Wenzel State Research Articles

Simple fabrication of asphalt-based superhydrophobic surface with controllable wetting transition from Cassie-Baxter to Wenzel wetting state

We report a simple method, using combustion of asphalt, to fabricate superhydrophobic surface that can change its wetting state from Cassie-Baxter state to Wenzel state. Although asphalt is a hydrophobic material mainly used in water-proof paving and construction, asphalt-coated surfaces exhibit hydrophilic behavior when exposed to heat and moisture. To change the wettability of asphalt, combustion reaction was used to create hierarchical roughness and low surface energy that are requirements for superhydrophobic surface. Combustion reaction of asphalt created carbon soot with hierarchical nano-structures by coating the rigid asphaltene layer with superhydrophobic soot layer, which changed the hydrophobic asphalt surface into Cassie-Baxter type superhydrophobic surface. Additional heat treatment with ethanol changed the wettability from Cassie-Baxter to Wenzel state. Changes in surface morphology and chemical composition of the fabricated surface were observed after the wetting transition. After the wetting transition to the Wenzel state superhydrophobic surface, the micro-sized structures on the surface was removed and the hydrogen bonding was increased. Droplet control using the fabricated Cassie-Baxter and Wenzel state surfaces was demonstrated. The fabricated Cassie-Baxter superhydophobic surface showed the static water contact angle of 157.2° and contact angle hysteresis of 8.4°, while Wenzel state superhydrophobic surface showed static contact angle of 150.2° and contact angle hysteresis of 48.6°. • Superhydrophobic surface was fabricated using combustion of asphalt only. • Carbon soot was deposited on asphaltene layer without additional fixation step. • Using ethanol, wetting transition from Cassie-Baxter to Wenzel state was enabled.

Maximizing electrochemical hydrogen peroxide production from oxygen reduction with superaerophilic electrodes

This study investigated the effects of electrode wettability on hydrogen peroxide (H2O2) production from oxygen reduction reaction (ORR) during electrolysis with superaerophilic electrodes. When the electrode was in the underwater Wenzel-Cassie (UWC) state, it could quickly adsorb aerated oxygen microbubbles, which significantly enhanced oxygen transfer. Meanwhile, H2O2 reduction was effectively inhibited. Consequently, high ORR currents and current efficiencies (CEs) of H2O2 production could be obtained in the UWC state. However, oxygen can only be transferred to the electrode by dissolved oxygen (DO) diffusion when the electrode was in the underwater Wenzel (UW) state. Due to the slow DO diffusion and enhanced H2O2 reduction at the wetted electrode, the rate and CEs of H2O2 production decreased dramatically in the UW state. Maintaining a stable UWC state by controlling the rate of O2 bubbling and rate of O2 consumption in ORR is thus critical to maximizing H2O2 electrosynthesis with the superaerophilic electrodes.

Roughness-dependent wetting and surface tension of molten lead on alumina

Surface roughness is an important factor that affects the wetting of molten metal on ceramics. The effect of surface roughness of the alumina substrate on the contact angle, contact diameter, drop height and surface tension of molten lead was investigated in the temperature range of 923–1123 K. The microstructure of the lead/substrate interface was observed by SEM. The surface free energy of alumina substrates was calculated by the geometrical average method. When the surface roughness of the substrate increased from 0.092 to 2.23 μm, the surface free energy increased gradually, ranging from 13.356 to 39.998 mJ/m2. The contact diameter of lead droplets decreased from 9.111 to 7.19 mm. The lead drop height increased from 3.41 to 3.85 mm. The contact angle increased from 113.05° to 137.15°. Moreover, the surface depression of the alumina substrate was filled with lead, and no obvious change was observed. The results demonstrated that the wetting of lead drop on alumina substrates was consistent with the Wenzel state.

Molecular Insights on the Wetting Behavior of a Surface Corrugated with Nanoscale Domed Pillars.

