**6. Potential strategies for condensation enhancement**

In addition to the condensation enhancement on the superhydrophobic surfaces as mentioned above, several other potential strategies and surfaces have recently been proposed to enhance condensation by manipulating droplet behaviors [5, 27, 28, 30, 69, 72, 100–105]. For example, a hybrid structured surface with wettability contrast was proposed to spatially control nucleation, where droplets preferentially form on the hydrophilic region [27]. By confining the hydrophilic regions on the top of the micropillars that are surrounded by superhydrophobic nanowires, such a hybrid surface can achieve both efficient droplet nucleation and jumping removal. Similar design of hybrid micro/nanostructured surfaces with wetting contrast can be found in the literature [69, 106, 107]. The slippery liquid-infused porous surface has been widely studied to promote droplet mobility [5, 28, 108–112]. A hydrophilic directional slippery rough surface was recently fabricated to improve droplet nucleation and removal [5]. Such surface consists of nanotextured directional microgrooves in which the nanotextures alone are infused with hydrophilic liquid lubricant. The surface has hydrophilic surface chemistry, a slippery interface, directional structures, and large surface areas for droplet nucleation and motion. Further experimentation under atmospheric pressure and pure vapor are needed to elucidate the condensation enhancement. Recently, another strategy was presented to enable thin film condensation on a porous superhydrophilic surface with a hydrophobic coating as a nucleation deterrent and energy barrier for the liquid film to propagate above the surface [101, 113, 114]. Further experimental demonstration of heat transfer enhancement of such thin film condensation is needed. It is noted that most of the surfaces mentioned above require precision fabrication, which is difficult to scale up cost-effectively to meet the large area applications.

Some emerging strategies and surface design provide the potential to achieve condensation enhancement by using macrotextures or low-cost commercial materials [72, 100, 103, 104, 115–117]. The plain hybrid surfaces, consisting of plain hydrophilic and hydrophobic regions, were proposed to achieve dropwise-filmwise condensation for reducing the droplet departure size [23, 100, 115, 116, 118–123].

### *Advances in Dropwise Condensation: Dancing Droplets DOI: http://dx.doi.org/10.5772/intechopen.92689*

nanowires, droplet wetting transition still occurs on both the uniform and hierar-

subcooling, resulting in the heat transfer degradation [21, 65]. This is because the active nucleation size decreases with the increase of surface subcooling while the straight nanowires tend to agglomerate and form a large number of micro-defects on the surface in fabrication processes [65, 73]. For condensation on such structured surfaces, vapor prefers to nucleate first in the micro-defects with a smaller energy barrier, resulting in the formation of large pinned droplets and flooding phenomenon [67, 68]. To avoid the agglomeration of straight nanowires, a 3D copper nanowire network fabricated has recently been demonstrated to eliminate the micro-defects between nanowires [71]. Benefiting from the interconnections between the nanowires, the surface morphology of the 3D nanowire network appears to be homogeneous without micro-defects. Compared to the droplet wetting transition on the uniform and hierarchical nanowired surfaces (**Figure 9c,d**), sustainable droplet jumping phenomena is obtained on the superhydrophobic surface with 3D nanowire networks even at a large surface subcooling of 28 K (**Figure 9e**). Benefiting from the formation of suspended and partially wetting droplets, a significant enhancement in heat transfer is realized on the 3D nanowire

chical nanowired surfaces (**Figure 9c,d**) with the increase of the surface

*21st Century Surface Science - a Handbook*

networks throughout the wide range of surface subcooling experimented

difficult to scale up cost-effectively to meet the large area applications.

Some emerging strategies and surface design provide the potential to achieve condensation enhancement by using macrotextures or low-cost commercial materials [72, 100, 103, 104, 115–117]. The plain hybrid surfaces, consisting of plain hydrophilic and hydrophobic regions, were proposed to achieve dropwise-filmwise condensation for reducing the droplet departure size [23, 100, 115, 116, 118–123].

