Wenzel State Research Articles

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.

Functionalized super-hydrophobic nanocomposite surface integrating with anti-icing and drag reduction properties

Anti-icing and drag reduction are two significant issues for aircraft, but functional surfaces integrating both properties are rarely reported. Here, we propose a functional super-hydrophobic nanocomposite surface with sharkskin-like groove structures, which achieves the combination of anti-icing and drag reduction properties. The nanocomposite surface was prepared by stripping polydimethylsiloxane doped with carbon nanotubes from a laser-prepared sharkskin-like structure mold and then spraying a layer of polytetrafluoroethylene hydrophobic coating. The microstructures and nanoparticles with low surface energy provide the surface with hydrophobicity and icephobicity, enabling droplet transitions between Cassie and Wenzel states during icing-melting process. Carbon nanotubes enhance the Joule heating performance when voltage is applied. Ice at interface melts into water film, significantly reducing adhesion and enabling efficient anti-icing. Under the influence of the bionic sharkskin riblet structure, the vortices are lifted and pinned above the riblet tips, which reduces the near-wall velocity gradient and total wall shear stress, ultimately leading to lower drag. In addition, the flexible surface undergoes adaptive deformation to more effectively conform to the fluid flow during motion. The riblet protrusions increase the contact area between the flexible surface and the fluid, thereby enabling the absorption of more turbulent energy and fluctuations, further reduces drag. This study presents a new method for manufacturing functional surfaces for practical anti-icing and drag reduction applications.

Modulating Wetting States of Molten Aluminum Droplets on SiO2 Surfaces via Vertical Sinusoidal Vibrations.

Vibration affects the wetting behavior of droplets, and it is feasible to use vibration to modulate the adhesion characteristics of droplets. In this paper, the effect of vertical sinusoidal vibrations on the wettability of molten aluminum droplets on the substrate surfaces of smooth and with nanopillars is investigated. The increase in the frequency or amplitude of the vibration leads to a rise in the interfacial potential energy between the molten droplets and the substrate, which in turn leads to the occurrence of the Wenzel-Cassie transition. Once the vibration frequency reaches the threshold values, the molten droplets leave from the substrate, that is, dewetting occurs. The molten droplets in the Wenzel state undergo a Wenzel-Cassie transition before dewetting occurs. A phase diagram describing the frequency thresholds at which the molten aluminum droplets undergo dewetting and the Wenzel-Cassie transition at different amplitudes is plotted. For a specific amplitude, the frequency of vibration required for dewetting to occur in molten aluminum droplets in the Young state is lower than that in the Wenzel state. The needed vibrational frequency for dewetting or the Wenzel-Cassie transition decreases with increasing amplitude.

Effect of surface micro-texture morphology on the wettability of aluminum alloy on B4C surface

Abstract To enhance the wettability of AlSi10Mg on the B4C surface, a 2D laminar two-phase flow, phase-field coupled multi-field model is utilized to investigate the effect of surface micro-texture morphology on the wettability of AlSi10Mg on B4C surfaces and optimize the morphology of the micro-texture with the best improvement of the interfacial wetting effect. The results of the study show that compared to the square texture, the conical texture has a better effect on the improvement of the wettability of the interface. The best wettability of AlSi10Mg on the B4C surface was obtained at a 1:1 depth-to-width ratio of the conical texture, and the contact angle of the Al droplet is 47.9, which is improved by 21.22% compared to that of the smooth interface. The interfacial wetting state is transformed from the Wenzel state to the Cassie state with the increasing depth-to-width ratio of the textures, and the wettability of the interface deteriorates.

Lattice Boltzmann study of droplet dynamic behaviors impinging on textured surfaces under an external electric field

Textured surfaces contribute to enhancing the cooling effectiveness of electrostatic spray, while the droplet impacting dynamics on such substrates under the influence of electric field are crucial for cooling efficiency. This study utilized a multiphase lattice Boltzmann method combined with the leaky dielectric model to systematically examine the dynamics of droplet impingement on textured surfaces when exposed to electric field. The impact of Weber number, microstructural surface parameters, and electric field strength on droplet impact behavior was discussed in detail. Simulation outcomes reveal that, without the presence of an electric field, the impingement of droplets on textured surfaces results in three distinct deposition states: the Cassie state, partial penetration state, and Wenzel state, primarily contingent upon the surface solid fraction and the droplet impingement velocity. In the Cassie impact regime influenced by an applied electric field, the droplet spreading behaviors exhibit minimal sensitivity to the electric field, with surface tension and inertia primarily governing the spreading dynamics. Throughout the retraction stage, the droplet elongated the direction of the electric field as a result of electric field forces, and eventually, as the electric field strength grows, it bounces off the surface. In the Wenzel impact regime, as the strength of the electric field escalates, the droplet undergoes upward stretching and splits into satellite droplets during the retraction phase, attributed to the dynamic pressure and electrostatic pressure at the apex exceeding the capillary pressure and gravity. These findings could aid in advancing electrostatic spray cooling technology.

