With the help of biomimetics, superficial characteristics were transposed, through various methods, onto artificially obtained materials. Many industrial fields applied surface architecture modifications as improvements of classic materials/methods. The medico-pharmaceutical, biochemical, transportation, and textile fields are few examples of industrial areas welcoming a “structural change.” Anti-bioadhesion was widely exploited by means of antibacterial or self-cleaning fabrics and cell culturing/screening/isolation. Anti-icing, antireflective, and anticorrosion materials/coatings gained attention in the transportation and optical device fields. Interdisciplinary approaches on extreme wettability include “solid-fluid” formations called liquid marbles, which will be further discussed as a superhydrophobic behavior exponent.
- superficial phenomena
- extreme wettability
- special surface architecture
- liquid marble
Since ancient times, humans observed special features which helped plants and animals survive in harsh environments. These properties were unraveled with the help of microscopical investigative techniques, which led to a more thorough understanding of superficial phenomena. Natural unique superficial architectures, like the lotus and rose petal effect, became iconic. Empirical models of wettability were developed (Young, Cassie, and Wenzel) to fully explain the behavior of liquids in contact with special surfaces.
Along with the fulminant expansion of technology during the last decades, a huge progress was also registered in surface sciences. Microscopical analysis techniques revealed surfaces’ special architectures and solved many mysteries regarding plants and animals’ adaptation to harsh environments (e.g., Namib beetles’ survival in the desert). Extreme wettability (superhydrophobicity/superhydrophilicity) was assigned to many natural phenomena, such as raindrops not collapsing while dropping onto ash-covered soil and moss storing the exact amount of water needed to survive. In particular, superhydrophobic surfaces which display a contact angle higher than 150°, a sliding angle smaller than 10°, and no hysteresis attracted researchers’ attention. Apart from theoretical aspects on wettability, which are a part of the paper, natural extreme wettability models will be discussed (lotus leaf, rose petal, and insect wings).
Principles of biomimetics are included in this chapter, as special superficial properties were adapted to human necessities and used as a model in many industrial areas, including nanotechnologies. Biomimetics is, in this case, “the thread that makes the dress complete,” or, in other words, “the scene that completes the movie.” Applications related to superhydrophobicity will be presented: development of self-cleaning and low friction surfaces, satellite antennas, solar and photovoltaic panels, exterior glass, etc. Studies concerning superhydrophobic surfaces’ applications in various domains will also be submitted: prevention of bacterial adhesion, of metal corrosion, of surface icing in humid atmosphere and low-temperature conditions, blood type determination techniques, etc. Efficient, cost-effective, ecological, and reproductible methods are still developing so that mass production of quality materials becomes a fact.
The chapter will also bring into attention an important exponent of superhydrophobicity: special structures called liquid marbles. The unique “solid-fluid” formations are regarded as soft objects, due to the microliter droplet encapsulated in hydrophobic particles. Practical uses include micro-reactors, miniature cell culturing, or screening devices, successfully replacing classical methods with cost- and reactive-efficient, low-toxicity analysis techniques. Other properties will be submitted along with applications in various fields.
2. Biomimetics: biology vs. technology
As human kind evolved, passing through the test of time, many necessities turned out as a result of convenience in everyday life activities. Thus, classical materials like wood, metal, and ceramic became no longer suitable and efficient, lacking performance in many domains (e.g., pharmaceutical, medical devices, weaponry, etc.). Aiming a more complex approach on artificial materials, the concept “materials by design” came to life (Bernadette Bensaude-Vincent, 1997) . The concept refers to developing “composite” materials, which reunite properties of already known ones: heat resistance and time durability of ceramics, lightness of plastic, hardness, and breaking resistance of metals. Depending on quality requirements of the final product, many design possibilities came out, exhibiting improved sustainability, cost-effectiveness, durability, and an environmentally friendly character.
Undoubtedly, a much older concept, “biomimetics,” also led to obtaining performant structural materials and is intricated to the “materials by design” concept. The term itself (“biomimetics”) was firstly introduced by Otto Schmitt , but its principles are considered to be used since Leonardo da Vinci (1452–1519) while designing flying machines after analyzing bird’s ability to fly .
