Showing papers on "Wetting transition published in 2019"
TL;DR: It is concluded that tissue spreading constitutes a prominent example of active wetting—a novel physical scenario that may explain morphological transitions during tissue morphogenesis and tumour progression.
Abstract: Development, regeneration and cancer involve drastic transitions in tissue morphology. In analogy with the behavior of inert fluids, some of these transitions have been interpreted as wetting transitions. The validity and scope of this analogy are unclear, however, because the active cellular forces that drive tissue wetting have been neither measured nor theoretically accounted for. Here we show that the transition between 2D epithelial monolayers and 3D spheroidal aggregates can be understood as an active wetting transition whose physics differs fundamentally from that of passive wetting phenomena. By combining an active polar fluid model with measurements of physical forces as a function of tissue size, contractility, cell-cell and cell-substrate adhesion, and substrate stiffness, we show that the wetting transition results from the competition between traction forces and contractile intercellular stresses. This competition defines a new intrinsic lengthscale that gives rise to a critical size for the wetting transition in tissues, a striking feature that has no counterpart in classical wetting. Finally, we show that active shape fluctuations are dynamically amplified during tissue dewetting. Overall, we conclude that tissue spreading constitutes a prominent example of active wetting --- a novel physical scenario that may explain morphological transitions during tissue morphogenesis and tumor progression.
102 citations
TL;DR: In this article, the transition between two-dimensional epithelial monolayers and three-dimensional spheroidal aggregates can be understood as an active wetting transition whose physics differs fundamentally from that of passive wetting phenomena.
Abstract: Development, regeneration and cancer involve drastic transitions in tissue morphology. In analogy with the behavior of inert fluids, some of these transitions have been interpreted as wetting transitions. The validity and scope of this analogy are unclear, however, because the active cellular forces that drive tissue wetting have been neither measured nor theoretically accounted for. Here we show that the transition between two-dimensional epithelial monolayers and three-dimensional spheroidal aggregates can be understood as an active wetting transition whose physics differs fundamentally from that of passive wetting phenomena. By combining an active polar fluid model with measurements of physical forces as a function of tissue size, contractility, cell-cell and cell-substrate adhesion, and substrate stiffness, we show that the wetting transition results from the competition between traction forces and contractile intercellular stresses. This competition defines a new intrinsic lengthscale that gives rise to a critical size for the wetting transition in tissues, a striking feature that has no counterpart in classical wetting. Finally, we show that active shape fluctuations are dynamically amplified during tissue dewetting. Overall, we conclude that tissue spreading constitutes a prominent example of active wetting - a novel physical scenario that may explain morphological transitions during tissue morphogenesis and tumor progression.
83 citations
TL;DR: A novel self-cleaning mechanism of textured surfaces attributed to a spontaneous coalescence-induced wetting transition is demonstrated, 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.
Abstract: 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...
81 citations
TL;DR: It is shown that above a critical receding contact line speed during drying, a previously not observed wetting transition occurs and is elucidated with high resolution force field measurements thereby determining its dependence on substrate properties.
Abstract: Droplet interactions with compliant materials are familiar, but surprisingly complex processes of importance to the manufacturing, chemical, and garment industries. Despite progress—previous research indicates that mesoscopic substrate deformations can enhance droplet drying or slow down spreading dynamics—our understanding of how the intertwined effects of transient wetting phenomena and substrate deformation affect drying remains incomplete. Here we show that above a critical receding contact line speed during drying, a previously not observed wetting transition occurs. We employ 4D confocal reference-free traction force microscopy (cTFM) to quantify the transient displacement and stress fields with the needed resolution, revealing high and asymmetric local substrate deformations leading to contact line pinning, illustrating a rate-dependent wettability on viscoelastic solids. Our study has significance for understanding the liquid removal mechanism on compliant substrates and for the associated surface design considerations. The developed methodology paves the way to study complex dynamic compliant substrate phenomena. It has been shown previously that substrate viscoelasticity affects surface wettability. Here the authors observe a wetting transition during drying of droplets on such substrates and elucidate it with high resolution force field measurements thereby determining its dependence on substrate properties.
50 citations
TL;DR: The objective of this work was to explore alternatives to predict SLL wettability, and found the extended EQS (e-EQS) method produced reasonable predictions of γo-aq·cosθO for all the available experimental and literature data.
