Cassie-Baxter State Research Articles

Enhancing Resistance to Wetting Transition through the Concave Structures.

Water-repellent superhydrophobic surfaces are ubiquitous in nature. The fundamental understanding of bio/bio-inspired structures facilitates practical applications surmounting metastable superhydrophobicity. Typically, the hierarchical structure and/or reentrant morphology have been employed hitherto to suppress the Cassie-Baxter to Wenzel transition (CWT). Herein, a new design concept is reported, an effect of concave structure, which is vital for the stable superhydrophobic surface. The thermodynamic and kinetic stabilities of the concave pillars are evaluated by continuous exposure to various hydrostatic pressures and sudden impacts of water droplets with various Weber numbers (We), comparing them to the standard superhydrophobic normal pillars. Specifically, the concave pillar exhibits reinforced impact resistance preventing CWT below a critical We of ≈27.6, which is ≈1.6 times higher than that of the normal pillar (≈17.0). Subsequently, the stability of underwater air film (plastron) is investigated at various hydrostatic pressures. The results show that convex air caps formed at the concave cavities generate downward Laplace pressure opposing the exerted hydrostatic pressure between the pillars, thus impeding the hydrostatic pressure-dependent underwater air diffusion. Hence, the effects of trapped air caps contributing to the stable Cassie-Baxter state can offer a pioneering strategy for the exploration and utilization of superhydrophobic surfaces.

Application of a thermal circuit model for the prediction of interfacial thermal resistance between water and a nanostructure surface using molecular dynamics simulations

The present study proposes various thermal circuit models (TCMs) to predict interfacial thermal resistances (ITRs) between water and a nanostructure surface. The effects of water models, nanopillar widths on nanostructure surfaces, and composite surfaces on the relationship between ITRs calculated by non-equilibrium molecular dynamics simulations and ITRs predicted by TCMs are investigated in water-copper and water-graphene-copper systems. The results reveal that the ITRs predicted by some TCMs are slightly affected by water pressure, while MD-calculated ITRs of the nanostructure surfaces in the Wenzel states agree with most of the predicted ITRs. Additionally, it is demonstrated that water molecules in the groove of a nanostructure surface play a crucial role in TCMs in the Cassie-Baxter states. Considering the effective contact region of water molecules in the groove in the Cassie-Baxter states, the use of TCMs can effectively reduce the prediction error of ITRs.

Adhesion Reduction at Solid/Liquid Interfaces Based on Topologically Optimized Microtextures.

Artificial microtextures adopted to achieve adhesion reduction help avoid the vulnerability associated with chemical coatings. Most current microtextures strongly rely on biological inspiration or designers' physical intuition. There are also manufacturing challenges due to the complex geometrical configurations. Topology optimization can determine the structural configurations encompassing geometric information on topology, shape, and size and ensure the manufacturability of the optimized microtextures by controlling the feature size corresponding to a specified fabrication process. Herein, we present an approach to reduce the liquid adhesion on solid surfaces by employing artificial microtextures with hexagonal periodicity, where the microtextures are inversely designed through topology optimization. The microtextures are fabricated of polydimethylsiloxane by using a soft lithography process. The liquid adhesion on the microtextures is measured via the tilting plate method. Experimental results demonstrate that the topologically optimized microtextures can significantly reduce the liquid adhesion by 45.0%, which is achieved by the robust Cassie-Baxter state of the wetting behavior. The topologically optimized microtextures can also support the robust Cassie-Baxter state underwater and accelerate the speed when the droplets slide off the surface with them. The findings can be utilized in the context of the reduction of underwater drag and bioadhesion.

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.

