The Cassie equation: how it is meant to be used

Locomotion behavior of air bubbles on solid surfaces

Air bubbles are a common occurrence in both natural and industrial settings and are a significant topic in the fields of physics, chemistry, engineering, and medicine. The physical phenomena of the contact between bubbles and submerged solid surfaces, as well as the locomotion behavior of bubbles, are worth exploring. Bubbles are generated in an unbounded liquid environment and rise due to unbalanced external forces. Bubbles of different diameters follow different ascending paths, after which they approach, touch, collide, bounce, and finally adsorb to the solid surface, forming a stable three-phase contact line (TPCL). The bubbles are in an unstable state due to the unbalanced external forces on the solid surface and the effects generated by the two-phase contact surface, resulting in different locomotion behaviors on the solid surface. Studying the formation, transport, aggregation, and rupture behaviors of bubbles on solid surfaces can enable the controllable operation of bubbles. This, in turn, can effectively reduce the loss of mechanical apparatus in agro-industrial production activities and improve corresponding production efficiency. Recent research has shown that the degree of bubble wetting on a solid surface is a crucial factor in the locomotion behavior of bubbles on that surface. This has led to significant progress in the study of bubble wetting, which has in turn greatly advanced our understanding of bubble behavior. Based on this, exploring the manipulation process of the directional motion of bubbles is a promising research direction. The locomotion behavior of bubbles on solid surfaces can be controlled by changing external conditions, leading to the integration of bubble behavior in various scientific and technological fields. Studying the dynamics of bubbles in liquids with infinite boundaries is worthwhile. Additionally, the manipulation process and mode of these bubbles is a popular research direction.

Influence of Mixed Solvent in the Morphology and Hydrophobicity of Electrospun Polystyrene Porous Fibers

Electrospun polystyrene (PS) fibers are produced using a mixed solvent of chloroform and n,n-dimethylformamide (DMF) to investigate the influence of the solvent ratio on the fiber surface morphology and contact angle of the obtained mats. Electrospinning is a simple processing technique for producing fibers with diameters in the range of nanometers to a few micrometers. When using the different solvent ratios for this process, porous PS membranes are created due to nonsolvent and thermally induced phase separation (N-TIPS). The morphology of the fibers is analyzed through scanning electron microscopy (SEM), which showed that fibers have diameters between 200 nm and 6 µm. SEM also revealed that the pores present on the surface of the fibers vary from densely compacted and well-formed nanopores with diameters in the range of 20-50 nm to larger pores with sizes of ≈100-200 nm. The pore size varied with different solvent ratios. The surface area is analyzed using the Brunauer-Emmett-Teller (BET) method and found that the polymeric fibers have a high surface area (≈35 m2 g-1). The fibers with such morphology are highly hydrophobic, with a contact angle higher than 143°. These materials are excellent candidates for applications in textiles, filtration, and biomedical fields.

Droplet Friction on Superhydrophobic Surfaces Scales With Liquid-Solid Contact Fraction

It is generally assumed that contact angle hysteresis of superhydrophobic surfaces scales with liquid-solid contact fraction, however, its experimental verification has been problematic due to the limited accuracy of contact angle and sliding angle goniometry. Advances in cantilever-based friction probes enable accurate droplet friction measurements down to the nanonewton regime, thus suiting much better for characterizing the wetting of superhydrophobic surfaces than contact angle hysteresis measurements. This work quantifies the relationship between droplet friction and liquid-solid contact fraction, through theory and experimental validation. Well-defined micropillar and microcone structures are used as model surfaces to provide a wide range of different liquid-solid contact fractions. Micropillars are known to be able to hold the water on top of them, and a theoretical analysis together with confocal laser scanning microscopy shows that despite the spiky nature of the microcones droplets do not sink into the conical structure either, rendering a diminishingly small liquid-solid contact fraction. Droplet friction characterization with a micropipette force sensor technique reveals a strong dependence of the droplet friction on the contact fraction, and the dependency is described with a simple physical equation, despite the nearly three-orders-of-magnitude difference in liquid-solid contact fraction between the sparsest cone surface and the densest pillar surface.

Influence of chemistry and topography on the wettability of copper

To understand the complex interplay of topography and surface chemistry in wetting, fundamental studies investigating both parameters are needed. Due to the sensitivity of wetting to miniscule changes in one of the parameters it is imperative to precisely control the experimental approach. A profound understanding of their influence on wetting facilitates a tailored design of surfaces with unique functionality. We present a multi-step study: The influence of surface chemistry is analyzed by determining the adsorption of volatile carbonous species (A) and by sputter deposition of metallic copper and copper oxides on flat copper substrates (B). A precise surface topography is created by laser processing. Isotropic topography is created by ps laser processing (C), and hierarchical anisotropic line patterns are produced by direct laser interference patterning (DLIP) with different pulse durations (D). Our results reveal that the long-term wetting response of polished copper surfaces stabilizes with time despite ongoing accumulation of hydrocarbons and is dominated by this adsorption layer over the oxide state of the substrate (Cu, CuO, Cu2O). The surfaces' wetting response can be precisely tuned by tailoring the topography via laser processing. The sub-pattern morphology of primary line-like patterns showed great impact on the static contact angle, wetting anisotropy, and water adhesion. An increased roughness inside the pattern valleys combined with a minor roughness on pattern peaks favors air-inclusions, isotropic hydrophobicity, and low water adhesion. Increasing depth of the primary topography can also induce air-inclusions despite increasing peak roughness while time dependent wetting transitions were observed.