42 citations
TL;DR: Molecular dynamics simulations are employed to understand the mechanism behind the CB-to-W transition for a nanoscale water film placed on a surface decorated with a single nanogroove when an external electric field is applied and it is found that the maximum of free energy always occurs after the film touches the groove bottom.
Abstract: When droplets are placed on hydrophobic textured surfaces, different wetting states Cassie–Baxter (CB) state or Wenzel (W) state may occur depending on materials and structures of surfaces, types a...
41 citations
TL;DR: This work introduces force zones across the droplet base and uses a local force balance model to explain wetting transition on engineered nanoporous microstructures, utilizing a critical force per unit length (FPL) value, and establishes 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.
Abstract: 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.
35 citations
TL;DR: In this paper, a three-dimensional free energy (FE) model was developed to investigate the effect of transition between various states of wetting on adhesion of a droplet onto a surface.
Abstract: Transitions between various states of wetting can have a profound effect on adhesion of a droplet onto a surface. In this study, a three-dimensional free energy (FE) model was developed to investig...
28 citations
TL;DR: The role of substrate stiffness in tissue spreading is studied by means of an active polar fluid model and it is shown that the tissue can wet the substrate on the stiffer side while dewetting from the softer side.
Abstract: Living tissues undergo wetting transitions: On a surface, they can either form a dropletlike cell aggregate or spread as a monolayer of migrating cells. Tissue wetting depends not only on the chemical but also on the mechanical properties of the substrate. Here, we study the role of substrate stiffness in tissue spreading, which we describe by means of an active polar fluid model. Taking into account that cells exert larger active traction forces on stiffer substrates, we predict a tissue wetting transition at a critical substrate stiffness that decreases with tissue size. On substrates with a stiffness gradient, we find that the tissue spreads faster on the stiffer side. Furthermore, we show that the tissue can wet the substrate on the stiffer side while dewetting from the softer side. We also show that, by means of viscous forces transmitted across the tissue, the stiffer-side interface can transiently drag the softer-side interface toward increasing stiffness, against its spreading tendency. These two ...
28 citations
TL;DR: Wang et al. as mentioned in this paper reported a simple and novel approach to achieve reversible and switchable wettability through the vibration of substrate, which showed that with the increase of the vibration frequency (f), the wetting state of the liquid metal gradually transform into wetting/dewetting mixed state, and finally becomes complete dewetting state.
24 citations
TL;DR: This work provides guidance for designing nanostructure surfaces to effectively control the droplet wetting state and enhance its mass transfer performance of phase change.
Abstract: 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-in...
TL;DR: In this paper, the static and dynamic wetting behaviors of copper thin films deposited by DC magnetron sputtering were investigated from the analysis of height topography images acquired by atomic force microscopy, and 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.
Abstract: 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.
TL;DR: A repeated Cassie-Wenzel-Cassie wetting state transition is studied at the microscale when a drop impacts a two-tier superhydrophobic surface to facilitate the design of functional superrepellent materials.
Abstract: Superrepellency is a favorable nonwetting scenario featuring a dramatic reduction of the solid/liquid contact area. The robustness of superhydrophobicity plays a central role in self-cleaning and anti-icing. Drop impacts happen ubiquitously in natural environments and often cause a notable extension of the solid/liquid contact area. This is associated with an enhanced affinity between water and the microtextures and therefore leads to irreversible breakdowns in the superhydrophobicity. This problem remains a major challenge and limits the practical applications of superrepellent materials. In order to find a solution, in this paper, a repeated Cassie-Wenzel-Cassie wetting state transition is studied at the microscale when a drop impacts a two-tier superhydrophobic surface. In this case, the surface is completely dry without any liquid residue after the drop rebounds. The present results exhibit a striking contrast to the conventional perspective. The influence of geometrical parameters of the textured surface on the spreading and retracting behaviors of the impact drops is quantified, as well as the time-dependence scaling laws. From a practical point of view, it is demonstrated that the self-cleaning and dropwise condensation may significantly benefit from this repeated wetting transition. Dirt particles or small droplets in deep textures are able to be taken away so that the functionality and the robustness of the superhydrophobicity may be significantly strengthened. The results reported in this study facilitate the design of functional superrepellent materials.
