal hole diameter condition. The reduced flow transverse can suppress the cavitation and vortex level under the micro-hole condition, and it is also impeded the fuel to enter into the micro-holes from the sac volume of the multi-hole nozzle. These could be

**Figure 11.**

*Close-up view of sprays injected from different injectors.*

used to explain the reason why the micro-hole multi-hole nozzle has a relatively narrower spray width and steady spray morphology.

## **3.3 Internal flow inside different nozzle configurations**

The computational study is used to illustrate the different internal flow characteristics between the two nozzles. In this transient simulation analysis, according to the needle lift curve applied in the study, three typical timings (0.1, 0.318, and 0.5 ms ASOI) are selected to represent the initial, full needle lift, and post stages of the injection, respectively.

*Internal Flow and Spray Dynamics of Multi-Hole Nozzle DOI: http://dx.doi.org/10.5772/intechopen.104601*

**Figure 12.**

*Close-up spray behaviors injected from different injectors.*

**Figure 14.** *Temporal variation of fuel velocity at the monitoring points on the hole sections.*

**Figure 13** shows the pressure variation inside different nozzles at the typical injection stages. The time-resolved averaged sac pressure variation of the two nozzles is also plotted in this figure. Attention should be paid to the nozzle with micro holes. Its sac and hole pressures are all higher than those of the one with larger orifices throughout the injection duration, which coincides with the experimental analysis shown in **Figure 9**. The lower theoretical effective flow area and sac pressure discharge rate of the micro-hole nozzle are mainly attributed to this issue.

The temporal velocity variation on the inlet and outlet sections of the two nozzles is shown in **Figure 14**. Four monitoring points (P1, P2, P3, and P4) were set along the horizontal diameter line (Line A-B) symmetrically.

The injection velocity on the outlet of the micro-hole nozzle is higher than that of the normal-hole one, especially in the initial stage of injection, which agrees with the discussion about the injection rate and sprays penetration results in the experiments. Furthermore, no matter on the inlet or on the outlet sections, the flow velocity fluctuation of the nozzle with larger orifices is the most intense one, which corresponds with the spray pulsating instance. Moreover, a reverse variation tendency appears at the symmetrical monitoring points with the needle moving up, which implies that there is a spiral and asymmetrical flow pattern inside the hole. However, under the micro-hole diameter conditions, the amplitude of the velocity wave decreases dramatically, which coincides with the neater spray profiles and the reduced spray pulsating phenomenon discussed in the experimental results.

The streamlines inside different holes, which are shown in **Figure 15**, can interpret the instance analyzed in the optical experiments. There are much more complicated streamlines with stronger curvatures and counter-rotating flow inside the hole volume *Internal Flow and Spray Dynamics of Multi-Hole Nozzle DOI: http://dx.doi.org/10.5772/intechopen.104601*

#### **Figure 15.**

*Streamline and streamwise vorticity distribution along the horizontal and vertical diameter lines on the outlets of different multi-hole nozzles at full needle lift timing.*

under the larger orifice condition. The vorticity distributions results along the orthogonal orifice lines also show higher stream-wise vorticity under 0.1 mm hole diameter conditions. This spiral flow is also observed by Gavaises and Andriotis [11], Lai et al. [32], and Hayashi et al. [35]. It has been proven that there are close correlations between this swirling motion and the wider spray cone angle. However, when attention is paid to the nozzles with micro-orifices, the vorticity decreases, and the streamline is stable and smooth relatively. This agrees with the reduced fluctuation of its spray behaviors observed in the experiments.

As shown in **Figure 16**, the velocity resolution of fuel jet injection on the hole exit section of the multi-hole nozzle is conducted, and *Va* is the vector on the hole axis. The

**Figure 16.**

*Distribution of the ratio of velocity components (*Vr*/*Va*) along the horizontal diameter of the exit of different nozzles at typical timings.*

**Figure 17.** *Temporal variation of liquid volume fraction distribution inside different nozzle holes.*

*Internal Flow and Spray Dynamics of Multi-Hole Nozzle DOI: http://dx.doi.org/10.5772/intechopen.104601*

**Figure 18.** *Distribution of turbulence kinetic energy along the horizontal diameter of the exit of different nozzles at typical timings.*

other velocity component *Vr* is on the outlet section, which could be divided into two components. In the current result, the ratio of *Vr* to *Va* along the horizontal hole diameter line is shown in this figure. The normal-hole nozzle at three typical timings all has a much higher ratio than that of the micro-hole nozzle, which is coincided with the result observed in the experiments that the nozzle with 0.10 mm holes has a wider spray cone angle.