Estimation of the Structure of Hydrophobic Surfaces Using the Cassie-Baxter Equation

The effect of extreme water repellency, called the lotus effect, is caused by the formation of a Cassie-Baxter state in which only a small portion of the wetting liquid droplet is in contact with the surface. The rest of the bottom of the droplet is in contact with air pockets. Instrumental methods are often used to determine the textural features that cause this effect-scanning electron and atomic force microscopies, profilometry, etc. However, this result provides only an accurate texture model, not the actual information about the part of the surface that is wetted by the liquid. Here, we show a practical method for estimating the surface fraction of texture that has contact with liquid in a Cassie-Baxter wetting state. The method is performed using a set of ethanol-water mixtures to determine the contact angle of the textured and chemically equivalent flat surfaces of AlSI 304 steel, 7500 aluminum, and siloxane elastomer. We showed that the system of Cassie-Baxter equations can be solved graphically by the wetting diagrams introduced in this paper, returning a value for the texture surface fraction in contact with a liquid. We anticipate that the demonstrated method will be useful for a direct evaluation of the ability of textures to repel liquids, particularly superhydrophobic and superoleophobic materials, slippery liquid-infused porous surfaces, etc.

A facile method for separating fine water droplets dispersed in oil through a pre-wetted mesh membrane

To achieve the successful separation of emulsions containing fine dispersed droplets and low volume fractions, a membrane with pore sizes comparable to or smaller than the droplet size is typically required. Although this approach is effective, its utilization is limited to the separation of emulsions with relatively large droplets. To overcome this limitation, a secondary membrane can be formed on the primary membrane to reduce pore size, but this can also be time-consuming and costly. Therefore, a facile and effective method is still required to be developed for separating emulsions with fine droplets. We introduce a pre-wetted mesh membrane with a pore size significantly larger than droplets, easily fabricated by wetting a hydrophilic stainless-steel mesh with water. Applying this membrane to emulsion separation via gravity-driven flow confirms a high efficiency greater than 98%, even with droplets approximately 10 times smaller than the pore size.

Investigation of the Influence of Machining Parameters and Surface Roughness on the Wettability of the Al6082 Surfaces Produced with WEDM

Electrical Discharge Machining (EDM) is a non-conventional machining technique, capable of processing any kind of conductive material. Recently, it has been successfully utilized for producing hydrophobic characteristics in inherently hydrophilic metallic materials. In this work, Wire Electrical Discharge Machining (WEDM) was utilized for producing hydrophobic characteristics on the surface of the aluminum alloy 6082, and various parameters that can affect wettability were investigated. Adopting an orthogonal Taguchi approach, the effects of the process parameter values of peak current, pulse-on time, and gap voltage on the contact angles of the machined surfaces were investigated. After machining, all samples were observed to have obtained hydrophobic properties, reaching contact angles up to 132°. The peak current was identified as the most influential parameter regarding the contact angle, while the gap voltage was the less influential parameter. A contact angle variation of 30° was observed throughout different combinations of machining parameters. Each combination of the machining parameters resulted in a distinct surface morphology. The samples with moderate roughness values (3.4 μm > Sa > 5.7 μm) were found to be more hydrophobic than the samples with high or low values, where the contact angle was measured under 115°. In addition, the finite element modeling of the experimental setup, with parametric surfaces of uniform random and Perlin noise types of roughness, was implemented. Time dependent simulations coupling phase field and laminar flow for the modelingof the wetting of surfaces with different surface roughness characteristics showed that an increase in the Sa roughness and total wetted area can lead to an increase in the contact angle. The combination of experimental and computational results suggests that the complexity of the wettability outcomes of aluminum alloy surfaces processed with WEDM lies in the interplay between variations of the surface chemical composition, roughness, micro/nano morphology, and the surface capability of forming a composite air/water interface.

Fabrication of CP-Ti structure with controllable wettability using powder bed fusion and eco-friendly post-process

Hydrophobic surfaces have a wide range of applications, such as water harvesting, self-cleaning, and anti-biofouling. However, traditional methods of achieving hydrophobicity often involve the use of toxic materials such as fluoropolymers. This study aims to create controllable wettability surfaces with a three-dimensional geometry using a laser base powder bed fusion (PBF) process with commercially pure titanium (CP-Ti) and silicone oil as non-toxic materials. The optimal PBF process parameters for fabricating micropillar structures, which are critical for obtaining the surface roughness necessary for achieving hydrophobic properties, were investigated experimentally. After fabricating the micropillar structures using PBF, their surface energy was reduced by treatment with silicone oil. Silicone oil provides a low-surface-energy coating that contributes to the water-repellent nature of hydrophobic surfaces. The wettability of the treated CP-Ti surfaces was evaluated based on the diameter of the pillars and the space between them. The structure with the optimal diameter and spacing of micropillars exhibited a high contact angle (156.15°). A pronounced petal effect (sliding angle of 25.9°) was achieved because of the morphology of the pillars, indicating the controllability of wetting. The micropillar diameter, spacing, and silicone oil played crucial roles in determining the water contact and sliding angle, which are key metrics for surface wettability.

Analysis of Cleaning Power Using New Cleaning Kinetics and Interfacial Science Studies on Aquatic Toxicity of Surfactants

Studies devised through the fusion of cleaning and environmental sciences can be summarized as follows: new cleaning kinetics applying a probability density function and a surface chemical approach to the aquatic toxicity of surfactants. Cleaning power analysis using the probability density functional method combines conventional cleaning kinetics using a first-order reaction equation with a risk analysis method using a probability density function. It is possible to analyze the cleaning mechanism from the obtained parameter values. It is also possible to determine whether the interaction between two different cleaning elements corresponds to a synergistic, additive, or offsetting effect. Studies on the aquatic toxicity of surfactants have focused on the surface tension at which surfactants exhibit toxicity, changes in toxicity due to water quality, and biodegradation, and the presence of adsorbed substances have been identified.

Perspectives on the Wetting of Solids in Pharmaceutical Systems

PURPOSE

The ability of water and aqueous solutions to wet relatively nonpolar pharmaceutical solids during the processing and administration of solid dosage forms is an important part of development.

RESULTS

Various factors, both fundamental and technological, which are important to wettability are reviewed and analyzed. Initially, the ideal thermodynamic importance of liquid surface tension and solid surface energetics, determined by the contact angle and the polarity of the solid surface, are established. Then, emphasis is placed on various factors that change the surface energetics due to crystal defects, polymorphism, varying Miller Indices, crystal habit, amorphous structure, variable surface concentration of components in a formulation mixture, surface roughness, and complex pore structure. Case studies cover single component systems (APIs and excipients), binary mixtures (amorphous solid dispersions and physical mixtures), multicomponent systems (granules and tablets), as well as disintegration and dissolution of solid oral dosage forms.