Wetting of soap bubbles on topographic surfaces

HypothesisThere is a relationship between the static contact angle of droplets and soap bubbles on flat homogeneous surfaces, therefore, it should be possible to derive a relationship between the static contact angle of a soap bubble on a periodic topographic surface and a droplet on a flat homogeneous surface. ExperimentsA free energy model of the static contact angle of soap bubbles on a topographic surface in the Cassie-Baxter state was derived. Polydimethylsiloxane surfaces of varying area fraction (0.125, 0.250, 0.500, 0.750, and 1.00) and periodic topographies (lined and pillared) were fabricated using 3D printed moulds for pattern transfer. A bubble goniometer was developed to accommodate bubbles of 40,000 ± 5,000 mm3 and 50,000 ± 5,000 mm3 volumes. Then, the static contact angle of bubbles of both volumes were measured on the varying topographic surfaces. FindingsThe derived predictions imply that the relationship between the static contact angle for bubbles on a flat homogeneous surface and on a composite surface, has the same form as the Cassie-Baxter equation for a droplet. The experimental results for the measured static contact angle for both bubble volumes on the varying surfaces had good agreement with the predicted trends.

Super photothermal/electrothermal response and anti-icing/deicing capability of superhydrophobic multi-walled carbon nanotubes/epoxy coating

Ice accumulation on wind turbine blades poses a significant threat to the efficiency and safety of wind turbines, necessitating urgent solutions through effective anti-icing/deicing strategies. In this study, superhydrophobic multi-walled carbon nanotubes (MWCNTs)/epoxy coating for anti-icing application was developed. The coatings exhibited super photothermal and electrothermal response by integrating the superhydrophobic capability of MWCNTs, which are ideal for all-weather anti-icing/deicing applications. The superhydrophobic coating in the ratio of MWCNTs/epoxy resin 1:4 exhibited a water CA of 154.3° and SA of 5.7°, respectively. The temperature of the superhydrophobic coating increased to 45.4 °C (300 s) and 89.5 °C (200 s) at 1.5 sun and 25 V, respectively. The coating exhibited a static delayed frost time of 1790 s and achieved photothermal fast defrosting (1.5 sun, 237 s) and electrothermal fast defrosting (25 V, 35 s), respectively. Furthermore, it demonstrated delayed icing times of 988 s, 579 s, and 264 s at − 10 °C, −15 °C, and − 20 °C, respectively. The coating showed efficient abilities of photothermal fast deicing (1.5 sun, 175 s) and electrothermal fast deicing (25 V, 65 s) as well. The superhydrophobic coating restored the Cassie-Baxter state after melting ice droplets into water droplets through photothermal and electrothermal stimulation. Moreover, the superhydrophobic coating demonstrated exceptional dynamic anti-icing performance even in a low-temperature and humid environments.

Underwater Acoustic Camouflage by Wettability Transition on Laser Textured Superhydrophobic Metasurfaces

AbstractThe superhydrophobicity of submerged surfaces typically pertains to the trapped air film at the liquid–solid interface, subject to wettability transitions from a Cassie–Baxter state to more unstable states that gradually collapse to high retention regimes, which are energetically more favorable. In this work, the dynamic evolution of those transient metastable states is correlated to the underwater acoustic performance of laser textured superhydrophobic surfaces, resolving the dependence of the ultrasound spectral response with the immersion time to capture the genuine contribution of the hierarchical subwavelength morphology, regardless of the air layer effects. Acoustic wave attenuation of the incident ultrasound energy is extensively quantified in transmission, accounting for instantaneous broadband sound blocking (>30 dB) within the spectral range 0.5–1.5 MHz. As a result of the air layer detachment with the immersion time, transmission coefficients increase accordingly, while acoustic fields in reflection unexpectedly evolve toward stealthiness and naïve acoustic camouflage, mostly ascribable to dissipative mechanisms at air layer interfaces. The intrinsic decay of the air layer effect is tentatively determined at different frequencies, since quantitative understanding of the transient lifetime governing underwater surface wettability is critical to design stable superhydrophobic character of laser induced subwavelength metastructures on the most promising acoustic materials – from eco‐friendly natural to artificial.