In addition to the condensation enhancement on the superhydrophobic surfaces as mentioned above, several other potential strategies and surfaces have recently been proposed to enhance condensation by manipulating droplet behaviors [5, 27, 28, 30, 69, 72, 100–105]. For example, a hybrid structured surface with wettability contrast was proposed to spatially control nucleation, where droplets preferentially form on the hydrophilic region [27]. By confining the hydrophilic regions on the top of the micropillars that are surrounded by superhydrophobic nanowires, such a hybrid surface can achieve both efficient droplet nucleation and jumping removal. Similar design of hybrid micro/nanostructured surfaces with wetting contrast can be found in the literature [69, 106, 107]. The slippery liquid-infused porous surface has been widely studied to promote droplet mobility [5, 28, 108–112]. A hydrophilic directional slippery rough surface was recently fabricated to improve droplet nucleation and removal [5]. Such surface consists of nanotextured directional microgrooves in which the nanotextures alone are infused with hydrophilic liquid lubricant. The surface has hydrophilic surface chemistry, a slippery interface, directional structures, and large surface areas for droplet nucleation and motion. Further experimentation under atmospheric pressure and pure vapor are needed to elucidate the condensation enhancement. Recently, another strategy was presented to enable thin film condensation on a porous superhydrophilic surface with a hydrophobic coating as a nucleation deterrent and energy barrier for the liquid film to propagate above the surface [101, 113, 114]. Further experimental demonstration of heat transfer enhancement of such thin film condensation is needed. It is noted that most of the surfaces mentioned above require precision fabrication, which is

**6. Potential strategies for condensation enhancement**

(**Figure 9f,g**).

**242**

On such a hybrid surface (**Figure 10a**), the growing droplets in the hydrophobic regions are removed when they grow large enough to contact with the liquid film in the hydrophilic regions, resulting in a smaller removal size than that of gravitydriven departure. Another attempt in coupling droplets with a liquid film for enhanced condensation was recently reported on a hierarchical mesh-covered surface [72]. The typical structural feature of the hierarchical mesh-covered surface is composed of an interweaving microchannel network between a superhydrophobic woven mesh layer and a flat substrate, and a large number of micropores among mesh wires (**Figure 10b**). Vapor permeates freely through the mesh layer and condenses on the substrate to form a thin liquid condensate film that is confined in the interconnected channel network. When the condensate film grows out of the micropores due to the increased condensation heat flux, the surrounding liquid condensate can be drained out by the liquid film flow and eventually leaves the superhydrophobic hierarchical mesh-covered surface in the form of gravity-driven falling droplets. The thin liquid film in the interweaving microchannel network not only provides efficient low-thermal resistance condensation interfaces but also continuously transports liquid condensate to be drained out from the substrate.

When the NCG presences in water vapor, condensation performance is highly dependent on the initial nucleation and mass transfer of vapor molecules in the diffusion layer near the condensing surface [103, 124, 125]. A hydrophilic copper surface with interval fluorocarbon-coated hydrophobic bumps to enable falling droplet-enhanced filmwise condensation (**Figure 10c**). Due to the decreased nucleation energy barrier on the hydrophilic regions, water vapor can rapidly nucleate, grow, and form a thin liquid film on the vertical surface. To prevent continuous thickness growth of condensate liquid film along the vertical tube, interval hydrophobic bumps are designed to remove liquid film periodically, which is enabled by the reduction of surface adhesion for condensate liquid on the hydrophobic bumps. More importantly, the condensate liquid departing from the hydrophobic bumps in the form of falling droplets can strongly disturb the NCG diffusion boundary layer. High-performance condensation heat transfer in the presence of NCG was

#### **Figure 10.**

*Emerging strategies and surface design for enhancing condensation heat transfer. (a) Hybrid surface with hydrophobic and hydrophilic stripes to enable dropwise-filmwise condensation [100]. (b) Sucking flow condensation on a hierarchical mesh structured surface [72]. (c) A hydrophilic copper tube surface with interval fluorocarbon-coated hydrophobic bumps [103]. (d) A superhydrophobic surface covered with macrogroove arrays [104].*

experimentally demonstrated on the vertical hydrophilic copper tube with hydrophobic fluorocarbon-coated bumps, which is better than both the conventional filmwise and dropwise condensation while avoiding the durability issues of ultrathin hydrophobic coatings. Recently, a structured surface with macrogroove arrays was proposed to improve droplet jumping dynamics in the presence of NCG by coupling rapid droplet growth and efficient droplet jumping relay (**Figure 10d**) [104]. The droplets formed on top of the cones and the bottom of the grooves play different roles during condensation process. Specifically, the cones can promote droplet formation and growth by breaking through the limitation of NCG layer. The droplets with higher mobility can be formed on the bottom of the grooves, resulting in series of coalescence-induced droplet jumping. Such a droplet jumping relay can enable a considerable vibration to trigger the jumping removal of droplets on top of the cones. Research Funds for the Central Universities (No. DUT20RC(3)016), and thank their collaborators who made important contributions to the work reviewed in this chapter including Ronggui Yang, Zhong Lan, Yung-Cheng Lee, Benli Peng, Wei Xu,

Sifang Wang.