CONCLUSIONS

This perspective and analysis indicates the primary importance of understanding and modifying solid surface energetics, surface chemical and physical heterogeneities, and pore structure to promote wettability in pharmaceutical systems.

Roles of aqueous nonsolvents influencing the dynamic stability of poly-(n-butyl methacrylate) thin films at biologically relevant temperatures

Poly-(n-butyl methacrylate) (PnBMA) is an important polymer in biomedical applications. Here we study the stability of PnBMA thin films prepared on top of slippery silicon substrates and exposed to nonsolvent aqueous incubation media like water and phosphate-buffered saline (PBS) at temperatures relevant to biological applications (37 °C, 25 °C and 4 °C). Dewetting hole growth experiments allowed us to probe the instability in PnBMA films upon incubation followed by thermal annealing. From the early stage of dewetting hole growth dynamics, we inferred that the stability of the thin PnBMA films decreases as a function of the duration and temperature of incubation, even though the films were found not to readily dewet at room temperature after incubation. It is also observed that water incubation makes films more unstable than incubation in PBS. We explained our observations as a combined effect of (i) an increase in surface energy of the PnBMA film due to incubation, (ii) an increased destabilizing effect due to the dominant polar interactions between the incubation medium and the PnBMA film and (iii) the plasticization effect of PnBMA films by the incubation media. Plasticization resulted in a decrease in the modulus of PnBMA thin films as a function of incubation time. The viscosity of PnBMA films upon incubation was found to be coupled to the decreasing modulus. Thus we infer that incubation in common aqueous nonsolvents can detrimentally affect the stability of polymers limiting their specific usages through a complex interplay of multiple molecular level phenomena.

Cicada Wing-Inspired Transparent Polystyrene Film Integrating Self-Cleaning, Antifogging, and Antibacterial Properties

A transparent film integrating antifouling, antifogging, and antibacterial properties is crucial for its application as a protective mask, goggles, or lens. Herein, applying dynamic injection molding coupled with a bionic gradient template, a fast and efficient method is proposed for the preparation of the bionic polystyrene surface (BNPPS) with a cicada wing-inspired nanopillar structure. The contact angle of the BNPPS film increases continuously along the wing vein, while the sliding angle decreases continuously, mimicking the gradient wetting state of a cicada wing and providing excellent self-propelled removal properties for tiny water droplets. Notably, the BNPPS film has a transmittance higher than 90% and a reflectivity lower than 5% in the visible light range. Dyeing water, milk, juice, cola, and ink can slide smoothly from the BNPPS film surface without leaving any residue. Importantly, the nanopillars on the BNPPS film surface can penetrate and kill most of the Escherichia coli within 20 min. Therefore, the prepared BNPPS film with sufficient mechanical strength gathers the unique properties of the cicada wing together. The proposed research is expected to offer valuable guidance for fabricating self-cleaning, antifogging, and antibacterial optical devices that could be utilized in medical and vision systems operating in harsh environments.

Droplet impacting dynamics: Recent progress and future aspects

Droplet impact behaviours are widely applied in a variety of domains including self-cleaning, painting and coating, corrosion of turbine blades and aircraft, separation and oil repellency, anti-icing, heat transfer and droplet electricity generation, etc. The wetting behaviours and impact dynamics of droplets on solid and liquid surfaces involve complex solid-liquid and liquid-liquid interfacial interactions. The modulation of droplet dynamics by means of specific surface morphology and hydrophobic/hydrophilic patterns, which in turn can be derived to related applications, is one of the current promising interests in the interfacial effect modulating droplet dynamics. This review provides a detailed overview of several scientific aspects of droplet impact behaviours and heat transfer processes influenced by multiple factors. Firstly, the essential wetting theory and the fundamental parameters of impinging droplets are introduced. Secondly, the effects of different parameters on the dynamic behaviours and heat transfer of impinging droplet are discussed. Finally, the potential applications are listed. Existing concerns and challenges are summarized and future perspectives are provided to address poorly understood and conflicting issues.

Unveiling the Relationship of Surface Roughness on Superliquid-Repelling Properties with Randomly Distributed Rough Surface Structures

Though superliquid-repelling surfaces are universally important in the fields of fundamental research and industrial production, the understanding and development of these surfaces to impacting liquid droplets remain elusive, especially the changes of wettability states. Surface roughness is required to obtain superliquid-repelling surfaces. However, the effect of surface roughness on the transition of these surfaces' wettability states is uncertain. Herein, we unveiled the relationship of surface roughness on regulating the wettability states of superliquid-repelling surfaces with randomly distributed rough structures through experiment and calculations. The roughness was controlled via regulating the size of surface rough structures, which were formed by a facile coating method. The results indicated that the surface rough structures could impact the value of the polar component (γ_s^p) and then impact the wettability states of superliquid-repelling surfaces. Quantitatively, when the increment of surface roughness was low, the decrement of γ_s^p was low and the wettability state of the superliquid-repelling surface was superhydrophobicity. When the increment of surface roughness was high, the decrement of γ_s^p was high and the wettability state of the superliquid-repelling surface converted to superamphiphobicity. The findings will shed light onto the development of superliquid-repelling surfaces in future studies.