TL;DR: In this article, the authors have achieved superhydrophobicity in plain copper using picosecond laser treatment and silane coating through vaporization technique and the effects of the same in condensation heat transfer has been studied.
TL;DR: In this article, a molecular dynamics method was used to evaluate the contact angle and potential energy of water droplet on nanostructured surfaces with different morphologies and geometries.
TL;DR: A pyramid shaped Cu2S film with hierarchical micro/nanostructures is formed on a commercial copper mesh, which enables reversible wetting transition between superhydrophilicity to superHydrophobicity and renders it as a potentially useful mesh material for switchable surfaces.
Abstract: 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.
TL;DR: In this article, the authors investigated the dynamic stability of retained air layers on three different superhydrophobic surfaces against repeated immersion and motion through various viscous liquids, and the directionality of the Cassie-to-Wenzel wetting transition on air-retaining surfaces was demonstrated.
Abstract: Development of superhydrophobic surfaces is of great interest for drag-reducing applications as air layers retained underwater greatly reduce fluidic drag. However, liquid flow over these surfaces can result in the collapse of the lubricating air layer. Here, we investigate the dynamic stability of retained air layers on three different superhydrophobic surfaces against repeated immersion and motion through various viscous liquids. The three surfaces investigated are a highly ordered polytetrafluoroethylene micropillar array, a two-level hierarchical random polycarbonate nanofur, and a double-scale hierarchical Teflon AF wrinkled surface. Both repeated immersions and contamination by viscous liquids accelerated the rate of plastron decay on the pillar array and the nanofur, while the Teflon wrinkles remained dry. Five topographical features were identified as correlated to a dynamically stable retained air layer, and a relation between these stability-enhancing parameters and the drag-reducing capabilities is found. Furthermore, resistance of superhydrophobic surfaces against contamination is studied and the directionality of the Cassie-to-Wenzel wetting transition on air-retaining surfaces is demonstrated. Together, an understanding of these properties allows for the rational design of new superhydrophobic surfaces fit for application.Development of superhydrophobic surfaces is of great interest for drag-reducing applications as air layers retained underwater greatly reduce fluidic drag. However, liquid flow over these surfaces can result in the collapse of the lubricating air layer. Here, we investigate the dynamic stability of retained air layers on three different superhydrophobic surfaces against repeated immersion and motion through various viscous liquids. The three surfaces investigated are a highly ordered polytetrafluoroethylene micropillar array, a two-level hierarchical random polycarbonate nanofur, and a double-scale hierarchical Teflon AF wrinkled surface. Both repeated immersions and contamination by viscous liquids accelerated the rate of plastron decay on the pillar array and the nanofur, while the Teflon wrinkles remained dry. Five topographical features were identified as correlated to a dynamically stable retained air layer, and a relation between these stability-enhancing parameters and the drag-reducing capabilitie...
TL;DR: It was demonstrated that the wetting transition from a metastable Cassie-Baxter state to a Wenzel state as well as the penetration of a droplet into the surface crevices occur at the smaller local surface slopes for the higher surface energy material.
Abstract: We studied the wetting behavior of silver and copper thin films versus their kinetic roughening upon deposition at room temperature on glass substrates. Time-dependent height-height correlation functions were extracted from atomic force microscopy images, and they demonstrated a nonstationary growth front of the film roughness associated with a temporal evolution of the local surface slope. As a result, we tried to correlate the roughness statistical properties such as the root-mean-square (rms) roughness $\ensuremath{\sigma}$, the correlation length $\ensuremath{\xi}$, and the local surface slope $(\ensuremath{\rho}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}\ensuremath{\sigma}/\ensuremath{\xi})$ with the wetting behavior of the films' surfaces. The contact angle behavior was also studied by analyzing the variation of the energy of the system with water penetrating into the surface cavities, and the associated Laplace pressure induced by the local surface curvature. Hence, it was demonstrated that the wetting transition from a metastable Cassie-Baxter state to a Wenzel state as well as the penetration of a droplet into the surface crevices occur at the smaller local surface slopes for the higher surface energy material.
TL;DR: In this article, the formation of hierarchical copper fractals on stainless steel meshes and their superhydrophobicity without any physical or chemical modification were studied, and the improvement of super hydrogenobicity of surfaces during storing in a glass bottle for a long time (> one year) is reported.