**Conflict of interest**

*D*<sup>12</sup> diffusion constant Δ*G*<sup>e</sup> nucleation energy, J

*J* nucleation rate *J*<sup>0</sup> kinetic constant *k* Boltzmann constant

*p*<sup>r</sup> vapor pressure, Pa *p*<sup>s</sup> saturation pressure, Pa

*q* heat flux, W/m<sup>2</sup> *r* surface roughness

*Sc* Schmidt number *T* temperature, K Δ*T* surface subcooling, K *U* free-stream velocity, m/s *w* mass transfer flux, mol/m<sup>2</sup>

*θ* contact angle, °

*ψ* solid fraction

**245**

*h*fg latent heat of vaporization, J/kg

*N* droplet population density, m�<sup>3</sup>

*r*max droplet departure radius, m

*θ*adv advancing contact angle, ° *θ*<sup>C</sup> contact angle of Cassie droplet, °

*θ*rec receding contact angle, °

Δ*θ* contact angle hysteresis, °

*μ* kinematic viscosity, N s/m2 *ρ*l liquid density, kg/m<sup>3</sup>

*δ***<sup>0</sup>** thickness of boundary layer, m

*θ*<sup>W</sup> contact angle of Wenzel droplet, °

*σ*lv liquid-vapor interfacial tension, N/m *σ*sv solid-vapor interfacial tension, N/m *σ*sl solid-liquid interfacial tension, N/m

*q*<sup>d</sup> heat transfer through individual droplet, W

*r*min minimum droplet nucleation radius, m

**Nomenclature**

The authors declare no conflict of interest.

*Advances in Dropwise Condensation: Dancing Droplets DOI: http://dx.doi.org/10.5772/intechopen.92689*

#### **7. Summary**

This chapter reviewed the recent advances in the fundamental understanding and performance enhancement of dropwise condensation by dancing droplets, as well as some other emerging enhancement strategies and surface design. Various micro/ nanostructured surfaces, along with functional wetting coatings, have been developed with designed morphology for diverse surface features. Addressing the intrinsic requirements on multiple length scales in the nucleation, growth, merge/coalescence and departure of the dynamic droplets, unprecedented enhancement in heat transfer performance has been demonstrated for dropwise condensation processes.

An efficient condensing surface should enable both rapid droplet growth and frequent surface refreshing. Due to the excellent surface refreshing capability, jumping droplet condensation on the superhydrophobic surfaces is one of the most active research areas over the last decade on enhancing condensation heat transfer. However, as surface subcooling increases, the mobile droplets in the suspended Cassie state can transition to the highly pinned Wentzel state due to the nucleation occurring within the structures, resulting in the flooding phenomenon and performance degradation. By decreasing the structure scale to be comparable with critical nucleation size, superhydrophobic surfaces with closely spaced nanowires have been demonstrated to minimize droplet nucleation within the structures and to promote the formation of mobile droplets on the surface. On such a nanowired surface, excellent water repellency has been demonstrated to enable efficient jumping droplets without flooding phenomenon, even at a large surface subcooling. A significant enhancement in heat transfer was also achieved under a very wide range of surface subcooling experimented. In addition to the superhydrophobic nanowired surfaces, several other strategies have also been proposed to enhance condensation processes, for example, improving droplet nucleation and jumping by designing hydrophilic patterns on the superhydrophobic surfaces, accelerating droplet removal through liquid film sucking on the hybrid surfaces with hydrophilic and hydrophobic strips, promoting thin film condensation using hybrid nanostructured surfaces, enhancing droplet mobility and transport using slippery liquidinfused porous surfaces, and improving liquid condensate removal using hierarchical mesh-covered surfaces.

#### **Acknowledgements**

The authors acknowledge the continuous support from National Natural Science Foundation of China (Nos. 51836002, 51706031, and 51236002), the Fundamental

*Advances in Dropwise Condensation: Dancing Droplets DOI: http://dx.doi.org/10.5772/intechopen.92689*

Research Funds for the Central Universities (No. DUT20RC(3)016), and thank their collaborators who made important contributions to the work reviewed in this chapter including Ronggui Yang, Zhong Lan, Yung-Cheng Lee, Benli Peng, Wei Xu, Sifang Wang.