SLAP@g-C3N4 Fluorescent Photocatalytic Composite Powders Enhance the Anti-Bacteria Adhesion Performance and Mechanism of Polydimethylsiloxane Coatings

Fluorescent antifouling and photocatalytic antifouling technologies have shown potential in the field of marine antifouling. SLAP@g-C₃N₄/PDMS (SLAP@CN/PDMS) composite antifouling coatings were designed and prepared using g-C₃N₄, sky-blue long afterglow phosphor (SLAP), and polydimethylsiloxane (PDMS). The fluorescence emitted by SLAP under dark conditions was used to excite g-C₃N₄ for fluorescent photocatalysis and to prolong the photocatalytic activity of g-C₃N₄. Key data were collected by testing and characterization and are presented in this work. The results showed that g-C₃N₄ was successfully coated on the SLAP surface and formed a heterogeneous structure. After the composite powder was added to the PDMS coating, the coating maintained low surface energy but enhanced the surface roughness of the coating. The experimental results of degraded Rhodamine B (RhB) showed that SLAP prolonged the g-C₃N₄ photocatalytic activity time. The anti-marine bacterial adhesion performance of the coating was investigated by bacterial adhesion experiments. The results showed that SLAP@CN could effectively improve the anti-bacterial adhesion performance of PDMS coating, in which the anti-bacterial adhesion performance of SLAP@CN-2.5/PDMS was improved by nearly 19 times. This antifouling coating introduces fluorescent antifouling, photocatalytic antifouling, and fluorescence-driven photocatalytic antifouling based on the low surface energy antifouling of silicones and achieves "all-weather" fluorescent photocatalytic antifouling.

Review of droplet dynamics and dropwise condensation enhancement: Theory, experiments and applications

Droplet dynamics and condensation phenomena are widespread in nature and industrial applications, and the fundamentals of various technological applications. Currently, with the rapid development of interfacial materials, microfluidics, micro/nano fabrication technology, as well as the intersection of fluid mechanics, interfacial mechanics, heat and mass transfer, thermodynamics and reaction kinetics and other disciplines, the preparation and design of various novel functional surfaces have contributed to the local modulation of droplets (including nucleation, jumping and directional migration) and the improvement of condensation heat transfer, further deepening the understanding of relevant mechanisms. The wetting and dynamic characteristics of droplets involve complex solid-liquid interfacial interactions, so that the local modulation of microdroplets and the extension of enhanced condensation heat transfer by means of complex micro/nano structures and hydrophilic/hydrophobic properties is one of the current hot topics in heat and mass transfer research. This work presents a detailed review of several scientific issues related to the droplet dynamics and dropwise condensation heat transfer under the influence of multiple factors (including fluid property, surface structure, wettability, temperature external field, etc.). Firstly, the basic theory of droplet wetting on the solid wall is introduced, and the mechanism of solid-liquid interfacial interaction involving droplet jumping and directional migration on the functional surfaces under the various influencing factors is discussed. Optimizing the surface structure for the local modulation of droplets is of guidance for condensation heat transfer. Secondly, we summarize the existing theoretical models of dropwise condensation applicable to various functional surfaces and briefly outline the current numerical models for simulating dropwise condensation at different scales, as well as the fabricating techniques of coatings and functional surfaces for enhancing heat transfer. Finally, the relevant problems and challenges are summarized and future research is discussed.

Achieving Superhydrophobic Surfaces via Air-Assisted Electrospray

Superhydrophobic surface is an enabling technology in numerous emerging and practical applications such as self-cleaning, anticorrosion, antifouling, anti-icing coatings, and oil-water separation. Here, we report a facile air-assisted electrospray approach to achieve a superhydrophobic surface by systematically studying spray conditions and the chemistry of a coating precursor solution consisting of silicon dioxide nanoparticles, polyacrylonitrile, and N,N-dimethylformamide. The wettability behavior of the surface was analyzed with contact angle measurement and correlated with surface structures. The superhydrophobic coating exhibits remarkable water and oil repellent characteristics, as well as good robustness against abrasion and harsh chemical conditions. This air-assisted electrospray technique has shown great control over the coating process and properties and thus can be potentially used for various advanced industrial applications for self-cleaning and anticorrosion surfaces.

Hydrophobic surface patterning with soft, wax-infused micro-stamps

HYPOTHESIS

Waxy hydrocarbons diffuse freely in polydimethylsiloxane (PDMS), and this capability can be leveraged to generate inexpensive surface micropatterns that modify adhesion and wetting.

EXPERIMENTS

Patterns are created by placing a waxy Parafilm sheet on the back of a PDMS stamp containing microscale surface features. When heated, the paraffin liquefies and diffuses through the stamp, creating a thin liquid layer on the micropatterned stamp surface; when placed in contact with a target surface, the layer solidifies and is retained on the target when the stamp is removed. Micropatterns were generated on different materials and surface topographies; pattern geometry was evaluated using optical profilometry and changes in wetting were evaluated using contact angle goniometry. Diffusion of paraffin through PDMS was evaluated using XPS.

FINDINGS

Wax micropatterns have submicron lateral resolution and thickness ranging from 85 to 380 nm depending on contact time. By using XPS analysis to track paraffin diffusion within the PDMS stamp during this process, we estimate the diffusion coefficient to be 5.3 × 10^-7 cm²/s at 65 °C. This means that the paraffin layer at the stamp surface replenishes in less than a second after stamping, so it can be used multiple times without re-inking to deposit complex, multi-layer paraffin patterns.

Robust and durable liquid-repellent surfaces

This review provides a comprehensive summary of characterization, design, fabrication, and application of robust and durable liquid-repellent surfaces.

Zein-Based Films Containing Monolaurin/Eugenol or Essential Oils with Potential for Bioactive Packaging Application

Zein is renewable plant protein with valuable film-forming properties that can be used as a packaging material. It is known that the addition of natural cross-linkers can enhance a film’s tensile properties. In this study, we aimed to prepare antimicrobial zein-based films enriched with monolaurin, eugenol, oregano, and thyme essential oil. Films were prepared using the solvent casting technique from ethanol solution. Their physicochemical properties were investigated using structural, morphological, and thermal techniques. Polar and dispersive components were analyzed using two models to evaluate the effects on the surface free energy values. The antimicrobial activity was proven using a disk diffusion method and the suppression of bacterial growth was confirmed via a growth kinetics study with the Gompertz function. The films’ morphological characteristics led to systems with uniform distribution of essential oils or eugenol droplets combined with a flat-plated structure of monolaurin. A unique combination of polyphenolic eugenol and amphiphilic monoglyceride provided highly stretchable films with enhanced barrier properties and efficiency against Gram-positive and Gram-negative bacteria, yeasts, and molds. The prepared zein-based films with tunable surface properties represent an alternative to non-renewable resources with a potential application as active packaging materials.