Using all-atom molecular dynamics simulation, we investigated the wettability of a surface texturized with nanoscale pillars of domed, rectangular, or cylindrical shapes. The dewetted and wetted states of the gaps between the pillars were related to the Cassie-Baxter (CB) and Wenzel (WZ) states of a macroscopic water droplet resting on top of the pillars. We uncovered the structures and free energies of the intermediate states existing between the CB and WZ states. The contact line of the liquid-vapor-solid interface could not be depinned for the domed pillars due to their smooth curvatures unlike for the rectangular or cylindrical pillars. The liquid symmetrically penetrated down into the gap between the domed pillars by a liquid-vapor interface shape like a paraboloid, while the penetration for the rectangular or cylindrical pillars was often asymmetrical, giving a half-tubular liquid-vapor interface.

Wettability of complex Long-Chain alkanes droplets on Pillar-type surfaces

To reveal the difference in wettability of long-chain alkane and simple molecular materials on the structural surfaces, we employ long-time molecular dynamics simulation to study the wettability of complex alkane droplets on flat and pillar-type surfaces by comparing the contact angle and the transition of the wetting state. The results showed that the transition of the wetting state depended on the joint action of potential coefficient, pillar spacing, and pillar height, and the wetting state of droplets could transform into each other among Wenzel state, Cross state, and Cassie state. The accurate ranges of the predicted values of the Cassie formula were the weaker fluid–solid attraction and smaller pillar spacing. In contrast, the accurate ranges of the predicted values of the Wenzel formula were the stronger fluid–solid attraction and smaller roughness. When the fluid–solid attraction was strong and the pillar spacing was small, both the Wenzel and Cassie formula could accurately predict the corresponding contact angle. By giving the corresponding conditions of the Cross state and revealing the differences in alkane droplets with variable chain lengths in the transition of the wetting state, it could serve as a reliable reference for subsequent simulations of complex materials on nanostructure surfaces.

Electrowetting-based control of wetting transition of a nanodroplet on pillar-arrayed surfaces

Two extreme wetting states, a highly non-wetting Cassie state and a wetting Wenzel state, can coexist or even mutually convert on patterned surfaces. Such a conversion process may be spontaneous or induced by external stimuli. This work studies the wetting transition of a nanodroplet on pillar-arrayed surfaces induced by an external electric field via an energy-minimization method in conjunction with molecular dynamics (MD) simulations. The simulation results reveal that driven by the electric field, the initial Cassie state could go through a partial wetting state, and eventually converts to the Wenzel state. The free-energy landscape reveals that there are multiple local energy minima, corresponding to multiple metastable states. For the metastable Cassie state, the wetting transition is irreversible, i.e., the droplet would remain in the Wenzel state when the electric field is removed. Conversely, the spontaneous dewetting transition from the Wenzel to the Cassie state can occur, if only the Cassie droplet is in a global energy minimum configuration. Thus, the stable Cassie wetting configuration is essential for triggering the spontaneous dewetting transition.

Understanding superhydrophobic behaviors on hydrophilic materials: a thermodynamic approach

Some experiments have proved that superhydrophobic behaviors can be achieved on inherently hydrophilic substrates without low surface energy modification at micro-scale. However, the thermodynamic mechanisms about these results have not been well-understood. In this work, a 2D analytical model was reported to analyze this unexpected experimental observations and wetting behaviors on trapezoidal, vertical and inverse-trapezoidal microstructure surfaces. Theoretical results showed that intrinsic contact angle, which was restricted by sidewall angle of micropillars, was not an independent parameter to affect superhydrophobicity. And re-entrant structures were critical in the realization of microstructures alone inducing transition from hydrophilicity to superhydrophobicity. The wetting transition criterion was that sidewall angle should be less than intrinsic contact angle. On this occasion, a positive energy barrier could support liquid/vapor interfaces and separate Wenzel and Cassie state on hydrophilic substrates. And the physical explanations can be found that the positive energy barrier mainly came from the growth of the high-energetic solid/vapor interfaces to be wetted by the drop with liquid/vapor interfaces moving down inverse-trapezoidal pillars. As for the optimal design of microstructures, considering the limitation of pillar width and the ‘sag’ transition caused by pillar height, T-shape microstructures could be a good choice.