The temporal variations of the liquid volume fraction are shown in **Figure 17**. Affected by its special configuration, the cavitation distribution in the multi-hole nozzle is asymmetrical. Under the low need lift condition, when the injection starts, the film-type cavitation generates at a lower field of the hole inlet because of the aspects changing the flow direction. With the needle lifting up, the cavitation moves to the upper flow field and gradually develops into the string-type cavitation, which is mainly affected by the spiral and streamwise counter-rotating vortices flow structure inside the hole. Finally, the string-type cavitation can even reach the central area of the exit section, where intense mass and momentum transfer happens in the downstream region. The issues discussed above are all conducive to the wider spray cone angle [36]. However, under the micro-hole diameter condition, the cavitation intensity is much smaller, which is attributed to the higher pressure and smoother flow structure inside the hole volume of the nozzle.

The turbulence intensity increases with the enhancement of the dissipation of the spray momentum transferring to the turbulence energy, resulting in more intense liquid/gas interactions, lower spray velocity, and wider spray diffusion [37]. **Figure 18** shows the turbulence kinetic energy (TKE) distribution along the horizontal hole diameter line of different nozzles at the typical timing. The gradient of this value

between the hole boundary locations and the hole central area of different nozzles is all prominent during the injection durations. However, they present different relations in the three injection stages. Initially, affected by the higher pressure increasing rate in the sac and the consequent higher velocity in the hole, the TKE value in the boundary location of the micro-hole is a little higher than that of the normal hole, but it is lower in the central area of the hole. After that, the TKE value under the normalhole diameter condition increases a lot, and maintains a high level. However, the TKE value under the micro-hole diameter condition does not increase so much in the following injection stages, which indicates that the spray behaviors of the micro-hole nozzle would be more stable, resulting in the narrower spray angle and cone angle observed in the experiments.

According to the results of the internal flow of the different nozzles, except for the aerodynamic factors, it is the hydrodynamic factors and the unique properties of the internal flow in different nozzles that dominate their spray behaviors. The different characteristics of the flow patterns inside different nozzles mainly contribute to the deviation between their spray properties.

## **4. Conclusions**

In the current work, the differences in the spray morphology between the two realistic multi-hole diesel nozzles under different hole diameter conditions were analyzed by the high-speed video observation method during the transient injection processes. The relationship between the internal flow, cavitation variation, and spray behaviors was investigated by the numerical simulations. The main conclusions are summarized as follows:


prolongs the injection duration. Moreover, the effect of hole diameter plays different roles in the spray properties (penetration, spray angle, and cone angle) of the multi-hole nozzles. The implications of these results have practical significance when considering the diesel fuel spray trajectory within the combustion chamber.

## **Acknowledgements**

This research was supported by the Basic Research Operation Program of Dalian University of Technology (grant number 82232029).

## **Notes/thanks/other declarations**

The authors would like to thank the Mazda Corporation for technical support and AVL-Japan for providing the numerical software.

## **Author details**

PengBo Dong School of Energy and Power Engineering, Dalian University of Technology, DaLian, China

\*Address all correspondence to: pengbo.dong@dlut.edu.cn

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## *Edited by Hongliang Luo*

This book examines the fundamental research and application of droplet dynamics. It includes six chapters in four sections. Section 1 introduces the concepts of droplet dynamics in powertrain systems. Section 2 reviews the optical methods for investigations in droplet dynamics. Section 3 examines the fundamental research on liquid droplet behaviors, such as droplet impact in internal combustion engines and the movement of liquid droplets in atmospheric pressure plasmas. Finally, Section 4 deals with the application of droplet behaviors not only in spray and combustion but also in bioinspired smart surfaces. The information contained herein is useful for engineers and students looking to broaden their knowledge of droplet behaviors and dynamics, especially for their development and application in low-carbon engines.

Published in London, UK © 2022 IntechOpen © 4maksym / Dollarphotoclub

Fundamental Research and Application of Droplet Dynamics

Fundamental Research

and Application of Droplet

Dynamics

*Edited by Hongliang Luo*