Laser-based functionalization for superhydrophobic silicon carbide with mechanical durability, anti-icing and anti-fouling properties

The further applications of silicon carbide (SiC) are limited due to the ease of being contaminated and tendency for icing at low temperature caused by its intrinsically hydrophilicity. In recent years, superhydrophobic surface has been widely employed due to the potential value in anti-fouling and passive anti-icing. In this work, a novel laser ablation-silicone oil heat treatment (LASH) composite process is proposed to fabricate superhydrophobic SiC surface. Many microscale “fence” structures and nanoscale “broccoli” fragmental structures are successfully obtained, which can be adjusted by using different laser processing parameters. Besides, the X-ray photoelectron spectroscopy analysis demonstrates that the effective deposition of low surface energy chemical functional groups is the key to realize the superhydrophobic properties of SiC surface. The coupling effect between the micro/nano structures and low surface energy is confirmed to be important for the stability of Cassie-Baxter state at low temperature. In addition, the freezing process of water droplets on the LASH surface at low temperature and the mechanism of the rapid transition from Wenzel state to Cassie-Baxter state are analyzed. The experimental results indicate that the LASH surface possesses a longer static icing time and keeps good hydrophobicity at −5 °C. The icing temperature of the LASH surface decreases to −17.6 °C while the temperature of the untreated surface is −3.7 °C. Moreover, the LASH surface possesses a lower ice adhesion strength which means it is easier for the ice droplets to break away from the surface. After 20 cycles of icing-deicing experiments, the WCA of the LASH surface is still higher than 150° and its ice adhesion strength slightly increases to 240 kPa. Finally, the mechanical robustness of the LASH surface is studied through the abrasive belt and tape stripping tests, and the anti-fouling property is also evaluated. The change of ice adhesion strength on the LASH surface under cyclic icing experiment is analyzed. The experimental results show that the LASH surface has great mechanical durability and anti-fouling property, which is expected to further expand the application prospects of SiC in different fields.

Mechanistic investigation of nucleation kinetics in heterogeneous ice crystallization: the role of cooling rate, surface energy, surface nanostructure, and wetting state

The mechanism of nucleation in heterogeneous ice crystallization is important but remains unclear in spite of decades-long research. This study aims at investigating the nucleation kinetics in heterogeneous ice crystallization through molecular dynamics simulations, and comprehensively elucidating the effect of cooling rate, surface energy, surface nanostructure, and wetting state on the nucleation behaviours. More than 300 cases were simulated using monatomic water (mW) model based on a modified Stillinger-Weber (SW) potential. The dynamic nucleation and crystallization behaviours, along with the evolution of the largest ice nucleus, total ice clusters, kinetic energy, and interaction energy between nanodroplet and substrate, were investigated. Increasing the cooling rate and higher surface energy could promote the ice nucleation, but may lead to a tendency to transform into metastable interfacial ice and 4-coordinated molecule types, and create ice-free layer near the substrate due to the strong interaction strength. What's more, the effect of size match between the accessible width and ice lattice constant depends on the wetting states. In Cassie-Baxter state, the size match effect could promote the ice nucleation, while in Wenzel state it is not pronounced and decreasing the contact area by increasing the nanogroove width would inhibit the nucleation process. This work will be valuable in understanding the mechanism of nucleation kinetics in heterogeneous ice crystallization and provide guidance in promoting or inhibiting ice formation.

Freezing-Melting Mediated Dewetting Transition for Droplets on Superhydrophobic Surfaces with Condensation.

The water-repellence properties of superhydrophobic surfaces make them promising for many applications. However, in some extreme environments, such as high humidities and low temperatures, condensation on the surface is inevitable, which induces the loss of surface superhydrophobicity. In this study, we propose a freezing-melting strategy to achieve the dewetting transition from the Wenzel state to the Cassie-Baxter state. It requires freezing the droplet by reducing the substrate temperature and then melting the droplet by heating the substrate. The condensation-induced wetting transition from the Cassie-Baxter state to the Wenzel state is analyzed first. Two kinds of superhydrophobic surfaces, i.e., single-scale nanostructured superhydrophobic surface and hierarchical-scale micronanostructured superhydrophobic surface, are compared and their effects on the static contact states and impact processes of droplets are analyzed. The mechanism for the dewetting transition is analyzed by exploring the differences in the micro/nanostructures of the surfaces, and it is attributed to the unique structure and strength of the superhydrophobic surface. These findings will enrich our understanding of the droplet-surface interaction involving phase changes and have great application prospects for the design of superhydrophobic surfaces.