According to some beliefs, biomimetics is a transfer of ideas between biology and technology, aiming to obtain superior device. A more complex approach refers to it as being “a study of the formation, structure or function of biologically produced substances, materials, biological mechanisms and processes especially for the purpose of synthetizing similar products by artificial mechanisms which mimic natural ones” .
As expected, a lot of controversial interpretations arise from different approaches of biology and engineering, considered “baselines” of biomimetics. On the one hand, biology relates to living organisms (cells, plants, and animals) which evolve following a natural DNA-embedded cycle. On the other hand, engineering relies on human intelligence which develops successive steps in order to obtain a final product .
Conflicts between biology and technology interpretations are classified through a Russian problem-solving system TRIZ (
Therefore, it became appropriate to say that “superficial properties” is a concept which can be regarded in the light of both “biology” and “technology.” Structural investigation of natural (biological) surfaces is performed using microscopical techniques. Technical interpretations of these surfaces and empirical models arise. Examples include special wettable surfaces like the lotus leaf, rose petal, Salvinia leaf, insect eyes, wings, fish scales, etc. They provide templates in designing new engineered materials exhibiting improved properties compared to classical materials. Such artificially obtained materials and coatings can be considered results of “materials by design” and “biomimetics” concepts, as a reunion of biological inspiration and human engineering. Even though many contradictory assessments take place, it is important to state that biology and technology functioned perfectly together when inventing the Velcro closure system according to the way burdock (
3. Extreme wettability: special patterns
3.1 Understanding wettability
As is well known, surface wettability characterizes interfacial phenomena between a liquid and a solid support. The liquid’s behavior on the studied surface is in fact an indicator of wettability, a superficial property which helps evaluate hydrophilicity/hydrophobicity of a solid. The quantitative indicator of wettability is represented by the contact angle, given by Young’s equation (Eq. (1)):
where θ is the contact angle, γSV is the solid-vapor superficial energy, γSL is the solid-liquid superficial energy, and γLV is the liquid-vapor superficial energy .
The equation establishes an equilibrium between superficial energies at the solid-liquid-air interface. However, adaptations of Young’s equation were proposed by Wenzel  and Cassie-Baxter , after it was proven that the original equation only applies to homogenous, smooth surfaces and that the contact angle is influenced by the support’s rugosities, as a surface roughness indicator.
Wenzel’s equation (Eq. (2)) applies to non-smooth surfaces. Surface rugosity is interpreted through the roughness factor r, defined as ratio of the actual rough surface area to the geometric area projected on a relatively smooth surface. This adapted equation refers to an apparent contact angle θ′, as follows (Eq. (2)):
Another relationship defining an apparent contact angle θ′ is similar to Wenzel’s equation, with the difference that the surface’s rugosities are separated by impenetrable air pockets (Cassie-Baxter wetting model). The surface f in direct contact with the liquid is considered, as follows (Eq. (3)):
where a and b are the contact areas with the drop (a) and, respectively, air (b). Considering (1 − f) the drop-air contact area and a contact angle of 180°, the calculation formula corresponding to the Cassie-Baxter wetting regime is shown in Eq. (4):
Empirical models of the Young, Wenzel, and Cassie-Baxter wetting states are presented in Figure 1.
Other interpretations by Quéré et al. [6, 7] consider the Wenzel wetting regime as an equilibrium state of the Cassie model: a critical value of the fraction f determines a critical contact angle θc, determined by the following equation (Eq. (5)):
Since wettability studies continue to unfold, researchers recently proved that Wenzel and Cassie wetting regimes actually co-occur on the same support surface. Hydrophobic surfaces with linear or pillar patterns exhibit both a Cassie levitating state corresponding to drops placed on the support and also a Wenzel pinned state for drops which come into contact with the surface after the impact. Transitions between these states were also reported as a result of external stimuli influence [8, 9]. Wenzel to Cassie and Cassie to Wenzel transitions were analyzed through sequential squeezing and releasing between texture surfaces of nonadhesive plate. Results indicate that both regimes exist at the same time on a double-scaled textured surface, resembling natural micro- and nano-surface architecture: the Wenzel state is characteristic for the larger texture and Cassie to the smaller one . Further investigations consisted in exploiting these characteristics and developing super-repellent materials, also based on natural models, following the principles of biomimetics.
