Wetting Transition Research Articles

Thickness-dependent fast wetting transitions due to the atomic layer deposition of zinc oxide on a micro-pillared surface.

Smart surfaces promote the fundamental understanding of wetting and are widely used in practical applications for energy and water collection. Light-induced switchable wettability facilitated by ZnO coatings, for instance, was developed for liquid manipulation at the surface. However, the transition of wetting states was reported to follow a hydrophobic–hydrophilic cycle in an hour, which is very long and may limit its future applications. We recently discovered that the cycle of the wetting state transitions on inorganic coatings can be shortened to less than 100 seconds by using ALD-coated ZnO on a pillared surface. However, the mechanisms are still unclear. Here, we investigated the effects of coating thickness on the transition speed and found that it significantly depended on the thickness of the coating with the optimal thickness less than 50 nm. We found that the minimum critical time for a wetting state transition cycle was less than 50 seconds with a thickness of 40 nm. Although the transition time of surfaces with coatings over 70 nm thickness remained constant at 10 min for a cycle, it was shorter than those of other deposition techniques for a coarse surface. Here, we propose a “penetration–diffusion” model to explain the fast and thickness-dependent wetting transitions. Our study may provide a new paradigm for fast wetting transition surfaces with cycle time within tens of seconds using a homogeneous thin layer coated on a rough surface.

Wetting state transition of a liquid gallium drop at the nanoscale.

Wetting state transition regulated by surface roughness has increasing importance for its wide applications. Molecular dynamics simulations have been performed to study the wetting state transition induced by surface roughness in the gallium-carbon system. There is a transition from the Wenzel state to the Cassie state when the roughness is changed. When the surface roughness is more than 1.8, the gallium droplet is in a Cassie state, but when it is less than 1.6, it is in the Wenzel state. The substrate composed of irregular pillars has a similar effect on the wetting state transition. Besides, distinctive variations occur in the interface tension, the mean-squared displacement, the wetted surface and the interaction energy as the wetting state changes, which are further explained by the proposed model. This study would provide significant guidance for designing superhydrophobic surfaces in the future.

Protective Fabrics: Metal-Organic Framework Textiles for Rapid Photocatalytic Sulfur Mustard Simulant Detoxification

Summary Metal-organic frameworks (MOFs) can catalyze toxic chemical decontamination, but new MOF materials and synthesis strategies are needed to improve performance, particularly in field-usable MOF-textile formats. This article reports for the first time the exceptional photocatalytic reactivity of Al-PMOF (Al-porphyrin-based MOF), composed of an earth-abundant metal-containing Al(OH)O4 cluster bridged by H2TCPP (5,10,15,20-tetrakis(4-carboxyphenyl)porphyrin) chromophores, against the toxic sulfur mustard simulant 2-chloroethyl ethyl sulfide (CEES) under visible-light irradiation. Furthermore, Al-PMOF is strongly immobilized into polymeric fibers via well-controlled Al2O3 solid film conversion using dimethylformamide/water cosolvent. The approach enables a secure integration of conformal Al-PMOF films onto polymer fibers at a relatively low synthesis temperature (120°C). In addition, on a per-unit mass of MOF basis, the surface-bound Al-PMOF films enable extremely rapid CEES detoxification turnover frequency, up to 170 mol CEES mol chromophore − 1 min − 1 , more than 10-fold faster than the best MOF powders and 2-fold better than MOF films reported to date.

Optically Induced Rapid Wetting Transition on Zn-Polar and O-Polar Zinc Oxide.

The design and fabrication of surfaces that support rapid wetting transition remain technologically challenging. Here, we examine the effects of optical illumination on the wetting behaviors of zinc oxide (ZnO) single crystals. We find that ultraviolet irradiation above the band gap energy promotes a rapid wetting transition, characterized by sliding of the water droplet, within a few seconds. Notably, the transition for Zn-polar (0001) ZnO surfaces is even faster than that for O-polar (0001̅) ZnO surfaces. We confirmed that process is dependent on power, surface polarity, and solution pH and reversible through illumination by near-infrared light, which restores the water contact angle back to its initial value. Surface chemical analysis revealed that the instantaneous photocatalytic formation of surface-terminated hydroxyl (-OH) groups is responsible for the observed rapid wetting transition. Density functional theory calculations with the inclusion of onsite Coulomb interactions revealed that both the Zn-polar and O-polar surfaces can be easily covered with -OH groups through the adsorption of -OH groups or hydrogen atoms, respectively. This study develops a route to fabricate optically active and controllable microfluidic devices that support rapid wetting transitions for water droplet manipulation.