Ultrasonic Healing of Plastrons.

Superhydrophobic surfaces (SHS) exhibit a pronounced ability to resist wetting. When immersed in water, water does not penetrate between the microstructures of the SHS. Instead, a thin layer of trapped gas remains, i.e., plastron. This fractional wetting is also known as the Cassie-Baxter state (CB). Impairment of superhydrophobicity occurs when water penetrates the plastron and, when complete wetting is achieved, a Wenzel state (W) results. Subsequent recovery back to CB state is one of the main challenges in the field of SHS wetting. Current methods for plastron recovery require complex mechanical or chemical integration, are time-consuming or lack spatial control. Here an on-demand, contact-less approach for performing facile transitions between these wetting states at micrometer length scales is proposed. This is achieved by the use of acoustic radiation force (ARF) produced by high-intensity focused ultrasound (HIFU). Switching from CB to W state takes <100 µs, while the local recovery back to CB state takes <45 s. To the best of authors knowledge, this is the first demonstration of ARF-induced manipulation of the plastron enabling facile two-way controlled switching of wettingstates.

The influence of geometric boundary features on droplet wetting and directional motion

Structured surfaces with micro and nanostructures have shown great potential for a wide range of applications, such as self-cleaning, deicing, and drag reduction. Previous research has extensively explored the interactions between droplets and microstructures, particularly in terms of Cassie-Baxter and Wenzel states. However, there is still a notable gap in systematically studying the interactions between droplets and various types of geometrical boundary features. In this study, we employed molecular dynamics simulations to investigate how different geometric boundary features affect the wetting behavior and motion of water droplets. Our detailed analysis led to the derivation of a formula to calculate the critical contact angle for a droplet interacting with any geometric boundary feature from a mechanical perspective. Based on these findings, we successfully controlled motion direction of water droplets along a separation membrane. We observed that due to the influence of geometric boundary features at the holes, droplets exhibited unidirectional motion on the separation membrane. By considering the influence of geometric boundary features, our research provides valuable insights for preventing wetting transition, enhancing the storage of lubricating oil in grooves, facilitating efficient oil–water separation, and regulating fluid flow in microchannels.

Transition of Liquid Drops on Microstructured Hygrophobic Surfaces from the Impaled Wenzel State to the "Fakir" Cassie-Baxter State.

Low adhesion of liquids on solid surfaces can be achieved with protrusions that minimize the contact area between the liquid and the solid. The wetting state where an air cushion forms under the drop is known as the Cassie-Baxter state. Surfaces where liquids form macroscopic contact angles above 150° are called superhydrophobic and superhygrophobic, if we refer to water or any liquid, respectively. The Cassie state is desirable for applications, but it is usually unstable compared to the Wenzel state, where the drop is in direct contact with the rough surface. The Cassie-to-Wenzel transition can be triggered by an increase in pressure and vibrations, but the inverse Wenzel-to-Cassie is much more difficult to observe. Here, we examine under what conditions the Wenzel-to-Cassie transition is triggered when the microscopic contact angle changes abruptly. For this, we applied a lubricant of low surface tension around drops that were in the Wenzel state on microstructured surfaces. The increase of the microscopic contact angle lifted the drop from the rough surface, when the pillar height and spacing are large and small, respectively. Numerical calculations for the drop-lubricant interface showed that the surface geometry requirements for the Wenzel-to-Cassie transition are stricter than the ones for the stability of the Cassie state. A surface geometry where the Cassie state is more stable than the Wenzel for a given Laplace pressure of the drop may not always allow the Wenzel-to-Cassie transition to take place. Therefore, the stability of the Cassie state is a necessary but insufficient condition for the inverse transition.

Dual-Energy-Barrier Stable Superhydrophobic Structures for Long Icing Delay.

