**4. Small exponent—big impact: liquid marbles**

#### **4.1 State of the art**

Liquid marbles are non-wettable structures, formed as a result of physical interactions between solid particles and a liquid drop. The formations are in fact represented by a liquid core covered in a particle shell (**Figure 6**) and exhibit a superhydrophobic-like behavior, without the intervention of surface modifications.

Among the first intents to obtain liquid marbles were carried out by Aussillous and Quéré [69], by rolling water droplets (1–10 mm3 ) in a hydrophobic silicacovered *Lycopodium* powder bed (20 μm), as presented in **Figure 7**.

When compared to plain water drops, the manufactured liquid marbles did not wet the support, due to the fact that the liquid-solid interface (water-glass) is replaced with a solid-solid interface (*Lycopodium* particles-glass). They resemble raindrops falling on lotus leaves and collecting dust particles while rolling off, as previously discussed (The Lotus Effect) [70].

 Liquid marbles' formulations are versatile, including various powders which differ in color, wetting degree, electrical charge, and even therapeutic activity.

 **Figure 6.**  *Liquid marble structure.* 

**Figure 7.**  *Obtaining liquid marbles by rolling water drops into a Lycopodium powder bed.* 

 Literature data indicates natural and synthetical powders such as *Lycopodium*, soot [71], respectively, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyethylene (PE) [72], polymethyl methacrylate (PMMA) [73], and hydrophobic copper powder [74]. The hydrophobic particle wall thickness varies depending on the particles, which are linked by van der Waals forces and distribute as mono- or a multilayers. In time, the wall undergoes changes depending on environmental conditions: multilayered shells correspond to "long surviving marbles," due to the possibility to stretch of particles surrounding the circumference, maintaining the system's integrity. Moreover, since large particles do not provide liquid core's flexibility and protection, nanoparticles are recommended as shell formers which fill in the gaps formed through compression [75].

 Similar to superhydrophobic surfaces, liquid marbles can also exhibit special structural architectures, depending on their components. Particular cases include porous shells made of hydrophobic poly-high internal phase emulsion (HIPE) polymer, with particles interconnected by "gigapores" of micronic dimensions, resembling natural organisms like radiolarians (protozoa-producing mineral microtubes) or diatoms (microalgae with cells interconnected by tubes) [76]. After the CuSO4 solution (core) evaporates through the shell, a CuSO4 shell remains. The method is proposed as the model in designing spherical objects. Among liquid marbles with curious properties are the ones guided using electric fields which resemble Janus particles. They are obtained by forcing together two marbles with different shells, resulting in a bigger marble: half covered in carbon black and the other in Teflon (**Figure 8**) [77].

Liquid marble's interior phase usually includes high-surface tension liquids like water or glycerol, but literature data also suggests low tension liquids such as ethanol, methanol, toluene, hexadecane, and 1,4-dioxane [78]. It is possible for the shells' particles to remain at the liquid-gas interface or to be engulfed by the liquid core, resulting in stable marbles [72]. Other particular liquid marbles include Galinstan (eutectic liquid mixture of gallium, indium, tin) covered in Teflon, isolators (SiO2), or semiconductors (CuO, ZnO, WO3). They are resistant to high temperatures, float on water, but must be obtained in a diluted hydrochloric acid solution, as an unwanted reduction reaction takes place in air [79].

Cases of hydrophilic particle-covered liquid marbles are possible due to air trapped between particles, resulting in aggregates which cover the droplets [80].

**Figure 8.**  *Liquid marbles resembling Janus particles.* 

#### *Natural and Artificial Superwettable Surfaces-Superficial Phenomena: An Extreme Wettability… DOI: http://dx.doi.org/10.5772/intechopen.84137*