One-step fabrication of soft calcium superhydrophobic surfaces by a simple electrodeposition process

A simple, one-step electrodeposition process was rapidly performed on a metal substrate to fabricate calcium superhydrophobic surfaces in an electrolyte containing calcium chloride (CaCl₂), myristic acid (CH₃(CH₂)₁₂COOH), and ethanol, which can avoid the intricate post-processing of surface treatment. The morphology and surface chemical compositions of the fabricated superhydrophobic surfaces were systematically examined by means of SEM, XRD, and FTIR, respectively. The results indicate that the deposited surfaces were mainly composed of calcium myristate, which can dramatically lower surface free energy. The shortest process for constructing a superhydrophobic surface is about 0.5 min, and the maximum contact angle of the as-prepared surfaces can reach as high as 166°, showing excellent superhydrophobicity. By adjusting the electrodeposition time, the structure of the cathodic surface transforms from the turfgrass structure, loose flower structures, larger and dense flower structures, secondary flower structures, and then into tertiary or more flower structures. The superhydrophobic surfaces showed excellent rebound performance with a high-speed camera. After a pressing force, their hardness increases, but the superhydrophobic performance is not weakened. Inversely, the bouncing performance is enhanced. This electrodeposition process offers a promising approach for large areas of superhydrophobic surfaces on conductive metals and strongly impacts the dynamics of water droplets.

We investigated a one-step method for calcium superhydrophobic surface preparation and researched the formation process of loose, flower-like microstructures. Also, we found that the pressing force strongly impacts the dynamics of water droplets.

Contact angle measurements: From existing methods to an open-source tool

Contact angle measurement is an effective way to investigate solid surface properties. The introduction of low-cost digital cameras, as well as software and libraries for image analysis, has made contact angle measurement potentially accessible to every laboratory. In this review, we provide a comparison of the main methods developed to evaluate contact angle from digital images, including the so-called Young-Laplace method, the circle and polynomial fittings, as well as the mask method. All methods have been implemented and compared analyzing virtual and real drop images in an open-source software, Dropen, developed as an app in MATLAB environment. The code enables single image analysis evaluation, for the robust automatic identification of the contact points and contact angle evaluation, with the goal of minimizing user inputs, automatizing the process and facilitating measurements for all users, from less experienced to advanced wetting experts. Dropen and its code are made available at BOA, the Bicocca Open Access public repository, for use and further development.

Wetting Behavior of a Surface with Dual-Scale Structures

Theoretical and numerical studies were conducted to investigate the transitional interpillar spacing for dual-scale structures, where wetting transition between the Wenzel and Cassie-Baxter states occurs in the primary and secondary pillars. A theoretical formula was derived for the transitional interpillar spacing based on the continuum picture of water. Molecular dynamics (MD) simulations were carried out by varying the interpillar spacing for the primary pillars for single- and dual-scale structures with various pillar heights. The results obtained from the theoretical formula agreed reasonably well with the results obtained from MD simulations, especially when the primary pillar height was relatively high. The transitional interpillar spacing increases as the pillar height and the number of secondary pillars increase. The effect of the secondary pillars on the transitional interpillar spacing was also evaluated using the difference in the grand potentials between the Wenzel and Cassie-Baxter states. These results show that the dual-scale structures increase the transitional interpillar spacing with an increase in the surface hydrophobicity.

Fog Collection Based on Secondary Electrohydrodynamic-Induced Hybrid Structures with Anisotropic Hydrophilicity

The outcomes of the study of plant surfaces, such as rice leaves or bamboo leaves, have led to extensive efforts being devoted to fabricating anisotropic arrays of micro/nanoscale features for exploring anisotropic droplet spreading. Nonetheless, precise engineering of the density and continuity of three-phase contact lines for anisotropic wetting remains a significant challenge without resorting to chemical modifications and costly procedures. In this work, we investigated secondary electrohydrodynamic instability in polymer films for producing secondary nanosized patterns between the micrometer-sized grooves by controlling the timescale parameter, 1/τ_m (>10^-4 s^-1). We experimentally demonstrated facile morphological control of anisotropic wettability without the use of any chemical modifications. Thus, anisotropic hydrophilic surfaces fabricated by the secondary phase instability of polymer films are advantageous for both droplet condensation and removal, thereby outperforming the water collection efficiency of conventional (isotropic) hydrophilic surfaces in water harvesting applications (∼200 mg·cm^-2·h^-1) with excellent durability.

A stomata-inspired superhydrophobic portable filter system

Stomata, specialized functional openings distributed on the leaf surface, are used for plant respiration by allowing gas exchange, i.e., taking in carbon dioxide and releasing oxygen, and for water content regulation. Their function is vital to plant survival. Leaves with different wettability exhibit different stomata densities. In this study, we find that stomata on Pistia stratiotes L. leaves are protected by superhydrophobic setae, which prevent direct contact between the stomata and water in humid environments by suspending water droplets on the top of the setae. Thus, oxygen and carbon dioxide are freely exchanged through the stomata. This structure inspired us to design and develop a mask for filtering solid particles and noxious gas from the atmosphere. The incoming gas is in convective contact with water, achieving a filtering efficiency. The solid particles and potential harmful gas in air are wetted and captured by water, leaving fresh air for healthy breathing. This novel design has potential applications in the treatment of respiratory diseases.