Using superhydrophobic surfaces (SHSs) with the water-repellent Cassie-Baxter (CB) state is widely acknowledged as an effective approach for anti-icing performances. Nonetheless, the CB state is susceptible to diverse physical phenomena (e.g., vapor condensation, gas contraction, etc.) at low temperatures, resulting in the transition to the sticky Wenzel state and the loss of anti-icing capabilities. SHSs with various micronanostructures have been empirically examined for enhancing the CB stability; however, the energy barrier transits from the metastable CB state to the stable Wenzel state and thus the CB stability enhancement is currently not enough to guarantee a well and appliable anti-icing performance at low temperatures. Here, we proposed a dual-energy-barrier design strategy on superhydrophobic micronanostructures. Rather than the typical single energy barrier of the conventional CB-to-Wenzel transition, we introduced two CB states (i.e., CB I and CB II), where the state transition needed to go through CB I and CB II then to Wenzel state, thus significantly improving the entire CB stability. We applied ultrafast laser to fabricate this dual-energy-barrier micronanostructures, established a theoretical framework, and performed a series of experiments. The anti-icing performances were exhibited with long delay icing times (over 27,000 s) and low ice-adhesion strengths (0.9 kPa). The kinetic mechanism underpinning the enhanced CB anti-icing stability was elucidated and attributed to the preferential liquid pinning in the shallow closed structures, enabling the higher CB-Wenzel transition energy barrier to sustain the CB state. Comprehensive durability tests further corroborated the potentials of the designed dual-energy-barrier structures for anti-icing applications.

Long‐Term Immersion Study for Durability of Interconnected Micropatterned Surfaces with Sustained Water Repellency

AbstractThe sustained water repellency of interconnected micropatterned surfaces is explored over an extended duration, with a focus on their resilience during a 90‐day water‐immersion test. Initially, the microstructure surfaces exhibit high water repellency, a characteristic of the Cassie–Baxter state. However, subsequent detailed temporal analyses reveal varying responses depending on the structural topology. The interconnected micropatterned surfaces exhibit remarkable long‐term resistance to water; this is attributed to the formation of large and stable air pockets enabled by their unique microcavity structures. In comparison, hierarchical microcavity surfaces with micropillars exhibit a notable decrease in water repellency, as evidenced by reduced contact angles, suggesting a transition to a wetting state owing to the emergence of surface hydrophilicity during long‐term water exposure. This study demonstrates the importance of stable air‐pocket effects, particularly in applications where the long‐term stability of liquid repellency is critical, and suggests the role of interconnected structures in maintaining water repellency over time.

Nano-structure and hydrophobicity co-determined barrier properties of corrosion protective ZnAl-LDH film in atmospheric environment

Salt spray is prone to convert superhydrophobic surface from Cassie-Baxter state to Wenzel state, which accelerates the metallic corrosion. Herein, corrosion process of superhydrophobic coatings were investigated by in-situ growth of LDH lamellae with different platelet spacing and modification of stearic acid. The corrosion protection mechanism of the superhydrophobic coating divided into two stages: the well-known air film blocking effect and the hydrophobicity-depended barrier performance, which is a new focus. Through investigating the influences of the nano-structure on corrosion protection via experiments and simulation, a new mechanism on corrosion protection failure of superhydrophobic surfaces in atmospheric environment is proposed.

Influence of surface roughness on the fluid flow in microchannel

Surface roughness is a crucial factor of fluid flow in microfluidic channels. Most of the existing studies focus on the effect of regular microstructured surfaces on fluid flow, while researchs about the impact of random rough surfaces on fluid flow remains limited. Therefore, in this study, a random rough surface model and two microstructured surface models were established, and the effects surface roughness on the fluid flow at Wenzel state and Cassie-Baxter state were studied using COMSOL multi-physics simulation software. Our findings reveal that both the flow velocity and the fluid flow rate at the microchannel outlet decreases with the surface roughness increases. Notably, the flow rate and velocity of the fluid flow at Cassie-Baxter state is higher than that of fluid flow at Wenzel state.