When discussing liquid marbles obtaining procedures, the most popular manufacturing method is the droplet rolling in a powder bed, as previously presented. Continuous research is developed concerning this domain since the proposed method is inefficient and time-consuming; irregularly covered marbles are formed and cannot be transposed at an industrial level. Methods including condensation and drop nucleation were recently reported: the liquid core is placed in a container, warmed by a heat source underneath. Hydrophobic particles (Cab-O-Sil fumed silica and micronic-sized Teflon) are distributed in a thin layer at the liquid-air interface. As the liquid boils, vapors condense and are covered by the particles. Micronic liquid marbles are formed. By heating these "parent-marbles," much smaller liquid droplets called "child-liquid marbles" are formed ("liquid marbles sweating"). The "child-marbles" roll off the "parent-marbles" and are more robust. Advantages of the method include industrially applicability of the technique and possibility to adapt conditions depending on the desired result [81]. Wrapping drops in transparent glass fibers, avoiding fluid evaporation, is a proposed design in developing new controlled drug release systems, water purification membranes [82]. Another automatized method is considered revolutionary by using instead of hydrophobic powders a superhydrophobic cloth of nanofibers. The drop is covered after impacting the cloth, resulting in highly resistant liquid marbles, with no internal phase loss [83].

Regarding their formulation, liquid marbles are versatile structures. The challenge is represented by choosing the appropriate components and experimental parameters of the fabrication/manufacturing process.

#### **4.2 Liquid marbles: superhydrophobic entities with unique properties**

Experimentally formed liquid marbles exhibit slightly different properties compared to naturally formed ones. Raindrops fall from big heights and get covered with particles due to internal currents and to kinetic energy [84]. Thus, the marbles exhibit *elastic-solid* and also *fluid* properties, known as a double "solid-fluid" character. The assumption that liquid marbles' elasticity is related to replacing liquidsolid (support) interface with solid (shell)-solid (support) interface is sustained by the absence of colored traces left by sodium hydroxide liquid marbles rolled on a phenolphthalein surface [85]. Moreover, shape changes occur for viscous marbles placed on an inclined plane: centrifugal forces determine marbles to slide off, while acceleration leads to the transformation of the spherical shape into "peanut," toroidal/"doughnut" shape [86], as presented in **Figure 9** [87].

Other experiments on liquid marbles' shape and elasticity proved how gradual compression of the marbles resulted in successive cracking and ultimately breaking of the shell, followed by collapse. Before the collapse, marbles allowed a compression up to 30% from the initial dimension [88].

*Coalescence* of the drops and possibility to engulf exterior objects may also be related to elasticity. As a result of applying exterior forces, two different liquid marbles connected through a glass bar undergo coalescence, forming a bigger structure and sharing a divided shell, as illustrated in **Figure 10(a)**. Regarding the possibility to "swallow" other objects, organic liquid covered in FD-POSS marbles is injected with another organic fluid. As long as the condition of immiscibility between the liquids is respected (proposed liquid pairs: toluene/DMSO, hexadecane/water), "encapsulating liquid marbles" are formed (**Figure 10 (b)**) [89].

Other liquid marbles' curious properties reside from freezing and drying in extreme temperature conditions. Experiments on PTFE-covered liquid marbles reveal surface aggregates, and multilayers are formed at the liquid-air interface, triggering wall thickening and shrinkage during *evaporation*. Thus, slow evaporation

#### **Figure 9.**

*Rolling liquid marbles: (a) "peanut" shape, (b) "doughnut" shape, and (c) transformation of "doughnut" into "peanut" shape, as the plane is removed [87].* 

#### **Figure 10.**

*(a) Liquid marble coalescence: (b) FD-POSS liquid marble encapsulating DMSO.* 

of water results in prolonged resistance of the microparticle-covered marbles, with emerging applications in microfluidics [90]. The liquid marbles' shell layering raised curiosities: a mono-stratified shell determines the marbles to dry faster than a plain drop. The explanation lies in the fact that heat generates shrinkage at the liquid-air interface in case of the uncovered drop, while solid particles block interface compression during drying. Marbles covered in a multilayered shell dry harder than uncovered drops, depending on the thickness of the particle layer. Studies reveal the importance of environmental temperature and humidity in investigating evaporation: humid air delays evaporation of the liquid marble's internal phase [91, 92].

 On the opposite pole of heating marbles are *freezing* ones, which were firstly reported as *Lycopodium*-covered water on a silicone support at −8°C. The marbles changed their shapes, as they flatten and extend sides: the "dome" shape evolves into a "flying saucer" shape, while the freezing process begins at the bottom and advances toward the top (**Figure 11**) [93].