Direct observation of wetting behavior of water drops on single micro-scale roughness surfaces of rose petal effect

Hypothesis: It has been verified that a surface of single micro-scale structures with certain roughness could exhibit petal effect. That is, water drops with a contact angle larger than 150° would pin on the petal effect surface. It is conjectured that the water drop could pin on the single micro-scale roughness petal effect surface by totally infiltrating into spaces (or grooves) between micro-pillars.Experiments: An inverted optical microscopy system is synchronically applied in the process of advancing/receding contact angle (ACA/RCA) measurements to directly observe the wetting behavior of water droplets on hydrophobic patterned surfaces with regular arrays of square micro-pillars.Findings: A sequence of wetting behavior evolution, Wenzel → petal → pseudo-lotus → lotus, is observed on the hydrophobic patterned surfaces along with increasing surface roughness. It is interesting to observe a Cassie-Wenzel transition for water drops on a petal substrate during the ACA measurement (embedded needle method), leading to two ACAs, one before (in Cassie state) and one after the transition (in Wenzel state). Thus, the petal substrates have large contact angle hysteresis (CAH) (with both ACA and RCA in Wenzel state) to pin the water drop in Wenzel state. A Cassie-Wenzel transition is consistently observed during the evaporation process of water drops on pseudo-lotus substrates, leading to two RCAs: one in Cassie state and one in Wenzel state. The pseudo-lotus substrates have CAH (with both ACA and RCA in Cassie state) small enough to make water drops easily slide off.

Long-Lived T-Shaped Micropillars with Submicron-Villi on PP/POE Surfaces with Grinding-Enhanced Water Repellency Fabricated via Hot Compression Molding.

Superhydrophobic properties are derived from the roughness of the surface of micro/nanostructures and low-surface-energy materials. However, they are both easy to damage on superhydrophobic surfaces after mechanical abrasion in practical applications, resulting in the transition from the Cassie-Baxter state to the Wenzel state and even the loss of water repellency. In this work, the mechanical properties of polypropylene (PP) toughened with poly(ethylene-co-octene) (POE) were improved for the fabrication of long-lived T-shaped micropillars with submicron-villi on top by a combined method of compression molding and grinding. A universal testing machine was modified as equipment for the precise control of the traveling distance of specimens on sandpaper in precise. The PP/POE blend possessed high tensile strength of up to ∼23.84 MPa as well as elongation at break of ∼533.60%. The abrasive grains on sandpaper reshaped their surface morphologies from micropillars to T-shaped microstructures, on which the submicron-villi as secondary structures formed. The abraded microstructured PP/POE surface exhibited the highest contact angle of 154.4° and the most stable wetting state with a bouncing height of 7.68 mm (3.2 times the diameter of the 7-μL droplet) after a traveling distance of 1000 mm on 3000-grit sandpaper among the abraded and unabraded PP/POE surfaces.

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.

Multiple Wetting-Dewetting States of a Water Droplet on Dual-Scale Hierarchical Structured Surfaces.

Surfaces with microscale roughness can entail dual-scale hierarchical structures such as the recently reported nano/microstructured surfaces produced in the laboratory (Wang et al. Nature2020, 582, 55−5732494077). However, how the dual-scale hierarchical structured surface affects the apparent wetting/dewetting states of a water droplet, and the transitions between the states are still largely unexplored. Here, we report a systematic large-scale molecular dynamics (MD) simulation study on the wetting/dewetting states of water droplets on various dual-scale nano/near-submicrometer structured surfaces. To this end, we devise slab-water/slab-substrate model systems with a variety of dual-scale surface structures and with different degrees of intrinsic wettability (as measured based on the counterpart smooth surface). The dual-scale hierarchical structure can be described as “nanotexture-on-near-submicrometer-hill”. Depending on three prototypical nanotextures, our MD simulations reveal five possible wetting/dewetting states for a water droplet: (i) Cassie state; (ii) infiltrated upper-valley state; (iii) immersed nanotexture-on-hill state; (iv) infiltrated valley state; and (v) Wenzel state. The transitions between these wetting/dewetting states are strongly dependent on the intrinsic wettability (Ein), the initial location of the water droplet, the height of the nanotextures (H1), and the spacing between nanotextures (W1). Notably, Ein–H1 and Ein–W1 diagrams show that regions of rich wetting/dewetting states can be identified, including regions where between one to five states can coexist.