Energy Dissipation during Wenzel Wetting via Roughness Scale Interface Dynamics.

A numerical method is proposed to simulate the roughness scale interface dynamics of a slow-moving fluid interface as it advances over a chemically homogeneous rough surface. Analysis of the governing augmented Navier-Stokes and Young's boundary condition equations shows how the local interface behavior can be represented via a series of incrementally advanced equilibrium interfacial morphologies. Combined with a roughness scale mechanical energy balance [Harvie, D. J. E. Contact-angle hysteresis on rough surfaces: mechanical energy balance framework. J. Fluid Mech. 2024, 986, A17], the simulations are used to calculate the energy dissipation associated with a surface decorated with a periodic array of round-edge square pillars. This dissipation is used to predict static contact angle hysteresis (CAH) from knowledge of just the surface roughness topography and equilibrium contact angle. We show that the energy dissipated varies approximately as ϕln ϕ (with ϕ being the area fraction), becoming zero as ϕ → 0. The CAH predicted by our method is in good agreement with the experimental results of Forsberg et al. [Forsberg, P. S.; Priest, C.; Brinkmann, M.; Sedev, R.; Ralston, J. Contact line pinning on microstructured surfaces for liquids in the Wenzel state. Langmuir 2010, 26, 860-865], thereby demonstrating that our numerical method of simulating interfacial dynamics adequately captures the real interface motion, as well as illustrating how far-field contact angle and energy dissipation approaches are consistent for this surface. We also compute CAH for an interface moving at 45° to the surface periodicity direction to show that the experimental measurements are bracketed by the 0° and 45° advance direction results. The proposed method opens up the field to quantitative analysis, surface functionalization, and design for different specific applications.

Understanding wetting behaviors on rough PTFE surfaces towards n-octane/water mixture separation by molecular dynamics simulation

The wetting behaviors of pure n-octane and water nanodroplets on rectangular pitted, grooved, and rectangular pillared polytetrafluoroethylene (PTFE) surfaces were examined by molecular dynamics simulation. The results showed that the three rough surfaces could achieve superlipophilicity and near superhydrophobicity by changing the rough morphology of surface. The n-octane droplets always presented the Wenzel state on the rough surfaces, and the water droplets were the Wenzel state, Transition state and Cassie state under different phase area fractions and/or roughness factors. Based on completely opposite wetting properties of the two droplets on the same surface, separation performances of mixture nanodroplets were further studied on the three rough PTFE surfaces. The results showed that the n-octane/water separation efficiency of pillared surface was above 98.4 %, significantly higher than those of pitted and grooved surfaces. The rough PTFE surface with special wettability enabled efficient separation of oil/water mixture. This work is expected to pave the way for the rational design and industrial application of oil/water separation materials.

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.

Unconventional Dually-Mobile Superrepellent Surfaces.

The ability of water droplets to move freely on superrepellent surfaces is a crucial feature that enables effective liquid repellency. Common superrepellent surfaces allow free motion of droplets in the Cassie state, with the liquid resting on the surface textures. However, liquid impalement into the textures generally leads to a wetting transition to the Wenzel state and droplet immobilization on the surface, thereby destroying the liquid repellency. This study reports the creation of a novel type of superrepellent surface through rational structural control combined with liquid-like surface chemistry, which allows for the free movement of water droplets and effective repellency in both the Cassie and Wenzel states. Theoretical guidelines for designing such surfaces are provided, and experimental results are consistent with theoretical analysis. Furthermore, this work demonstrates the enhanced ice resistance of the dually-mobile superrepellent surfaces, along with their distinctive self-cleaning capability to eliminate internal contaminants. This study expands the understanding of superrepellency and offers new possibilities for the development of repellent surfaces with exceptional anti-wetting properties.

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.

Harnessing Multiscale Physiochemical Interactions on Nanobiointerface for Enhanced Stress Resilience in Rice.

Nanoagrochemicals present promising solutions for augmenting conventional agriculture, while insufficient utilization of nanobiointerfacial interactions hinders their field application. This work investigates the multiscale physiochemical interactions between nanoagrochemicals and rice (Oryza sativa L.) leaves and devises a strategy for elevating targeting efficiency of nanoagrochemicals and stress resilience of rice. We identified multiple deposition behaviors of nanoagrochemicals on hierarchically structured leaves and demonstrated the crucial role of leaf microarchitectures. A transition from the Cassie-Baxter to the Wenzel state significantly changed the deposition behavior from superlattice assembly, ring-shaped aggregation to uniform monolayer deposition. By fine-tuning the formulation properties, we achieved a 415.9-fold surge in retention efficiency, and enhanced the sustainability of nanoagrochemicals by minimizing loss during long-term application. This biointerface design significantly relieved the growth inhibition of Cd(II) pollutant on rice plants with a 95.2% increase in biomass after foliar application of SiO2 nanoagrochemicals. Our research elucidates the intricate interplay between leaf structural attributes, nanobiointerface design, and biological responses of plants, fostering field application of nanoagrochemicals.