Bioinspired Metal-Intermetallic Laminated Composites for the Fabrication of Superhydrophobic Surfaces with Responsive Wettability

Hundreds of copper and titanium foils were applied to prepare biomimetic metal-intermetallic laminated composites by diffusion bonding. The cross sections of the obtained diffusion bonded bulks were etched selectively with FeCl₃ solution to get regular microarray structures. This kind of microstructure was controlled accurately and promptly by simple parameter adjustment. The etched surfaces were modified with 1-dodecanethiol, and the water contact angles (WCAs) were measured. The relationship between the microstructure and wettability of the achieved material was discussed, and the reason for the anisotropic wettability was also analyzed. Then etched surfaces were anodized in different electrolyte solutions to obtain different nanostructures. The morphology and chemical compositions of the surfaces were analyzed. The surfaces with CuO nanostructures by modification show superhydrophobicity with self-cleaning, on which the WCA and water sliding angle are 160.9° and 0.8°, respectively. The surfaces with TiO₂ nanostructures without modification show ultraviolet light-responsive wettability. After modification with 11-mercaptoundecanoic acid and 1-decanethiol, the surfaces also exhibit pH-responsive wettability. The superhydrophobic surfaces with responsive wettability have potential applications in biotechnology and microfluidics.

Recent advances in biomimetic surfaces inspired by creatures for fog harvesting

In this review, the recent advances in artificial surfaces for fog harvesting are introduced with emphasis on the surfaces and their mechanisms used to enhance water capture and transportation, providing prospects for coping with water shortages.

Dewetting of Polymer Films Controlled by Protein Adsorption

The stability of the film poly(n-butyl methacrylate) (PnBMA) with different tacticities, prepared on silicon oxide and exposed to aqueous phosphate-buffered saline with different concentrations of bovine serum albumin (CBSA between 0 and 4.5 mg/mL), was examined at temperatures close to the physiological limit (between 4 and 37 °C) with optical microscopy, contact angle measurements, atomic force microscopy, and time-of-flight secondary ion mass spectrometry. For PBS solutions with CBSA = 0, the stability of atactic PnBMA and dewetting of isotactic PnBMA was observed, caused by the interplay between the stabilizing long-range dispersion forces and the destabilizing short-range polar interactions. Analogous considerations of excess free energy cannot explain the retardation of dewetting observed for isotactic PnBMA in PBS solutions with higher CBSA. Instead, formation of a BSA overlayer, adsorbed preferentially but not exclusively to uncovered SiOx regions, is evidenced and postulated to hinder polymer dewetting. Polymer dewetting and protein patterning are obtained in one step, suggesting a simple approach to fabricate biomaterials with micropatterned proteins.

Preparation of a Flexible Superhydrophobic Surface and Its Wetting Mechanism Based on Fractal Theory

Substrates of the superhydrophobic surface are important for their application. Preparation of a flexible superhydrophobic surface has drawn more and more attention. In this work, a flexible substrate was made using a semicuring spray method to obtain a flexible superhydrophobic surface with excellent abrasion resistance on the surface of a room temperature vulcanized silicone rubber. Results show that under a bending condition, excellent superhydrophobic properties are still maintained. The Cassie-Baxter model and Wenzel model can be used to estimate the static water contact angle for regular roughness surfaces. There are few numerical theoretical models to predict contact angle or wetting mode for irregular micronanostructures superhydrophobic surfaces. The fractal theory can be used to transform the equation of the Wenzel model and obtain the fractal wetting theory suitable for fractal structures on irregular rough surfaces. However, this fractal-wetting model cannot be applied to the Cassie-Baxter state, which is always suitable for superhydrophobic surfaces. A new method was developed to calculate the static water contact angle of water droplets in the Cassie-Baxter model state. Using image identification and the splitting surface method, a new model is constructed based on the fractal theory. Experimental data for water contact angles on the flexible superhydrophobic surface with SiC/CNTs micronanostructures is in agreement with the simulated values.

Surprising Lack of Influence on Water Droplet Motion by Hydrophilic Microdomains on Checkerboard-like Surfaces with Matched Contact Angle Hysteresis

Chemically and spatially micropatterned surfaces have been successfully prepared for a number of diverse applications, including water/fog harvesting, screen printing, microfluidics, and cell/protein assays. While there have been quite some reports on micropatterned surfaces, less is known about the factors that influence dynamic surface wettability. To that end, smooth checkerboard-like micropatterned hydrophobic/(super)hydrophilic surfaces (2, 5, 10, 20 μm pattern sizes) with regions of matching/mismatching contact angle hysteresis (CAH) were prepared on the basis of a simple chemisorption/photopatterning of monolayers. The effects of regional wettability/CAH and pattern size on the overall dynamic wettability were then examined by measuring the dynamic contact angles (CAs) and substrate tilt angles (θ_T) of water. It was found that the dynamic wettability on samples with matching regional CAH remained unchanged, even when using hydrophilic regions or changing the pattern size. In contrast, surfaces containing mismatching CAH regions pinned water droplets, leading to overall dynamic wetting properties markedly dependent on pattern size. In addition, the experimental data did not match values predicted by the Cassie equation because dynamic wetting behavior is dominated not by interfacial area but by the interactions of the liquid and solid at the three-phase contact line.

How do chemical patterns affect equilibrium droplet shapes?

By utilizing a proposed analytical model in combination with the phase-field method, we present a comprehensive study on the effect of chemical patterns on equilibrium droplet morphologies. Here, three influencing factors, the droplet sizes, contact angles, and the ratios of the hydrophilic area to the hydrophobic area, are contemplated. In the analytical model, chemical heterogeneities are described by different non-linear functions. By tuning these functions and the related parameters, the analytical model is capable of calculating the energy landscapes of the system. The chemically patterned surfaces display complex energy landscapes with chemical-heterogeneity-induced local minima, which correspond to the equilibrium morphologies of the droplets. Phase-field (PF) simulations are accordingly conducted and compared with the predicted equilibrium morphologies. In addition, we propose a modified Cassie-Baxter (CB) model to delineate the equilibrium droplet shapes. In contrast to the classic CB model, our extension is not only restricted to the shape with a spherical cap. Both the energy landscape method and the modified CB model are demonstrated to have a good agreement with the PF simulations.

