3-D thermodynamic analysis of superhydrophobic surfaces

Some microtextured surfaces strongly repel water. In particular, surfaces with contact angle (CA) higher than 150 degrees are called superhydrophobic surfaces and many studies to obtain such surfaces have been reported. However, none of them could be a guide to achieve superhydrophobicity and the thermodynamic mechanisms are not well understood. In this study, two types of 3-D models--pillar-textured surfaces and cavity surfaces--were selected and thermodynamically analyzed focusing on the surface free energy. By calculations, equilibrium CA, free energy wall (FEW), and CA hysteresis are obtained. Based on these calculations, the factors to determine the sliding angle are indicated. Additionally, based on these results, one example of the optimal geometry for superhydrophobic surfaces is proposed.     

Microtextured superhydrophobic surfaces: a thermodynamic analysis

Superhydrophobic surfaces with a contact angle (CA) larger than 150 degrees have recently attracted great interest in both academic research and practical applications due to their water-repellent or self-cleaning properties. However, thermodynamic mechanisms responsible for the effects of various factors such as surface geometry and chemistry, liquids, and environmental sources have not been well understood. In this study, a pillar microtexture, which has been intensively investigated in experiments, is chosen as a typical example and thermodynamically analyzed in detail. To gain a comprehensive insight into superhydrophobic behavior, the roles of pillar height, width and spacing (or roughness and solid fraction), intrinsic CA, drop size, and vibrational energy are systematically investigated. Free energy (FE) and free energy barrier (FEB) are calculated using a simple and robust model. Based on the calculations of FE and FEB, various CAs, including apparent, equilibrium (stable), advancing and receding CAs, and contact angle hysteresis (CAH) can be determined. Especially, the design of practical superhydrophobic surfaces is emphasized in connection with the transition between noncomposite and composite states; a criterion for judging such transition is proposed. The theoretical results are consistent with the Wenzel's and the Cassie's equations for equilibrium CA values and experimental observations. Furthermore, based on these results and the proposed criterion, some general principles to achieve superhydrophobic performance are suggested.     

A thermodynamic approach for determining the contact angle hysteresis for superhydrophobic surfaces

Contact angle hysteresis (CAH) is critical to superhydrophobicity of a surface. This study proposes a free energy thermodynamic analysis (of a 2-D model surface) that significantly simplifies calculations of free energy barrier associated with CAH phenomena. A microtextured surface with pillar structure, typical of one used in experimental studies, is used as an example. We demonstrate that the predicted CAH and equilibrium contact angles are consistent with experimental observations and predictions of Wenzel's and Cassie's equations, respectively. We also establish a criterion for transition between noncomposite and composite wetting states. The results and methodology presented can potentially be used for designing superhydrophobic surfaces.     

Thermodynamic analysis on wetting behavior of hierarchical structured superhydrophobic surfaces

Superhydrophobicity of biological surfaces has recently been studied intensively with the aim to design artificial surfaces. It has been revealed that nearly all of the superhydrophobic surfaces consist of the intrinsic hierarchical structures. However, the role of such structures has not been completely understood. In this study, different scales of hierarchical structures have been thermodynamically analyzed using a 2-D model. In particular, the free energy (FE) and free energy barrier (FEB) for the composite wetting states are calculated, and the effects of relative pillar height (h(r)) and relative pillar width (a(r)) on contact angle (CA) and contact angle hysteresis (CAH) have been investigated in detail. The results show that if the geometrical parameter ratio is the same (e.g., a:b:h = 2:2:1), the equilibrium CA for the composite of the three-, dual-, and single- scale roughness structures is 159.8°, 151.1°, and 138.6°, respectively. Furthermore, the nano- to microstructures of such surfaces can split a large FEB into many small ones and hence can decrease FEB; in particular, a hierarchical geometrical structure can lead to a hierarchical "FEB structure" (e.g., for a dual-scale roughness geometrical structure, there is also a dual-scale FEB structure). This is especially important for a droplet to overcome the large FEBs to reach a stable superhydrophobic state, which can lead to an improved self-cleaning property. Moreover, for extremely small droplets, the secondary or third structure (i.e., submicrostructure or nanostructure) can play a dominant role in resisting the droplets into troughs, so that a composite state can be always thermodynamically favorable for such a hierarchical structured system.     