Lotus leaf inspired hierarchical micro-nano structure on NdYbZr2O7 ceramic pellet with enhanced volcanic ash repellence

Volcanic ash dispersed in the air adheres to thermal barrier coatings (TBCs) on the hot section components of gas turbines, posing a severe threat to aviation safety. Here, we report a novel strategy inspired by lotus leaf for mitigating the molten volcanic ash adhesion and corrosion. A hierarchical micro-nano structure composed of arrayed micro-conoids and numerous nanoparticles were constructed on the surface of NdYbZr2O7 ceramic pellet by ultrafast laser direct writing technology, whose morphology could be changed by optimizing the processing parameters. The laser-ablated pellet exhibits larger contact angle of molten volcanic ash than the original one, revealing an enhanced resistance repelling volcanic ash. This phenomenon is primarily attributed to the air trapped in the micro-grooves, together with these nanoparticles. Note that the evolution of viscous force and capillary force with time accelerates the transition of molten volcanic ash from the Cassie state to the Wenzel state and reduces its contact angle. Also, this structure effectively inhibits the infiltration of molten volcanic ash by rapidly developing a reaction layer. These findings are an important step toward the design and development of next-generation silicate ash-repellent TBCs.

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.

Wetting of nanoscale water films on hierarchically structured surfaces

Surfaces with hierarchical structures significantly enhance the hydrophobic properties of solids, proving crucial for diverse applications including self-cleaning, anti-icing, and contamination prevention. In this study, we directly observe the dynamic wetting transitions of nanoscale water films on desirable textured surfaces decorated with dual-scale roughness between various wetting states encompassing Cassie–Cassie, Wenzel–Cassie, Cassie–Wenzel, and Wenzel–Wenzel states. Additionally, detailed information on the wetting of the water film on desirable textured surfaces decorated with dual-scale roughness is obtained using atomistic simulations in conjunction with sampling techniques. Through observation of the dynamic wetting transition, two common types of wetting pathways are directly captured, dubbed the preferential primary intrusion and secondary intrusions. The wetting follows which pathway is dependent on Hs/Ss of the small-scale roughness. The mechanisms behind the wetting transitions are revealed based on corresponding free-energy pathways. Moreover, the effect of aspect ratio and intrinsic contact angle on the wetting behavior has been studied. Subsequently, we construct a wetting phase diagram to exhibit all the possible outcomes and identify different wetting regimes. This work paves the way to understanding the wetting mechanisms on nanoscale textured surfaces with two-tier roughness, which can help to design a hydrophobic surface with superior robustness.

Ultra-low friction system using special wetting interfaces: Bridging across various wetting regimes

Friction was the main way of energy loss in society, and reducing friction has become a focal point in current research. Inspired by special wetting phenomena, an asymmetric multiphase composite (AMPC) friction system was constructed. Tribological behavior of the friction system was evaluated using hydrogel on anodized aluminum as friction pair and superhydrophobic alumina with micro/nano-composite as substrate. Ultra-low friction coefficients (∼10−3) were measured under both Cassie and Wenzel regimes. Especially in Wenzel state, increased contact stress or extended shear rate promoted the formation of transfer lubricant film, thus enabling the ultra-low friction coefficient. Moreover, the system showed mechanical stability during prolonged water lubrication. This novel lubrication approach may provide valuable scientific solutions for specific tribology applications.

Enhancing carbon capture: Exploring droplet wetting and gas condensation of carbon dioxide on nanostructured surfaces

The investigation of carbon dioxide (CO2) condensation on heat exchange surfaces is essential to the advancement of carbon capture, utilization, and storage (CCUS) technology. Molecular dynamics simulations are employed to explore the wetting and condensation behavior of CO2 on nanostructured surfaces. The results indicate that the contact angle of CO2 droplets on nanostructured surfaces is greater than that on smooth surfaces. Increasing the pillar height (h) or solid fraction (f) induces the transition of CO2 droplets from the Wenzel to Cassie state, resulting in an increased contact angle. Nanostructured surfaces significantly enhance the nucleation rate, with molecules initially nucleating at the bottom of the pillars. A higher h or f accelerates the nucleation rate during the condensation. Droplets in the Wenzel state exhibit higher heat transfer efficiency than those in the Cassie state. Additionally, the formation of Cassie-state CO2 droplets undergoes a dewetting transition, altering the heat conduction mode. The dewetting behavior of CO2 droplets favors their detachment from the surface, potentially reducing the initial heat resistance. Therefore, appropriately increasing h and f can promote nucleation and droplet growth, enhancing dewetting transition and mass transfer performance. These simulation results hold great importance for inspiring the design of future CO2-phobic surfaces and guiding CO2 condensation production.