The “superwettability system,” briefly presented in Figure 2, includes a much extensive approach on wetting states, depending on the liquid type, the solid support’s architecture, and the environment in which the phenomenon is described. Thus, the terms discussed above (hydrophilicity/hydrophobicity) refer to water’s behavior in air and upon flat surfaces. Regarding low-surface liquids, such as oils, the “oleophilic/oleophobic” concepts are defining. Moreover, if the support exhibits a nano-/micro-rough architecture, then the behavior of liquids when contacting such a surface is known as “superhydrophobic/superhydrophilic” and “superoleophobic/superoleophilic.” Corresponding wetting behaviors under water for structured rough supports are known as “superoleophobic/superoleophilic” and “superaerophobic/superaerophilic.” If placed under oil, then the appropriate approach refers to “superhydrophobicity/superhydrophilicity” and also “superaerophobicity/superaerophilicity” .
3.2 Natural designs
Natural special surfaces transposed as survival skills in animals and plants captivated attention of researchers. They investigated and applied in practice what nature provided. Apart from scientists, novelists like Jules Verne were fascinated by certain elements from the environment and used them as inspirational sources to imagine innovative devices, mostly designed as transformational means and considered eccentrical in that era: the Nautilus submarine whose shape resembled a whale, the eponymous Steam House—a mechanical elephant, the helicopter imagined starting from insects’ flight mechanisms and shapes, etc.
Apart from mechanical devices, natures’ kingdom offered humans the possibility to improve artificial materials, based on the evolution of SEM analysis techniques in the 1960s. Detailed investigations of surface structure and properties were performed. As a result, surface architecture was held responsible for many phenomena which were not explained at that time: how plants maintain clean in marshy environments and how their water needs are satisfied during high-temperature exposure. From this category, two types of surface structures, designed as micro- and nano-scaled patterns, confer superhydrophobicity to the leaves of certain plant species: lotus, rice, and taro. Another model which confers water repellency was attributed to a unitary structure of 1–2 μm fibers (Chinese watermelon, Ramee leaves). Also, vertical/horizontal hairs were attributed in the property of water repellency in case of
The iconic plant superhydrophobic behavior belongs to the lotus leaf (
The other side of the lotus leaf presents no waxy crystals, but has tabular nano-groove convex lumps which confer inverse wettability .
An example of unique structural characteristics is the carnivorous plant
An exponent of plant adaptation to harsh environment conditions is represented by
“The Rose Petal Effect” reveals how nano-folds covered with micro-papillae of rose petals confer contact angle values of 152°, resembling the Cassie impregnating model: water droplets maintain their spherical shape, adhere to the surface, and do not slip when turned upside down. Compared to the lotus leaf architecture, this wetting regime is characterized by a liquid film which impregnates the papillae, leaving only some dry areas. A dependence was observed between the drops’ volume and surface tension: the equilibrium is ruined and the drop falls if it exceeds 10 μL in volume. Thus, smaller drops stay stable, while raindrops slide off, since they are bigger . Figure 4 illustrates a comparison between the rose petal (a) and the lotus leaf (b) surface structure .
Transitioning from the plant to the animal kingdom, it is important to state that apart from “slippery” surfaces discussed above, “adherent” superhydrophobic surfaces were also noted: the gecko lizard’s finger structures confer them the ability to climb even perfectly vertical walls, due to micrometric lamellae divided into nanometric setae. A drop placed on this surface retains its shape even in an antigravity position [23, 24]. The gecko feet model inspired climbing a glass building using Kevlar and polyurethane special gloves . The group of adhesive superhydrophobic natural surfaces includes also the rose petals, as previously discussed.
Regarded at first from a different angle, the insects’ ability to fly, to maintain impurity and water-free wings, was later attributed to superhydrophobicity. Microscales hierarchically disposed on insect wings are responsible for maintaining them dry (Figure 5) and also exhibit, in some cases, antibacterial activity (cicada wings are bactericidal against Gram-negative bacteria) [26, 27].