Polarity dependent electrowetting for directional transport of water through patterned superhydrophobic laser induced graphene fibers

The possibilities of the precise control of wetting properties of a series of laser-induced graphene (LIG) films consisting of microscale air pockets on top of nano-scale surface roughness using electrowetting are demonstrated. By application of a marginal DC bias (∼2 V), water can efficiently wet as well as can be pumped through the superhydrophobic LIG substrates. Interestingly, the electrowetting phenomenon is strongly dependent on the applied voltage polarity and it causes an abrupt wetting transition from superhydrophobic (contact angle ∼152°) Cassie state to superhydrophilic (contact angle ∼7°) Wenzel state on the LIG films. By analyzing the voltage polarity dependent electrowetting results with an equivalent electrical circuit model at the solid-liquid interface, and considering the hierarchical dual surface roughness (micro-nano scale), the transition between the “slippy” Cassie state and the “sticky” Wenzel states is explained. Furthermore, we demonstrate that the unique structural characteristics of the custom-designed micropatterned LIGs, with precisely tailored surface energy by simple post-annealing treatment, enable easy preparation of superhydrophobic LIG films. The approach to prepare stable superhydrophobic LIG with voltage polarity dependent wetting mode transition is used here to controllably transport of water through 3D porous LIG surfaces.

Numerical study of thermocapillary migration behaviors of droplets on a grooved surface with a three-dimensional color-gradient lattice Boltzmann model

Thermocapillary actuation is used extensively in droplet-based microfluidic devices to manipulate the dynamic behaviors of droplets. In this study, a three-dimensional color-gradient lattice Boltzmann model is used to investigate the migration behaviors of droplets in the Wenzel state on a grooved surface that is subject to a uniform temperature gradient. On the solid surface, the wetting boundary condition is used to improve the accuracy of the simulations and to suppress spurious velocities at the contact line. The model is used to simulate the thermocapillary migration of a three-dimensional deformable droplet and the thermocapillary migration of a two-dimensional droplet on a solid substrate, and its accuracy is verified against theoretical predictions. The migration behavior of droplets on a smooth surface is investigated, and the flow field and corresponding temperature field around the droplets are analyzed. The experimental findings numerically confirm that a surface with micro-grooves parallel to the temperature gradient can accelerate thermocapillary migration to a greater extent than a smooth surface, indicating the influence of the grooves. The influence of the viscosity ratio is investigated, and it is found that the use of high-viscosity fluids is an effective means of obstructing migration. To determine the influence of surface roughness, a systematic and parametric study of groove depth and width is conducted. Finally, the influence of the orientation of the surface topography is investigated, and it is demonstrated that a surface with micro-grooves perpendicular to the temperature gradient can obstruct migration.

Spontaneous Adsorption-Induced Salvinia-like Micropillars with High Adhesion.

Superhydrophobic surfaces with high adhesion provide high potential for underwater applications. Inspired by Salvinia leaf, here, we have reported a simple method for fabricating adhesive Salvinia-like micropillars via photolithography and spontaneous adsorption of organic molecules from the atmosphere. With continuous hydrocarbon adsorption on sputtered cerium dioxide (CeO2) films, the surface gradually evolved and eventually became chemically heterogeneous. Huge wetting contrast from superhydrophilic to superhydrophobic over exposure time was observed; meanwhile, the wetting mode changed from the Wenzel (W) state to Cassie-Baxter (C-B) state. As a result, hydrophobic hydrocarbons (C-C/C-H) and trapped air between adjacent pillars contributed to the high apparent contact angle (CA), while the hydrophilic domains of C-O/O═C-O and CeO2 on the top layer made the surface highly adhesive with water droplets. In comparison with traditional fluorinated superhydrophobic surfaces, CeO2-coated surfaces showed high adhesive force with water droplets and can be used as a "mechanical hand" for water droplet transport. The adsorption-induced Salvinia-like micropillars with high adhesion may find many other droplet-based applications in microfluidic fields.