 Liquid marbles' behavior while *floating* on a liquid surface was also considered in recent experiments, since not only the liquid support surface deforms but also the marble itself. Particles covering the marble are packed between two fluids (liquid marble's core and carrier liquid) and change distribution leading to the marbles' collapse and release of the core into the support liquid. This phenomenon happens in normal conditions. In humid atmosphere, marbles maintain their shape many days while floating [94]. As expected, the deformation of the interface increases, consecutive to larger drops [95].

 After floating investigations, *self-propelling* of liquid marbles became of interest, when an autonomous movement similar to Leidenfrost droplets was reported for water and alcohol marbles covered in extremely hydrophobic fumed silica. Supports include Petri dishes with water, and straight-line movement was observed. Taking into account the particles separating the core from the exterior, in the floating case,

*Natural and Artificial Superwettable Surfaces-Superficial Phenomena: An Extreme Wettability… DOI: http://dx.doi.org/10.5772/intechopen.84137* 

**Figure 11.**  *Shape changes in freezing liquid marbles: Initial, "dome," and "flying saucer."* 

a vapor layer is responsible for marbles' support, similar to the Leidenfrost effect. The layer forms as ethanol evaporates (from the core). The Marangoni flow is triggered by ethanol condensation on the water support surface. Thus, the marbles begin to move without rolling. Fumed silica and Teflon-covered Janus marbles present no black traces while moving [96].

## **4.3 Small-scaled superhydrophobicity with innovative applications**

Due to their versatile formulations and their special superhydrophobic-like properties, liquid marbles exhibit promising applications in various domains.

In the pharmaceutical domain, liquid marbles are known as precursors of hollow granules, microcapsules, and Pickering-like emulsions. Polytetrafluoroethylene (PTFE), aerosil, and Ballotini spheres as shells and binders (PVP, HPMC, HPC) are used to form liquid marbles which are dried through various methods: moist air at 24°C, freezing at −50°C, and dry air at 60°C, 80°C, 100°C). *Hollow granule*  formation is promoted by high drying temperatures, nanometric particles, and high binder concentrations. Therefore, aerosil proved successful regarding spherical shape generation, when used together with HPMC and drying at 100°C forming ideal-shaped hollow granules [97], as presented in **Figure 12** [87].

 Moreover, liquid marbles are able to include low solubility and hydrophobic active ingredients, representing formulation alternatives in case of substance incompatibilities and targeted release drugs (e.g., intestine and not stomach). The active ingredient's protection is mandatory against local acidity/enzymes and pathogens/other substances competing for binding sites and can be achieved by choosing the ideal development process while following Quality by Design Guidelines [87].

*Microcapsules* can also be obtained from liquid marbles: exposed to solvent vapors, submicrometer-sized polystyrene particles (PDEA-PS) covering

**Figure 12.**  *Hollow granule [87].* 

poly(2(diethylamino)ethyl methacrylate) cores undergo a polymerization reaction, forming a filled capsule. After the core evaporates, the now empty microcapsule maintains its shape, representing an important idea for further design of modified drug release systems [98].

Liquid marbles have also been reported as precursors of *Pickering-like emulsions*. The difference between classical and Pickering emulsions stands in the lack of added stabilizing agents. Pickering emulsions are stabilized by solid particles adsorbed at the internal and external phase interface. These adsorbed particles are attached to the drop they cover and are wetted in both the watery and the oily phases, conferring integrity to the immersed drop [99, 100]. Stabilizer particles include clays, latexes, calcium carbonate, carbon black, magnetic particles, proteins, and even bacteria. Hydrophobic particles stabilize water/oil emulsions, and hydrophilic ones stabilize oi/water emulsions [99]. Stable Pickering like emulsions containing *Lycopodium-*covered liquid marbles were obtained in PDMS, as presented in **Figure 13**.

Experiments show good stability of marbles immersed in less polar liquids (silicone fluids, aromatic solvents), while collapse is a trigger in polar solvents. Pickering-like emulsions find their applicability in cosmetic formulations due to no allergenic, cytotoxic, or hemolytic stabilizers. Topical use of caffeine Pickering emulsions in controlled studies revealed higher absorption than the other pharmaceutical forms, due to silica-covered liquid marbles, which promote epidermal caffeine absorption from the aqueous phase of the emulsion [101]. Also, retinol included in the oily phase of a Pickering-like emulsion is stabilized against UV radiation and only penetrates the corneous layers of the epidermis [100].