Characterization of Cabot BP280 with argon and nitrogen adsorption at temperatures above and below the triple point – Energetic vs Structural heterogeneities

Abstract Wetting transitions on surfaces are a ubiquitous phenomenon in nature because the wettability depends on the balance between the adsorbate-adsorbent (which in turn depend on the surface topology), adsorbate-adsorbate interactions and temperature. To understand the separate contributions of these interactions in a wetting transition for a specific adsorbate-adsorbent pair, molecular simulation is an invaluable tool. Here we illustrate this with a detailed experimental and molecular simulation study of argon and nitrogen adsorption on Cabot BP280 at temperatures above and below their respective triple point temperatures. We report new adsorption isotherm data at 60 K and at 87 K and 77 K which confirm previous measurements on this adsorbent. These results were also used to calculate isosteric heats. Our TEM study shows that the surface of BP280 has a geometrically corrugated graphitic structure which is not as planar as highly graphitized thermal carbon black, such as Carbopack F. Simulations were carried out with two models: an energetic model built from strips of different adsorption strength, and a structural model consisting of crevices embedded in a graphite surface. Adsorption at temperatures below the triple point facilitates the probing of the fine details on the surface, enabling the discrimination between the energetic and structural molecular models, as proposed in this paper. The simulated isotherms and the isosteric heats, generated with the structural molecular models, agree very well with experimental data. Analyses of the microscopic mechanism of adsorption include local density distributions and snapshots of molecular configurations, and local order parameters were used to understand the packing of the adsorbate layer.

Internal-Flow-Mediated, Tunable One-dimensional Cassie-to-Wenzel Wetting Transition on Superhydrophobic Microcavity Surfaces during Evaporation

ABSTRACTSuperhydrophobic textured surfaces are known to maintain a nonwetted state unless external stimuli are applied since they can withstand high wetting pressure. Herein, we report a new category of tunable, one-dimensional (1D) Cassie-to-Wenzel wetting transitions during evaporation, even on superhydrophobic surfaces. The transition initiates at the periphery of the evaporating drop, and the wetting transition propagates toward the center of the drop. The transitions are observed for surfaces with wetting pressures as high as ~ 7,568 Pa, which is much higher than the Laplace pressure, i.e., ~200 Pa. In situ high-contrast fluorescence microscopy images of the evaporating drop show that the transition is induced by preferential depinning of the air-water interface and subsequent formation of air bubbles in the cavities near the three-phase contact line. The evaporation-induced internal flow enhances the pressure within the water droplet and subsequently causes a Cassie-to-Wenzel wetting transition.

Reversibly switching water droplets wettability on hierarchical structured Cu2S mesh for efficient oil/water separation

Surfaces with reversible wettability have broad applications but remain challenging since the switching process is usually energy intensive and complex. In this paper, a pyramid shaped Cu2S film with hierarchical micro/nanostructures is formed on a commercial copper mesh. This film is formed by a spontaneous redox sulfuration reaction and results in a roughened surface, which enables reversible wetting transition between superhydrophilicity to superhydrophobicity. This switching occurs by simple processes such as alternately storing in air or using an ethanol solution treatment and yields cyclic wettability switching for many cycles. This convenient wetting transition behavior, as well as strong stability and efficient oil/water separation with efficiency exceeding 98%, renders it as a potentially useful mesh material for switchable surfaces.

Roughness dependent wettability of sputtered copper thin films: The effect of the local surface slope

Here, we investigated the static and the dynamic wetting behaviors of copper (Cu) thin films deposited by DC magnetron sputtering. The deposited films have random rough surfaces for which the rms roughness amplitude σ, the lateral correlation length ξ, and the roughness exponent α were obtained from the analysis of height topography images acquired by atomic force microscopy. The time-dependent height-height correlation functions indicated anomalous kinetic roughening with roughness exponents α ≈ 0.9 and evolving roughness parameters σ and ξ with deposition time. The latter yields a nonstationary local surface slope σ/ξ that has a crucial impact on the surface wettability. Indeed, static and dynamic contact angles’ (CAs) measurements revealed two wetting regimes associated with different growth stages leading to a transition from a metastable Cassie-Baxter to a Wenzel-like state for the roughest films. Moreover, the increasing roughness with well distributed peaks and valleys leads to increasing CAs due to trapped air in surface cavities, while after some point the larger surface features lead to a decrement of the CAs that vary only slightly with further roughening. Although the apparent wetting transition with increasing surface roughness is not favored by the local Laplace pressure estimation, the energy of the system decreases with surface roughening, or equivalently increasing local surface slope, favoring energetically a Wenzel state. Under these conditions, the water droplet can spontaneously fill the surface cavities once the impregnation is initiated by the hydrophilic nature of the surface, in agreement with our experiments for significantly large local surface slopes ρ (>0.1) and large roughness exponents α ∼ 1.