Evaluating Superhydrophobic Surfaces under External Pressures using Quartz Crystal Microbalance.

The performance of hydrophobic surfaces under hydraulic pressures is critical to a wide range of practical applications such as drag reduction of seaboard vessels and design of microfluidic devices. This research focuses on the evaluation of drag reduction and velocity slip of hydrophobic surfaces and coatings under external hydrostatic pressures using an acoustic wave device (i.e., quartz crystal microbalance, QCM). The correlation between the resonant frequency shift of a QCM device and drag reduction of hydrophobic surface coated on the QCM was theoretically developed and the model was validated by comparing the measurement results of the drag reduction of an epoxy-based superhydrophobic coating with those measured by a rheometer. The QCM device was further employed to study the wetting state transition and drag reduction of water on a micropillar array based superhydrophobic surface under elevated hydrostatic pressures. It was found that the transition from Cassie to Wenzel states occurred at a critical hydrostatic pressure which was indicated by a sudden frequency drop of the QCM device. In addition, the effective heights of the meniscus at the liquid/air interface increased with the external pressure before the transition took place. The drag reduction induced by the micropillar surface decreased with the increasing hydrostatic pressures. It was demonstrated that the developed QCM based technology provides a low cost, simple, and reliable tool for evaluating hydrophobic performance of various surfaces under external hydrostatic pressures.

Directional Metastable Wetting Evolution of Droplets on Artificial Patterned Microcavity Surfaces

AbstractControllable wetting transition on artificial microtextured surfaces has significant applications in many industrial fields. In this work, the droplet spreading and the directional wetting transition are investigated on the substrate surface with the patterned microcavities. The results show that the macroscopic inward wetting transition from the periphery to the center of the droplet strongly depends on the mesoscopic sequential transition from Cassie to Wenzel state in the microcavities on the substrate surfaces. The semiquantitative relationship between sagging depth of meniscus in the microcavities and the droplet spreading velocity is set up by utilizing scaling‐law analysis in terms of mechanical equilibrium of the meniscus. This finding is expected to help to clarify the issues on the mechanism and main affecting factors of the wetting transition on the patterned surface.

Sustainable superhydrophobic branched hierarchical ZnO nanowires: Stability and wettability phase diagram

Stability of Cassie-Baxter (CB) state is very critical in application of superhydrophobic surfaces. The most industrial applications of superhydrophobic surfaces are limited by the transition from the CB state to Wenzel (W) state. In this research, CB state stability of branched hierarchical ZnO nanowires (BH-ZnO NWs) was investigated as compared with ZnO nanowires (ZnO NWs) by using theoretical and experimental approaches. For this purpose, surface of the BH-ZnO NWs and ZnO NWs were modified by thin layers of methyltrimethoxysilane (MTMS). The MTMS thickness was optimized by varying NH4F (0, 5, 10, 20 μL) as used catalyst. The highest water contact angle (WCA) was measured at about 153 ± 3° with sliding angle of 15 ± 3° for the M (10)/BH-ZnO NWs. Based on the theoretical results, the critical pressure (Pc) for the transition from the CB to W state was obtained at about of 1155 ± 230 and 36770 ± 7350 Pa for the M (10)/ZnO NWs and first level of the M (10)/BH-ZnO NWs samples, respectively. Finally, it was found that the M (10)/BH-ZnO NWs sample showed the higher CB stability as compared to the M (10)/ ZnO NWs sample due to presence of hierarchical nanostructures.

Performance matching between the surface structure of cucumber powdery mildew in different growth stages and the properties of surfactant solution.