Fabrication of high-durability superhydrophobic coatings based on dual-sized SiC particles

In recent years, inspired by “biomimicry”, superhydrophobic surfaces have gained significant attention. Superhydrophobic surfaces demonstrate notable advantages in addressing interfacial issues, and superhydrophobic coatings exhibit excellent waterproofness, anti-fouling, self-cleaning, anti-corrosion, and additional capabilities, making them promising next-generation waterproof materials. However, the complex preparation process, coupled with poor wear resistance and environmental durability, severely limits their practical applications. Therefore, this article started from simplifying the preparation process and improving the durability of the coatings. Epoxy resin (E51) was used as the film-forming material, and carbon nanotubes (CNTs) and dual-sized SiC particles (nano-SiC and micro-SiC) were used as the fillers. Room temperature vulcanized silicone rubber (RTV) was used as a binder interacting with epoxy resin to promote the interface interaction between the fillers and the polymers. This process resulted in the successful preparation of superhydrophobic coatings with outstanding comprehensive performance. When the ratio of μ-SiC to n-SiC was 1:1, the prepared coating exhibited the best superhydrophobic properties with a water contact angle (WCA) of 167.4° and a sliding angle (SA) of 4.6°. Even after undergoing severe mechanical tests, such as sandpaper abrasion for 1000 cycles, sand impact for 100 cycles, cross-cut test, and tape-peeling for 70 cycles, the coatings still maintained their non-wetting Cassie-Baxter state. Furthermore, even after immersion in strong acid, strong alkali and 3.5 wt% NaCl solutions for 6 days, keeping at 500 ℃ for 2 hours, and exposure to ultraviolet for 6 days, the coatings still exhibited excellent superhydrophobicity. This suggested that the prepared coating had excellent chemical stability and high-temperature resistance. In addition, the superhydrophobic coating exhibited exceptional capabilities in self-cleaning, anti-corrosion, anti-icing, and de-icing properties. Furthermore, this coating, applicable to diverse substrates including board, steel, paper, and glass, demonstrated an impressive water contact angle (WCA) and sliding angle (SA). The spraying method offers the benefits of simplicity and cost-effectiveness. This is poised to significantly broaden its practical applications in various fields, including construction, transportation, and the chemical industry.

Nature-Inspired Superhydrophobic Coating Materials: Drawing Inspiration from Nature for Enhanced Functionality.

Superhydrophobic surfaces, characterized by exceptional water repellency and self-cleaning properties, have gained significant attention for their diverse applications across industries. This review paper comprehensively explores the theoretical foundations, various fabrication methods, applications, and associated challenges of superhydrophobic surfaces. The theoretical section investigates the underlying principles, focusing on models such as Young's equation, Wenzel and Cassie-Baxter states, and the dynamics of wetting. Various fabrication methods are explored, ranging from microstructuring and nanostructuring techniques to advanced material coatings, shedding light on the evolution of surface engineering. The extensive applications of superhydrophobic surfaces, spanning from self-cleaning technologies to oil-water separation, are systematically discussed, emphasizing their potential contributions to diverse fields such as healthcare, energy, and environmental protection. Despite their promising attributes, superhydrophobic surfaces also face significant challenges, including durability and scalability issues, environmental concerns, and limitations in achieving multifunctionality, which are discussed in this paper. By providing a comprehensive overview of the current state of superhydrophobic research, this review aims to guide future investigations and inspire innovations in the development and utilization of these fascinating surfaces.

Validating the Transition Criteria from the Cassie-Baxter to the Wenzel State for Periodically Pillared Surfaces with Lattice Boltzmann Simulations.

Microfabrication techniques allow the development and production of artificial superhydrophobic surfaces that possess a precisely controlled roughness at the micrometer level, typically achieved through the arrangement of micropillar structures in periodic patterns. In this work, we analyze the stability and energy barrier of droplets in the Cassie-Baxter (CB) state on such periodic patterns. In addition, we further develop a transition criterion using the CB equation and derive an improved version which allows predicting for which pillar geometries, equilibrium contact angles, and droplet volumes the CB state switches from a metastable to an unstable state. This enables a comparison with existing experiments and three-dimensional multiphase Lattice Boltzmann simulations for different pillar distances, two contact angles, and two droplet volumes, where a good agreement has been found.