Evaporation-Induced Wetting Transition of Nanodroplets on Nanopatterned Surfaces with Concentric Rings: Surface Geometry and Wettability Effects.

Droplet evaporation is widespread in natural and industrial application, and the rapid and efficient evaporation can significantly improve energy efficiency. However, the fundamental mechanism of contact line dynamics and the microscopic characteristics of evaporating nanodroplets are not well understood. Moreover, how to design a nanostructure surface to enhance nanodroplet evaporation remains unclear. Here, through molecular dynamics simulation, we investigated the evaporation dynamics of nanodroplets on various nanoring surfaces with different geometric parameters and wettability. By measuring the changes of contact radius and contact angle, the results showed that nanodroplets successively exhibit constant contact angle (CCA), constant contact radius (CCR), and mix mode during evaporation, and the evaporation-induced CCA-CCR transition, in essence, is a Cassie-Wenzel wetting transition, whose onset time is remarkably dependent on the surface roughness and wettability. We found that this evaporation-induced wetting transition is postponed on the surface with small nanostructure spacing and weak hydrophilicity, and the evaporation rate of nanodroplets improves accordingly. The dense and hydrophobic nanostructures can not only restrain the Cassie-Wenzel transition, but also enhance the evaporation rate of nanodroplets. Last, through the potential energy field analysis of nanoring substrates, we revealed that the Cassie-Wenzel wetting transition of nanodroplets is a process of molecule migration to low potential energy regions. Our work provides guidance for designing nanostructure surfaces to effectively control the droplet wetting state and enhance its mass transfer performance of phase change.

Wetting Transition at a Threshold Surfactant Concentration of Evaporating Sessile Droplets on a Patterned Surface.

Wetting transitions induced by varying the components in a solution of a drying droplet can lead to its evolving shape on a textured surface. It can provide new insights on liquid pattern control through manipulating droplet solutions. We show the pronounced transitions of wetting for surfactant solution droplets drying on a micropyramid-patterned surface. At low initial surfactant concentrations, the droplet maintains an octagonal shape until the end of drying. At intermediate initial surfactant concentrations, the early octagon spreads to a square, which later evolves to a stretched rectangle. At high initial surfactant concentrations, the droplet mainly exhibits the "octagon-to-square" transition, and the square shape is maintained until the end. The octagon-to-square transition occurs at similar temporal volume-averaged surfactant concentrations for the various initial surfactant concentrations. It results from the dependence of the surface energy change of spread over the micropyramid structure on the temporal volume-averaged surfactant concentration. At high initial surfactant concentrations, the accumulation of the surfactant near the contact line driven by outward flows could raise the local viscosity and enhance the pinning effect, leading to the great suppression of the "square-to-rectangle" transition.

Promoted reversible wettability transition by plasmonic effects at Ag/TiO2 heterointerface

We explore in this paper the photoinduced wetting/dewetting behaviors of silver/rutile heterointerfaces and demonstrate that silver nanoparticles can greatly increase the ultraviolet induced wetting and infrared induced dewetting transition rates. The results are interpreted based on the examination of the defect structures of the rutile surface before and after ultraviolet irradiation. The density functional theory calculation with the inclusion of the on-site Coulomb interaction reveals that the formation energy of an oxygen vacancy on the silver/rutile (110) interface is lower than that on the blank rutile (110) surface. We also discover that plasmonic nanostructures enable the partial wetting transition of the rutile (110) surface by irradiation with visible light at 473 nm and 532 nm. This work opens up a feasible route to the development of high-performance multifunctional materials via plasmonic nanostructures and defect engineering.

Explaining Evaporation-Triggered Wetting Transition Using Local Force Balance Model and Contact Line-Fraction

Understanding wettability and mechanisms of wetting transition are important for design and engineering of superhydrophobic surfaces. There have been numerous studies on the design and fabrication of superhydrophobic and omniphobic surfaces and on the wetting transition mechanisms triggered by liquid evaporation. However, there is a lack of a universal method to examine wetting transition on rough surfaces. Here, we introduce force zones across the droplet base and use a local force balance model to explain wetting transition on engineered nanoporous microstructures, utilizing a critical force per unit length (FPL) value. For the first time, we provide a universal scale using the concept of the critical FPL value which enables comparison of various superhydrophobic surfaces in terms of preventing wetting transition during liquid evaporation. In addition, we establish the concept of contact line-fraction theoretically and experimentally by relating it to area-fraction, which clarifies various arguments about the validity of the Cassie-Baxter equation. We use the contact line-fraction model to explain the droplet contact angles, liquid evaporation modes, and depinning mechanism during liquid evaporation. Finally, we develop a model relating a droplet curvature to conventional beam deflection, providing a framework for engineering pressure stable superhydrophobic surfaces.