Sticking to the field of topical application, liquid marble formulations represent a basis in foundation, antiperspirants/deodorants, solar protection products, and some drug formulation. Easy application is followed by a moisture and cooling sensation due to internal phase liberation. Among components, the most popular are deionized/floral water (50–90%) mixed with polymers/copolymers (PVP), wetting agents (hyaluronic acid), hydrosoluble vitamins, preservatives, and antioxidants. Such formulations are recommended for oily skins, due to a low oil content (<10%). Therapeutic agents may be added: antibacterial, antifungal, analgesic, keratolytic, corticosteroids, etc.

A novelty in blood typing is represented by liquid marbles as *biological microreactors*: hydrophobic-precipitated CaCO3-covered blood drops are injected with antibody solutions (anti-A, anti-B, anti-D). If the color changes from red to dark

**Figure 13.**  *Pickering-like emulsion.* 

*Natural and Artificial Superwettable Surfaces-Superficial Phenomena: An Extreme Wettability… DOI: http://dx.doi.org/10.5772/intechopen.84137* 

**Figure 14.** 

*Hemagglutination reaction inside a blood liquid marble [87].* 

red, the hemagglutination reaction is considered positive, as illustrated in **Figure 14**, and the blood type is immediately identified. If the color does not change, the reaction is negative, and another drop is tested using another antibody. The technique is considered innovative: low contamination risks and reagent costs and necessity of small blood samples, which is beneficial for patients [102].

 PTFE liquid marbles were also used as *cell culturing medium*: spheroids successfully developed from HepG2 hepatocellular cancer cells. Advantages include promoting cell aggregation due to restricted space and no human intervention. In order to screen cell development inside the marble, magnetic particles are proposed for shells, leaving an observation section open when a magnetic field is near. These methods are used in cell physiology screening or tissue engineering [103], stem cells evolution into embryoid bodies [104], and bacterial culturing especially for anaerobic species [105].

 Liquid marbles are providers of 3D spherical space with adjustable volume and formulation and can also be regarded as *chemical micro-reactors*, hosting different chemical reactions resulting in toxic/explosive unwanted products, in small amounts, and isolated. Intervention from the outside is possible for "intelligent marbles" covered in magnetic particles, in order to inoculate a new reagent into the reaction, collect a product, identify, or quantitatively evaluate a certain compound. Indicators of core chemical reactions include color changes, chemiluminescence, and precipitation reactions [106]. Besides hosting chemical reactions, some marbles called "plasmonic liquid marbles" are covered in Ag/Au nanoparticles and represent special analytical platform precursors. They function as qualitative and also quantitative detectors for waste products resulted from industrial spills, being able to trace compounds in concentration of femto- or ato-molar concentrations (10<sup>−</sup>15, 10−18) [107]. "Cleaning agent stimuli-responsive liquid marbles" detect pollutants and signal their presence by shell breakage. The core eliminates a detoxifying agent (1 N-oxone covered in Cab-O-Sil T-530 shells) which cleans oil-contaminated water [108].

## **5. Conclusions**

This chapter is an interdisciplinary approach on extreme wettability, granting particular attention to superhydrophobic natural and artificial surfaces and to liquid marbles, as exponent. Literature data is reunited in order to offer a unique and complex understanding of superficial properties from a theoretical point of view, in correlation with examples from the natural environment. An extensive picture illustrates how superhydrophobicity was initially interpreted, how its understanding evolved, becoming of large exploitation in many industrial fields. Superficial properties and liquid marbles are linked through conceptual similarities, as an opening gate to numerous applications.

#### *Wettability and Interfacial Phenomena - Implications for Material Processing*

 Liquid marble exploration substantially advanced during the last years, from the phase of basic understanding through wetting models to more complex interpretations, obtaining methods and applications. Studies revealed a "non-wetting" contact with solid supports and many unexpected properties, such as versatility in choice of cores and shells, recoverable deformability, ability to float on water, low evaporation rate, and significant advantages derived from a well-confined compartment. Emerging applications discussed in this chapter are diverse and offer a rich variety of further exploitation possibilities, arising from complex structural designs.