Synthesis and characterization of PMMA-based superhydrophobic surfaces

Recently, superhydrophobic surfaces are gaining much interest because they may be employed in a series of applications, spanning from the realization of self-cleaning surfaces to microfluidics to special water-impermeable tissues allowing perspiration. It is well-known that superhydrophobicity strictly depends on the combination of superficial micro- and nano-structures. Then, key factors in the process of surface synthesis are the parameters which will define the surface conformation. In this work, we deal with the fabrication of polymer-based superhydrophobic surfaces. We developed a new method to have a good control of the structure of the synthesised surface. A high stability of the superhydrophobic character during time was obtained. Moreover, the synthesis process is green and easily transferable to industry for large production.     

Fabrication of superhydrophobic surfaces of n-hexatriacontane

Superhydrophobic surfaces of n-hexatriacontane were fabricated in a single-step process. The low surface energy of n-hexatriacontane together with the randomly distributed micro- and nanoscale roughness features guarantees very large contact angles and a small roll-off angle for water drops. The advantage of n-hexatriacontane superhydrophobic surfaces is their stability in the sense that they are impervious to chemical reactions and retain their wetting characteristics over a long period of time, as confirmed by XPS analysis and contact angle measurements.     

Electrochemical synthesis of silver polyhedrons and dendritic films with superhydrophobic surfaces

Single-crystalline Ag dendrites are grown on a Ni/Cu substrate by using a simple templateless, surfactantless electrochemical technique. Controlling only the applied potential causes a change in the deposited silver morphology from polyhedrons to dendrites. Microstructure characterization suggests that preferential growth along the 211 directions by the oriented attachment of Ag nanocrystals leads to the formation of Ag dendrites, which are composed of trunks, branches, and leaves. Modifying as-grown Ag dendritic film with a thickness of about 10 microm with a self-assembled monolayer of n-dodecanethiol yields a superhydrophobic surface with a contact angle of 154.5 +/- 1.0 degrees and a tilt angle lower than 2 degrees.     

Simultaneous fabrication of superhydrophobic and superhydrophilic polyimide surfaces with low hysteresis

Polyimide is of great interest in the field of MEMS and microtechnology. It is often used for its chemical, thermal, mechanical, and optical properties. In this paper, an original study is performed on controlled variation of polyimide film wettability. A two-step microtexturing method is developed to transform hydrophilic polyimide surfaces into a superhydrophobic surface with low magnitude of hysteresis (Δθ ≈ 0° and contact angle θ ≈ 158°). This method is based on the conception of a new kind of fakir surface with triangular cross-section micropillars, the use of a two-scale roughening, and a C(4)F(8) coating. We demonstrate that the absence of hysteresis is related to a combination of two scales of structuring and the pillar shape. The technology that has been developed results in the simultaneous fabrication of adjacent superhydrophobic and superhydrophilic small areas, which allows an effect of self-positioning of water droplets when deposited on such a checkerboard-like surface.     

Dynamical force and imaging characterization of superhydrophobic surfaces

We devised a dangling cantilever optical lever setup with imaging that permits dynamical studies of superhydrophobic surfaces without the effects of gravitational acceleration for better insight into the mechanics. The setup enabled us to ascertain liquid loss and ascribe it to the interaction of liquid that just touched the superhydrophobic surface as it translated at various constant lateral speeds. At lower speeds (20-60 μm/s), the interactions were characterized by a strong initial liquid pin (at up to 0.6 nN force) and depin followed by a series of smaller force pin and depins before sufficient liquid loss led to total liquid detachment from the surface. At higher translation speeds (80-100 μm/s), the interactions were characterized by liquid pinning and depinning processes at a sustained force (around 0.7 nN) in which liquid loss was low enough to engender a much later liquid detachment (beyond 100 s). A linear reduction of the receding contact angle with time, but not with the advancing contact angle, was found up to the point of first liquid depinning. This suggested a stronger role played by the receding contact line in establishing liquid adherence to the superhydrophobic surface. The detachment process from the surface was also characterized by a liquid bridge driven to rupture by way of liquid being conveyed away from the bridge.     