TL;DR: This study provides a basis for a better understanding of adsorption in a range of systems because ammonia is an intermediate between simple molecules and strongly associating fluids, such as water.
Abstract: Simulations of ammonia adsorption on graphite were carried out over a range of temperatures to investigate the transition from nonwetting to wetting. The process is governed by a subtle interplay between the various interactions in the system and the temperature. At temperatures below the bulk triple point, the system is nonwetting; above the triple point, we observed continuous wetting, preceded by a prewetting region in which the so-called thin-to-thick film transition occurs. This system serves as an excellent example of wetting/nonwetting behavior in an associating fluid as a function of temperature because the heat of sublimation (or condensation) is greater than the isosteric heat of adsorption at zero loading. The nonwetting-to-wetting transition (NW/W) is also strongly affected by the adsorbate-adsorbate interaction, which becomes important when this contribution to the isosteric heat is of a similar magnitude to the heat of condensation. An appropriate indicator of a NW/W transition at a given loading is therefore the difference between the isosteric heat and the heat of sublimation (or condensation). Our simulation results show the "thin-to-thick" film transition in the temperature range between 195 and 240 K, which has not been previously explained. Above 240 K, continuous wetting occurs. This study provides a basis for a better understanding of adsorption in a range of systems because ammonia is an intermediate between simple molecules, such as argon, and strongly associating fluids, such as water.
TL;DR: In this article, a contact angle hypothesis traps (CAHTs)-assisted optical droplet-based reaction/analysis platform was developed, where the triple-phase contact line extension caused by the light-heating induced wetting transition was induced to facilitate the rapid mixing and detection.
Abstract: Coalescence and mixing of the complex chemical reagents on the droplet-based open microfluidics are considered to be critical steps for the sample processing and the follow-up reactions and chromogenic/fluorescence analyses. Recently, using the light to actuate the droplets coalescence has been demonstrated to be able to create the flexible, precise and high-throughput optical droplet-based reaction/analysis platforms. Particularly, the photothermal effect, which is one of the fluid-light interactions, is promising in the droplets coalescence by the light-induced phase change. In present study, a contact angle hypothesis traps (CAHTs)-assisted optical droplet-based reaction/analysis platform was developed. With the triple-phase contact line extension caused by the light-heating induced wetting transition, the coalescence of two neighboring droplets on the CAHTs was induced to facilitate the rapid mixing and detection. It was also confirmed that with the off-center light heating, the orientable extension of the irradiated droplet was realized, which could increase the critical CAHTs gap required for the occurrence of the droplets coalescence. With the developed platform, an ion detection by processing a chromogenic reaction was presented as a proof of concept. It is believed that this concept has the significant potential in many applications, including biomedicine, pharmacy, clinical diagnosis and chemosynthesis.
TL;DR: In this article, the authors 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.
Abstract: 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.
TL;DR: Results indicated that the reduction of actual contact area between the solid and liquid phases restricted ice nucleus formation, and this problem in the development of anti-icing surfaces needed for applications, such as aircraft wings and infrastructures.
Abstract: There have been conflicting reports as to whether surface wettability is effective in the freezing delay enhancement of attached water droplets. It is an important problem in the development of anti-icing surfaces needed for applications, such as aircraft wings and infrastructures. Here, we prepared precooled ambient conditions and surfaces which included smooth, microstructured, and two nanostructured surfaces with hydrophobic coatings to create an environment closer to the actual environment and to avoid frost formation, which enhances wetting transition and nucleation. Static and dynamic wetting characteristics of each surface were investigated as the fundamental properties and the freezing behavior of precooled water droplets were observed. A distinct elongation of the freezing delay time was observed for droplets on nanostructured surfaces which have static contact angles >150°, in contrast to those on smooth and microstructured surfaces. However, the difference in droplet adhesion induced by nanostructures showed a negligible impact on freezing delay. These results indicated that the reduction of the actual contact area between the solid and liquid phases restricted ice nucleus formation.
TL;DR: This work explores how addition of ethanol, which reduces the surface tension, influences the wetting properties of superhydrophobic and smooth hydrophobic surfaces and shows that the receding contact angle on the superhydrophic surface is of paramount importance for describing the interaction forces.