Understanding performance matching of pesticide droplets on the surface of cucumber leaves modified by powdery mildew is of practical importance for the agricultural sector. Here, the surface texture and wettability of cucumber leaves covered by powdery mildew were systematically examined using parameters such as micromorphology, physicochemical properties, and liquid droplet contact angle measurements. Our results show that powdery mildew growth can be divided into four distinct stages according to the surface texture characteristics of the diseased cucumber leaves. The three-dimensional (3D) surface structures of powdery mildew layers on cucumber leaves had individual characteristics at different mildew growth stages, among which powdery mildew was more easily spread in the last growth stage, and powdery mildew height was greatest in the NO. 2 growth stage (Sa =425.35 μm). Surface free energy values, static contact angle, and contact angle hysteresis all correlated strongly with the surface characteristics of powdery mildew layers at different growth stages. When the concentration of surfactant reached the critical micelle concentration, the wetting state of AEO-5 solution droplets on the surface of cucumber powdery mildew leaves reached the Wenzel state more easily. The wettability of a droplet on the leaf surface depends on the state of the monomer and micelle in the surfactant solution and the surface characteristics of the powdery mildew-covered leaf. The 3D structure and relative dielectric constant of powdery mildew-covered leaves influenced surface texture characteristics, which in turn controlled the wetting and matching ability of surfactant droplets on diseased leaves. This work provides valuable new insights into the matching of the structure of powdery mildew-covered plant leaves with the properties of surfactant solutions. © 2021 Society of Chemical Industry.

Mechanical Equilibrium Dynamics Controlling Wetting State Transition at Low-Temperature Superhydrophobic Array-Microstructure Surfaces

Superhydrophobic materials are significant for engineering applications in the anti-icing field because of their non-wetting property. The interface physical mechanisms of non-wetting properties are important to promote real applications of superhydrophobic surfaces, especially under low-temperature conditions. Here, we found that low temperature could induce the wetting state transition from a Cassie–Baxter state to a Wenzel state. This transition occurred at 14 °C (and 2 °C) on superhydrophobic surfaces with pillar heights of 250 μm (and 300 μm). As a consequence, the driving-force of the Cassie-Wenzel (C-W) wetting transition was induced by the contraction of air pockets on cooling, and the pressure of air pockets supporting the droplet decreased with the contraction degree. Decreasing the pressure of air pockets broke the mechanical equilibrium at the solid–liquid contact interface, and the continuous contraction overcame the resistance in the C-W wetting transition. Based on the analysis of work against resistance in the C-W wetting transition, lower C-W wetting transition temperature was mainly attributed to a higher pillar, which produced more work against resistance to require more energy. This energy was directly reflected by the energy required for continuous contraction of air pockets. Superhydrophobic surfaces with higher pillar structure remain stable non-wetting property at low-temperature conditions. This work provides theoretical support for the application of superhydrophobic materials in low-temperature environments.

Microscale mechanism of microstructure, micromorphology and Janus wettability of the banana leaf surface

As a result of natural selection, the adaxial and abaxial sides of banana leaves show different wetting states and anisotropy. Janus wettability between the adaxial and abaxial sides of the banana leaf surface is revealed for the first time in this work. This has relevance for the preparation of bionic materials and an important role in the efficient and high-quality production management of pesticide spraying in banana orchards. The main purpose of this research is to analyze and study the microscale mechanism and coupling relationship between the Janus wettability of banana leaf surface and the microstructure and micromorphology. We adopt advanced modern instrument analysis technology, such as contact angle (CA) measurements, field emission scanning electron microscopy (FESEM), X-ray spectrometric analysis (EDS), and Fourier transform infrared spectroscopy (FTIR), and performed tests on the adaxial and abaxial sides of banana leaves to investigate the cause of Janus wettability. The results show that banana leaves exhibit different degrees of anisotropy, mainly due to the surface micromorphology. Banana leaves exhibit a hydrophilic Wenzel state on the adaxial side and a weakly hydrophobic Cassie-Baxter state on the abaxial side. We focused on studying the coupling effect and found that the main coupling element impacting the Janus wettability of the banana leaf surface is the nanopillars microstructure, and the secondary coupling element is the content of hydrophilic functional groups on the surface. This work may lead to the design and fabrication of Janus wetting surfaces by mimicking the nanopillar structure on banana leaf surfaces and help explore the potential application of efficient and high-quality pesticide spraying in banana orchards.