Self-Cleaning of Hydrophobic Rough Surfaces by Coalescence-Induced Wetting Transition.

The superhydrophobic leaves of a lotus plant and other natural surfaces with self-cleaning function have been studied intensively for the development of artificial biomimetic surfaces. The surface roughness generated by hierarchical structures is a crucial property required for superhydrophobicity and self-cleaning. Here, we demonstrate a novel self-cleaning mechanism of textured surfaces attributed to a spontaneous coalescence-induced wetting transition. We focus on the wetting transition as it represents a new mechanism, which can explain why droplets on rough surfaces are able to change from the highly adhesive Wenzel state to the low adhesion Cassie-Baxter state and achieve self-cleaning. In particular, we perform many-body dissipative particle dynamics simulations of liquid droplets (with a diameter of 89 μm) sitting on mechanically textured substrates. We quantitatively investigate the wetting behavior of an isolated droplet as well as coalescence of droplets for both Cassie-Baxter and Wenzel states. Our simulation results reveal that droplets in the Cassie-Baxter state have much lower contact angle hysteresis and smaller hydrodynamic resistance than droplets in the Wenzel state. When small neighboring droplets coalesce into bigger ones on textured hydrophobic substrates, we observe a spontaneous wetting transition from the Wenzel state to the Cassie-Baxter state, which is powered by the surface energy released upon coalescence of the droplets. For superhydrophobic surfaces, the released surface energy may be sufficient to cause a jumping motion of droplets off the surface, in which case adding one more droplet to the coalescence may increase the jumping velocity by one order of magnitude. When multiple droplets are involved, we found that the spatial distribution of liquid components in the coalesced droplet can be controlled by properly designing the overall arrangement of droplets and the distance between them. These findings offer new insights for designing effective biomimetic self-cleaning surfaces by enhancing spontaneous Wenzel-to-Cassie wetting transitions, and additionally, for developing new noncontact methods to manipulate liquids inside the small droplets via multiple-droplet coalescence.

Direct synthesis of electrowettable nanostructured hybrid diamond

A low-voltage wetting transition from the Cassie–Baxter state to the Wenzel state was achieved by preparing a needle-like conductive diamond–graphite core–shell nanostructure.

Universality of active wetting transitions

Four on-lattice and six off-lattice models for active matter are studied numerically, showing that in contact with a wall, they display universal wetting transitions between three distinctive phases. The particles, which interact via exclusion volume only, move persistently and, depending on the model, change their direction either via tumble processes or rotational diffusion. When increasing the turning rate $\nu_T$, the systems transit from total wetting, to partial wetting and dewetted phases. In the first phase, a wetting film covers the wall, with increasing heights while decreasing $\nu_T$. The second phase is characterized by wetting droplets on the walls. And, finally, the walls dries with few particles in contact with it. These phases present two continuous non-equilibrium transitions. For the first transition, from partial to total wetting, the fraction of dry sites vanishes continuously when decreasing $\nu_T$, with a power law of exponent 1. And, for the second transition, an order parameter proportional to the excess mass in droplets decreases continuously with a power law of exponent 3 when approaching a critical value of $\nu_T$. The critical exponents are the same for all the models studied.

Forced dewetting in a capillary tube

Liquid films can be entrained when the dewetting velocity attains a threshold, and this dynamical wetting transition has been well studied in the situation of plane substrates. We investigate the forced dewetting in a capillary tube using diffuse-interface simulations and lubrication analysis, focusing on the onset of wetting transition and subsequent interface evolution. Results show that the meniscus remains stable when the displacing rate is below a threshold, beyond which film entrainment occurs and eventually leads to the formation of Taylor bubbles separated by liquid slugs, as has also been observed in the recent experiments of Zhao et al. (Phys. Rev. Lett., vol. 120, 2018, 084501). We derive an analytical solution of the critical capillary number, and demonstrate that the wetting transition is accompanied by a vanishing apparent contact angle and an abrupt drop of the contact-line velocity. Both the bubble and slug lengths are found to depend on the capillary number and the wettability of the wall. A theoretical formula for the bubble length is also proposed and compares favourably with numerical and experimental results.

Inducing wetting morphologies and increased reactivities of small Au clusters on doped oxide supports.