Condensation and freezing of droplets on superhydrophobic surfaces

Superhydrophobic coatings are reported as promising candidates for anti-icing applications. Various studies have shown that as well as having ultra water repellency the surfaces have reduced ice adhesion and can delay water freezing. However, the structure or texture (roughness) of the superhydrophobic surface is subject to degradation during the thermocycling or wetting process. This degradation can impair the superhydrophobicity and the icephobicity of those coatings. In this review, a brief overview of the process of droplet freezing on superhydrophobic coatings is presented with respect to their potential in anti-icing applications. To support this discussion, new data is presented about the condensation of water onto physically decorated substrates, and the associated freezing process which impacts on the freezing of macroscopic droplets on the surface.     

Wetting and self-cleaning properties of artificial superhydrophobic surfaces

The wetting and the self-cleaning properties (the latter is often called the "Lotus-Effect") of three types of superhydrophobic surfaces have been investigated: silicon wafer specimens with different regular arrays of spikes hydrophobized by chemical treatment, replicates of water-repellent leaves of plants, and commercially available metal foils which were additionally hydrophobized by means of a fluorinated agent. Water droplets rolled off easily from those silicon samples which had a microstructure consisting of rather slender spikes with narrow pitches. Such samples could be cleaned almost completely from artificial particulate contaminations by a fog consisting of water droplets (diameter range, 8-20 microm). Some metal foils and some replicates had two levels of roughening. Because of this, a complete removal of all particles was not possible using artificial fog. However, water drops with some amount of kinetic impact energy were able to clean these surfaces perfectly. A substrate where pronounced structures in the range below 5 microm were lacking could not be cleaned by means of fog because this treatment resulted in a continuous water film on the samples.     

Investigating the interface of superhydrophobic surfaces in contact with water

Neutron reflectivity (NR) is used to probe the solid, liquid, vapor interface of a porous superhydrophobic (SH) surface submerged in water. A low-temperature, low-pressure technique was used to prepare a rough, highly porous organosilica aerogel-like film. UV/ozone treatments were used to control the surface coverage of hydrophobic organic ligands on the silica framework, allowing the contact angle with water to be continuously varied over the range of 160 degrees (superhydrophobic) to <10 degrees (hydrophilic). NR shows that the superhydrophobic nature of the surface prevents infiltration of water into the porous film. Atomic force microscopy and density functional theory simulations are used in combination to interpret the NR results and help establish the location, width, and nature of the SH film-water interface.     

Preparation of superhydrophobic silicon oxide nanowire surfaces

The paper reports on the preparation of superhydrophobic amorphous silicon oxide nanowires (a-SiONWs) on silicon substrates with a contact angle greater than 150 degrees by means of surface roughness and self-assembly. Nanowires with an average mean diameter in the range 20-150 nm and 15-20 microm in length were obtained by the so-called solid-liquid-solid (SLS) technique. The porous nature and the high roughness of the resulting surfaces were confirmed by AFM imaging. The superhydrophobicity resulted from the combined effects of surface roughness and chemical modification with fluorodecyl trichlorosilane.     

Mimicking natural superhydrophobic surfaces and grasping the wetting process: a review on recent progress in preparing superhydrophobic surfaces

A typical superhydrophobic (ultrahydrophobic) surface can repel water droplets from wetting itself, and the contact angle of a water droplet resting on a superhydrophobic surface is greater than 150°, which means extremely low wettability is achievable on superhydrophobic surfaces. Many superhydrophobic surfaces (both manmade and natural) normally exhibit micro- or nanosized roughness as well as hierarchical structure, which somehow can influence the surface's water repellence. As the research into superhydrophobic surfaces goes deeper and wider, it is becoming more important to both academic fields and industrial applications. In this work, the most recent progress in preparing manmade superhydrophobic surfaces through a variety of methodologies, particularly within the past several years, and the fundamental theories of wetting phenomena related to superhydrophobic surfaces are reviewed. We also discuss the perspective of natural superhydrophobic surfaces utilized as mimicking models. The discussion focuses on how the superhydrophobic property is promoted on solid surfaces and emphasizes the effect of surface roughness and structure in particular. This review aims to enable researchers to perceive the inner principles of wetting phenomena and employ suitable methods for creation and modification of superhydrophobic surfaces.     