Delayed Frost Growth on Nanoporous Microstructured Surfaces Utilizing Jumping and Sweeping Condensates

Self-propelled jumping of condensate droplets (dew) enables their easy and efficient removal from surfaces and is essential for enhancing the condensation heat transfer coefficient and for delaying the frost growth rate on supercooled surfaces. Here, we report the droplet-jumping phenomenon using nanoporous vertically aligned carbon nanotube (VA-CNT) microstructures grown on smooth silicon substrates and coated with poly-(1H, 1H, 2H, 2H-perfluorodecylacrylate) (pPFDA). We also report droplet-sweeping phenomenon on horizontally mounted surfaces, concluding that the frost surface coverage area and the frost growth rates observed with the droplet-sweeping phenomenon are much lower than those that are observed with the droplet-jumping phenomenon alone. We also investigate the fundamentals of droplet-jumping and the frost growth phenomena using line-shaped, hollow-cylindrical, and cylindrical microstructures, comparing the frost surface coverage area and the ice-bridging times during condensation-frosting, prolonged condensation-frosting, and direct-frosting. We find that the closely spaced thin line-shaped microstructures and hollow-cylindrical microstructures are optimal for frost coverage reduction because of their ability to exhibit droplet-jumping and droplet-sweeping phenomena. We observe that adding nonuniform roughness on top of the microstructures leads to jumping-associated droplet-sweeping on supercooled surfaces. Here, we report the evaporation of an already frozen droplet because of freezing of a supercooled condensate droplet in its close vicinity, enabling the Cassie-Baxter state frost growth and enhancing defrosting efficiency. Finally, we discuss the dynamic defrosting behavior of the pPFDA-coated VA-CNT microstructures, concluding that the small gaps (spacings) between the microstructures not only enable dewetting transitions of droplets but also promote the Cassie-Baxter state frost formation.

Apparent Contact Angle around the Periphery of a Liquid Drop on Roughened Surfaces

The wetting of roughened surfaces is complicated since not all of the surface of the irregular surface is wetted and thus, the three-phase contact line for the liquid drop is a complex, three-dimensional line that varies according to the dimensions of the roughness and its spatial heterogeneity. This can cause the contact line to not sit within a constant height horizontal plane especially when air is trapped underneath the liquid layer. Here, we explore the effect of roughness on the effective contact angle of a water droplet on a roughened hydrophobic surface. The results show that the apparent contact angle varies around the periphery of the droplet due to the roughness of the surface on first contact. Also, repeated wetting of the droplet on the surface reveals that the apparent contact angle changes due to residual liquid remaining on the roughened surface. The results also show that the Wenzel and Cassie-Baxter models tend to overestimate the apparent contact angle on the roughened surfaces.

Design of Active Interfaces Using Responsive Molecular Components

Responsive interfaces are interfaces that show a defined and reversible change in physical properties in response to external stimuli. Typically, responsive interfaces result from the immobilization of responsive molecular components at the interface that translate a nanoscale signal into a macroscopic effect. Responsive interfaces can also be obtained if the topology of the interface can be reversibly changed using an external stimulus. As the surface of any material is its connection to the environment, responsive interfaces provide opportunities for interactive materials which are not only able to change properties upon demand, but also sense their environment and act autonomously. The application of responsive molecular components at interfaces, however, requires chemical and physical compatibility with the material surface of interest, posing a challenge not least in the retention of the responsive functionality. The state of the art in "active" interfaces which display responsive wettability, permeability, or adhesion is discussed, with a particular emphasis on microscale and nanoscale patterning since patterned interfaces can give rise to unique material properties. Finally, perspectives in the development of responsive interfaces, as well as promising approaches for bypassing the most prominent challenges are discussed.

Effect of a Cationic Surfactant on Droplet Wetting on Superhydrophobic Surfaces

We experimentally and theoretically examine the influence of a double-chain cationic surfactant, didodecyldimethylammonium bromide (DDAB), on the wetting states and contact angles on superhydrophobic (SH) surfaces made of hydrophobic microcylinders. We use two types of micropatterns of different surface roughness, r, and packing fraction, ϕ, and vary nine dimensionless surfactant concentrations (C_S), normalized by the critical micelle concentration (CMC), in the experiments. At low C_S, some of the surfactant-laden droplets are in a gas-trapping, Cassie-Baxter (CB) state on the high-roughness microstructures. In contrast, some droplets are in a complete-wetting Wenzel (W) state on the low-roughness microtextures. We found that the contact angle of CB drops can be well predicted using a thermodynamic model considering surfactant adsorption at the liquid-vapor (LV) and solid-liquid (SL) interfaces. At high C_S, however, all of the DDAB drops wet in a Wenzel mode. Based on a Gibbsian thermodynamic analysis, we find that for the two types of superhydrophobic surfaces used, the Wenzel state has the lowest thermodynamic energy and thus is more favorable theoretically. The CB state, however, is metastable at low C_S due to a thermodynamic energy barrier. The metastable CB wetting state becomes more stable on the SH microtextures with greater ϕ and r, in agreement with our experimental observations. Finally, we generalize this surface-energy analysis to provide useful designs of surface parameters for a DDAB-laden surfactant droplet on the SH surface with a stable and robust CB state.

A Volume-Corrected Wenzel Model

The Wenzel model, commonly used for predicting the equilibrium contact angle (CA) of drops which penetrate the asperities of a rough surface, does not account for the liquid volume stored in the asperities. Interestingly, many previous experimental and molecular dynamics studies have noted discrepancies between observed CAs and those predicted by the Wenzel model because of this neglected liquid volume. Here, we apply a thermodynamic model to wetting of periodically patterned surfaces to derive a volume-corrected Wenzel equation in the limit of small pattern wavelength (compared to drop size). We show that the corrected equilibrium CA is smaller than that predicted by the Wenzel equation and that the reduction in CA can be significant when the liquid volume within the asperities becomes non-negligible compared to the total droplet volume. In such cases, the corrected CAs agree reasonably well with experimental observations and results of molecular dynamics simulations reported in previous studies.