Au nanoparticles are promising catalysts for industrially important reactions. Their catalytic activity is known to depend on their charge state and morphology. Using density functional theory calculations, we have studied how the induced charge and dimensionality of small Au clusters can be tuned by doping the oxide support that they are deposited on. We have investigated Au n clusters of sizes n = 1, 2, 3, and 20 on Al-doped MgO and Mo-doped CaO. We show that substitutionally doping the oxide support with an electron donor changes the cluster morphology from an upright and/or three-dimensional geometry to a flat geometry. This structural wetting transition results in an increase in the negative charge induced on the cluster and a consequent lowering in the dissociation barrier for the O2 atoms adsorbed on the cluster. We find that the nature of Mo and Al dopants differs: only for the former is it true that the charge state of the dopant atoms depends on the presence or absence of Au nanoparticles and their size.

Distinct Photoinduced Wetting Transitions of Differently Doped GaN: An Indication of Collective Behavior

Water is a unique molecule made up of only three atoms. At room temperature, water molecules are linked together by hydrogen bonds; this arrangement is responsible for many unusual properties distinct from those of other liquids. We explore here the wetting behavior of water on gallium nitride (GaN) and reveal that UV irradiation promotes a wetting transition on n-type GaN but not on p-type GaN. A comprehensive investigation reveals that the oxygen impurities present in n-type GaN are responsible for the wetting transition. The photoluminescence (PL) and humidity-dependent behaviors also support the proposed explanation. In particular, an individual oxygen atom could affect ∼105 water molecules through the extended hydrogen bond network, which provides direct evidence that wettability by water molecules is a collective behavior.

Robust multifunctional superhydrophobic fabric with UV induced reversible wettability, photocatalytic self-cleaning property, and oil-water separation via thiol-ene click chemistry

Robust superhydrophobic fabric (SF) was successfully achieved by surface modification with anatase TiO2 sol and mercapto silanes, subsequently followed by hydrophobization with perfluorooctyl methacrylate (PFOMA) via thiol-ene click reaction. The resultant SFs not only exhibited excellent superhydrophobicity with a water contact angle of 157.7° and a water shedding angle of 4°, but also showed the considerable photocatalytic activity by the degradation of oil red O under UV irradiation. Moreover, the SFs exhibited a wetting transition from a superhydrophobic state to a superhydrophilic state by the alternation of UV exposure and thermal treatment. The SF could be utilized to separate oil-water mixture owing to its superhydrophobicity and superoleophilicity. Importantly, the SFs were tolerant towards strong acidic or alkaline environments, corrosive organic solvents, laundering, and mechanical abrasion. In addition, this facile preparation process could impart the superhydrophobicity to other substrates (such as PET fabrics, nonwoven fabrics, and sponges). Therefore, superhydrophobic materials with multi-functionality of UV induced reversible wettability, photocatalytic self-cleaning property, and oil-water separation are regarded as promising candidates for a wide range of applications.

Spontaneous wetting transition of droplet coalescence on immersed micropillared surfaces

• The possibility of spontaneous Wenzel-to-Cassie transition is verified by an energy analysis. • The wetting transition of droplets during coalescence depends on small interpillar spacing and wettability. • The dynamical contact angle hysteresis of droplets can be decreased via spontaneous Wenzel-to-Cassie transition. • Robust superhydrophobicity and efficient droplet transportation rely on surface geometry and wettability. The spontaneous wetting transition of droplet coalescence on immersed micropillared surfaces was investigated by using many-body dissipative particle dynamics (MDPD). The simulation results show that wetting states are influenced by both the interpillar spacing and intrinsic contact angle of surfaces during droplet coalescence. The overestimation of the contact angle by the Cassie equation was modified by introducing a penetration factor, and the energy analysis suggests that the spontaneous Wenzel-to-Cassie transition could be predicted from critical contact angles. For surfaces with small interpillar spacing and large intrinsic contact angle, the spontaneous Wenzel-to-Cassie transition occurs during droplet coalescence, irrespective of the size of the immersed area. The contact angle hysteresis was also studied, in which the droplet was driven on immersed micropillared surfaces. It was found that reducing interpillar spacing is a feasible option to maintain robust superhydrophobicity, and that it is possible to realize the spontaneous Wenzel-to-Cassie transition. Otherwise, larger interpillar spacing and larger size of the immersed region will result in longer contact lines pinned by pillars if the spontaneous Wenzel-to-Cassie transition fails to occur. Meanwhile, it leads to the weakening of droplet mobility during transportation. The outcomes demonstrate that appropriate geometrical structures and wettability properties should be taken into consideration in order to achieve robust superhydrophobicity and high efficiency of the droplet transportation.