For some insects, patterns joining superhydrophobicity in alternation with superhydrophilicity represent an adaptation to harsh environmental conditions:
3.3 Engineered superwettability—materials and coatings: practical applications
Moving on from the theoretical field, extreme wettability is regarded an open gate for numerous everyday life and also industrial applications.
Following biomimetic principles and varying surface templates, innovative materials are fabricated, depending on qualitative requirements. The first artificial superhydrophobic materials appeared in the early 1990s: the submicrometer-roughed glass plates hydrophobized with fluoroalkyl trichlorosilane (CA = 155°) , fractal surfaces covered in n-alkyl ketene (CA = 174°) [29, 30], and ion-plated polytetrafluorethylene (PTFE) coatings with nanometric rugosities . In the 2000s, surface topography studies were correlated with surface chemistry, leading to patterned silicone surfaces with low wettability .
Techniques used to confer surface roughness are still improving, along with transparency, permeability, resistance, and color change imparting methods [33, 34, 35]. Nature proved that hierarchical surface structures are responsible for surface special wettability, and not fluorocarbon derivatives, as it was considered at that time .
The most popular known procedures used to artificially obtain superhydrophobic surfaces include chemical reactions in a humid atmosphere , thermic reactions , electrochemical deposition , individual/layer-by-layer assembling , etching , chemical vapor deposition , and polymerization reactions . Substrates include glass, metals (Cu, Ti, Zn), and cotton, and resulted structures exhibit CA > 150°, mimicking natural patterns . For example, the rose petal was used as template in order to obtain polymeric coatings, resulting in “adhesive” superhydrophobicity . Patterns resembling surface design of the lotus leaf were also fabricated through eco-friendly methods, without toxic solvents .
Fluorocarbon and silicone derivatives were preferred as substrates in fabricating superhydrophobic surfaces, assuming that the larger the number of flor atoms, the higher is the hydrophobicity . Nowadays, these materials are replaced with biodegradable ones, such as agricultural residues . Recent studies indicate that lignocellulose can be successfully used in obtaining fire-proof coatings . Another example of eco-friendly superhydrophobic coatings includes waterborne resins from aqueous silanes and siloxane solutions with silica nanoparticles applied as protective coatings to cultural heritage (marble, sandstone, cotton, ceramic artifacts) .
The result of joining extreme wettability surfaces are patterns which promote development of cells planted as hydrogels/solutions in the hydrophilic zone. Advantages of the method include lack of lateral contamination risks due to hydrophobic separative borders, efficiency, economic analysis method, the possibility of real-time screening, and noninvasive diagnosis [56, 57, 58].
Other high-impact applications of superhydrophobic surfaces include
Interdisciplinary researches on surface extreme wettability will be continued by discussing an intrinsically superhydrophobic behavior, characteristic for versatile structures entitled liquid marbles.
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 , by rolling water droplets (1–10 mm3) in a hydrophobic silica-covered
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 (
Liquid marbles’ formulations are versatile, including various powders which differ in color, wetting degree, electrical charge, and even therapeutic activity. Literature data indicates natural and synthetical powders such as
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) . 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) .
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 . 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 . 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 .
Cases of hydrophilic particle-covered liquid marbles are possible due to air trapped between particles, resulting in aggregates which cover the droplets .
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 . Wrapping drops in transparent glass fibers, avoiding fluid evaporation, is a proposed design in developing new controlled drug release systems, water purification membranes . 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 .
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 . Thus, the marbles exhibit
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 .
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
On the opposite pole of heating marbles are
Liquid marbles’ behavior while
After floating investigations,
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).
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 .
Liquid marbles have also been reported as precursors of
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 . 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 .
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
PTFE liquid marbles were also used as
Liquid marbles are providers of 3D spherical space with adjustable volume and formulation and can also be regarded as
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.
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.
Conflict of interest
We, the authors of this paper Cristina Elena Dinu-Pîrvu, Roxana-Elena Avrămescu, Mihaela Violeta Ghica, and Lăcrămioara Popa, declare no conflicts of interests.