Wetting and dewetting transitions on hierarchical superhydrophobic surfaces

Many natural superhydrophobic structures have hierarchical two-tier roughness which is empirically known to promote robust superhydrophobicity. We report the wetting and dewetting properties of two-tier roughness as a function of the wettability of the working fluid, where the surface tension of water/ethanol drops is tuned by the mixing ratio, and compare the results to one-tier roughness. When the ethanol concentration of deposited drops is gradually increased on one-tier control samples, the impalement of the microtier-only surface occurs at a lower ethanol concentration compared to the nanotier-only surface. The corresponding two-tier surface exhibits a two-stage wetting transition, first for the impalement of the microscale texture and then for the nanoscale one. The impaled drops are subsequently subjected to vibration-induced dewetting. Drops impaling one-tier surfaces could not be dewetted; neither could drops impaling both tiers of the two-tier roughness. However, on the two-tier surface, drops impaling only the microscale roughness exhibited a full dewetting transition upon vibration. Our work suggests that two-tier roughness is essential for preventing catastrophic, irreversible wetting of superhydrophobic surfaces.     

Superhydrophilic and superhydrophobic nanostructured surfaces via plasma treatment

Polyethylene terephthalate (PET) films have been structured with isolated nanofibrils and fibril bundles using oxidative plasma treatments with increasing etching ratios. The transition from fibrils to bundles was smooth and it was associated with a significant reduction in the overall top area fraction and with the development of a second organisation level at a larger length scale. This increased complexity was reflected in the surface properties. The surfaces with two-level substructures showed superhydrophilic and superhydrophobic properties depending on the surface chemistry. These properties were preserved during prolonged storage and resisted moderate mechanical stress. By combining different contact angle and drop impact measurements, the optimum surface design and plasma processing parameters for maximizing stability of the superhydrophobic or superhydrophilic properties of the PET films were identified.     

Droplet motion on designed microtextured superhydrophobic surfaces with tunable wettability

Superhydrophobic surfaces have shown promising applications in microfluidic systems as a result of their water-repellent and low-friction properties over the past decade. Recently, designed microstructures have been experimentally applied to construct wettability gradients and direct the droplet motion. However, thermodynamic mechanisms responsible for the droplet motion on such regular rough surfaces have not been well understood such that at present specific guidelines for the design of tunable superhydrophobic surfaces are not available. In this study, we propose a simple but robust thermodynamic methodology to gain thorough insight into the physical nature for the controllable motion of droplets. On the basis of the thermodynamic calculations of free energy (FE) and the free-energy barrier (FEB), the effects of surface geometry of a pillar microtexture are systematically investigated. It is found that decreasing the pillar width and spacing simultaneously is required to lower the advancing and receding FEBs to effectively direct droplets on the roughness gradient surface. Furthermore, the external energy plays a role in the actuation of spontaneous droplet motion with the cooperation of the roughness gradient. In addition, it is suggested that the so-called "virtual wall" used to confine the liquid flow along the undesired directions could be achieved by constructing highly advancing FEB areas around the microchannels, which is promising for the design of microfluidic systems.     

Long-term and thermally stable superhydrophobic surfaces of carbon nanofibers

Carbon nanofibers were prepared from polyvinyl alcohol (PVA) by a simple wetting-compatible method. The surfaces of the aligned carbon nanofibers show excellent thermal- and time-stable superhydrophobicity. The average water contact angle (CA) values are about 153.1+/-2.2 degrees at room temperature, with little difference coming from experimental error. In addition, the CA of the aligned carbon nanofibers surface maintains 139.1+/-3.2 degrees after 10 months of exposure to the ambient environment. Nanostructure on the surface of carbon nanofibers and the intrinsic thermal resistance of carbon contribute to this unique surface property.     

Impact of picoliter droplets on superhydrophobic surfaces with ultralow spreading ratios

The impact of picoliter-sized water droplets on superhydrophobic CF(4) plasma fluorinated polybutadiene surfaces is investigated with high-speed imaging. Variation of the surface topography by plasmachemical modification enables the dynamics of wetting to be precisely controlled. Final spreading ratios as low as 0.63 can be achieved. A comparison of the maximum spreading ratio and droplet oscillation frequencies to models described in the literature shows that both are found to be much lower than theoretically predicted.