Thermodynamic analysis on wetting states and wetting state transitions of rough surfaces

Determining the equilibrium wetting states and exploring the conditions and mechanisms of the wetting state transition from the Cassie-Baxter (CB) state to the Wenzel (W) state (CB-W transition) have been a central topic in the study of superhydrophobic behavior on rough or textured surfaces. Although considerable progress has been made, some issues regarding this topic are still not completely understood. In this study, a systematic thermodynamic analysis has been performed to address several key issues related to this topic. Generalized theoretical expressions for determining the equilibrium wetting states (the threshold Young contact angle of the CB region) and evaluating the stability of the CB state (the energy barrier separating the CB and W states and the critical pressure for the CB-W transition) have been derived. Applying these expressions to four types of surfaces built with protrusions in paraboloid, truncated cone, inverted truncated cone and flat-top pillar shapes, the wetting equilibrium and resultant wetting states have been studied. The physical meanings of the threshold Young contact angle, the roles and mechanisms of the energy barrier and critical pressure in stabilizing the CB state have been discussed. Finally, a general guidance for achieving robust superhydrophobicity on the studied surfaces has been given.

Review on Blueprint of Designing Anti-Wetting Polymeric Membrane Surfaces for Enhanced Membrane Distillation Performance

Recently, membrane distillation (MD) has emerged as a versatile technology for treating saline water and industrial wastewater. However, the long-term use of MD wets the polymeric membrane and prevents the membrane from working as a semi-permeable barrier. Currently, the concept of antiwetting interfaces has been utilized for reducing the wetting issue of MD. This review paper discusses the fundamentals and roles of surface energy and hierarchical structures on both the hydrophobic characteristics and wetting tolerance of MD membranes. Designing stable antiwetting interfaces with their basic working principle is illustrated with high scientific discussions. The capability of antiwetting surfaces in terms of their self-cleaning properties has also been demonstrated. This comprehensive review paper can be utilized as the fundamental basis for developing antiwetting surfaces to minimize fouling, as well as the wetting issue in the MD process.

How Surfactants Affect Droplet Wetting on Hydrophobic Microstructures

Surfactants, as amphiphilic molecules, adsorb easily at interfaces and can detrimentally destroy the useful, gas-trapping wetting state (Cassie-Baxter, CB) of a drop on superhydrophobic surfaces. Here, we provide a quantitative understanding of how surfactants alter the wetting state and contact angle of aqueous drops on hydrophobic microstructures of different roughness (r) and solid fraction (ϕ). Experimentally, at low surfactant concentrations (C), some drops attain a homogeneous wetting state (Wenzel, W), while others attain the CB state whose large contact angles can be predicted by a thermodynamic model. In contrast, all of our high-C drops attain the Wenzel state. To explain this observed transition, we consider the free energy and find that, theoretically, for our surfaces the W state is always preferred, while the CB state is metastable at low C, consistent with experimental results. Furthermore, we provide a beneficial blueprint for stable CB states for applications exploiting superhydrophobicity.

Application of Bionic Technologies on the Fracturing Plug

The dissolvable bridge plug is one of the most important tools for multi-stage hydraulic fracturing in the field of oil/gas development. The plug provides zonal isolation to realize staged stimulation and, after fracturing, the plug is fully dissolved in produced liquids. A bionic surface was introduced to improve the performance of the plug. Surface dimples in the micron dimension were prepared on the dissolvable materials of the plug. The experimental results showed that the surface dimples changed the hydrophilic and hydrophobic properties of the dissolvable materials. The dissolution rate has a great relation with the parameters of the dimples and can be controlled by choosing the dimples’ parameters to some degree.

Multifunctional Magnetocontrollable Superwettable-Microcilia Surface for Directional Droplet Manipulation

In nature, fluid manipulations are ubiquitous in organisms, and they are crucial for many of their vital activities. Therefore, this process has also attracted widescale research attention. However, despite significant advances in fluid transportation research over the past few decades, it is still hugely challenging to achieve efficient and nondestructive droplet transportation owing to contamination effects and controllability problems in liquid transportation applications. To this end, inspired by the motile microcilia of micro-organisms, the superhydrophobicity of lotus leaves, the underwater superoleophobicity of filefish skin, and pigeons' migration behavior, a novel manipulation strategy is developed for droplets motion. Specifically, herein, a superwettable magnetic microcilia array surface with a structure that is switchable by an external magnetic field is constructed for droplet manipulation. It is found that under external magnetic fields, the superhydrophobic magnetic microcilia array surface can continuously and directionally manipulate the water droplets in air and that the underwater superoleophobic magnetic microcilia array surface can control the oil droplets underwater. This work demonstrates that the nondestructive droplet transportation mechanism can be used for liquid transportation, droplet reactions, and micropipeline transmission, thus opening up an avenue for practical applications of droplet manipulation using intelligent microstructure surfaces.

Cassie-Baxter Surfaces for Reversible, Barrier-Free Integration of Microfluidics and 3D Cell Culture

3D cell culture and microfluidics both represent powerful tools for replicating critical components of the cell microenvironment; however, challenges involved in the integration of the two and compatibility with standard tissue culture protocols still represent a steep barrier to widespread adoption. Here we demonstrate the use of engineered surface roughness in the form of microfluidic channels to integrate 3D cell-laden hydrogels and microfluidic fluid delivery. When a liquid hydrogel precursor solution is pipetted onto a surface containing open microfluidic channels, the solid/liquid/air interface becomes pinned at sharp edges such that the hydrogel forms the "fourth wall" of the channels upon solidification. We designed Cassie-Baxter microfluidic surfaces that leverage this phenomenon, making it possible to have barrier-free diffusion between the channels and the hydrogel; in addition, sealing is robust enough to prevent leakage between the two components during fluid flow, but the sealing can also be reversed to facilitate recovery of the cell/hydrogel material after culture. This method was used to culture MDA-MB-231 cells in collagen, which remained viable and proliferated while receiving media exclusively through the microfluidic channels over the course of several days.