Abstract: Superhydrophobic surfaces in the Cassie–Baxter wetting state retain an air layer at the surface which prevents liquid water from reaching into the porous surface structure. In this work we explore how addition of ethanol, which reduces the surface tension, influences the wetting properties of superhydrophobic and smooth hydrophobic surfaces. Wetting properties are measured by dynamic contact angles, and the air layer at the superhydrophobic surface is visualized by laser scanning confocal microscopy. Colloidal probe atomic force microscopy measurements between a hydrophobic microsphere and the macroscopic surfaces showed that the presence of ethanol strongly affects the interaction forces. When the macroscopic surface is superhydrophobic, attractive forces extending up to a few micrometers are observed on retraction in water and in 20 vol % ethanol, signifying the presence of a large and growing gas capillary. Submicrometer attractive forces are observed between the probe particle and a smooth hydrophobic...
TL;DR: In this article, Terlain et al. showed that CO adsorbed on graphite is nonwetting below the bulk triple-point temperature (T = 215 K) but wetting above this temperature and differs from noble gases, which form an adorbed film at all temperatures.
Abstract: Extensive simulations of CO adsorption on graphite were carried out over a range of temperature to investigate the effects of temperature on the nonwetting/wetting transition. CO adsorbed on graphite is nonwetting below the bulk triple-point temperature (T = 215 K) but wetting above this temperature and differs from noble gases, which form an adsorbed film at all temperatures. Our simulation results confirm that the CO /graphite system is nonwetting at temperatures below 90 K, as reported experimentally by Terlain, A.; et al. [Phase Diagrams of Films of Linear Molecules with Large Quadrupole Moments (CO , N O, C N ) Adsorbed on Graphite. Surf. Sci. 1983, 125, 304-311] and by Morishige, K. [The Structure of a Monolayer Film of Carbon Dioxide Adsorbed on Graphite. Mol. Phys. 1993, 78, 1203-1209]. For temperatures between the wetting temperature of 90 K and the bulk triple point, incomplete wetting occurs, and the adsorbed film has a finite thickness at the bulk coexistence pressure. This is a consequence of the fact that the isosteric heat of adsorption at zero loading, (0), is lower than the heat of sublimation of bulk CO . On the other hand, at temperatures greater than T , the adsorption isotherm exhibits continuous wetting as the pressure approaches the bulk coexistence pressure because qs(0) is greater than the bulk heat of condensation. We support these findings with detailed analysis of the molecular configurations along the canonical isotherms, the isosteric heat as a function of loading, and the local orientation-density distributions. The two-dimensional critical temperature of the first adsorbate layer was determined as 130 K, in excellent agreement with 127.5 K estimated experimentally by Terlain, A.; et al. [Phase Diagrams of Films of Linear Molecules with Large Quadrupole Moments (CO , N O, C N ) Adsorbed on Graphite. Surf. Sci. 1983, 125, 304-311]. In the final section, we present a parametric map showing regions of nonwetting, wetting, and incomplete wetting for CO adsorption on the surface of adsorbents of different strengths.
TL;DR: This study develops a route to fabricating optically active and controllable microfluidic devices that support rapid wetting transitions for water droplet manipulation through illumination by near infrared light.
Abstract: 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 (0001) 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.
TL;DR: 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.
Abstract: 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.
TL;DR: Complete wetting of solid walls that are patterned by parallel nanogrooves that implies that nanomodification of substrate surfaces can always change their wetting character from hydrophilic into hydrophobic, in direct contrast to the macroscopic Wenzel law.
Abstract: We study complete wetting of solid walls that are patterned by parallel nanogrooves of depth D and width L with a periodicity of 2L. The wall is formed of a material which interacts with the fluid via a long-range potential and exhibits first-order wetting transition at temperature T_{w}, should the wall be planar. Using a nonlocal density functional theory we show that at a fixed temperature T>T_{w} the process of complete wetting depends sensitively on two microscopic length scales L_{c}^{+} and L_{c}^{-}. If the corrugation parameter L is greater than L_{c}^{+}, the process is continuous similar to complete wetting on a planar wall. For L_{c}^{-}
TL;DR: The pronounced transitions of wetting for surfactant solution droplets drying on a micropyramid-patterned surface shows the octagon-to-square transition, which results from the dependence of the surface energy change of spread over the micropyramids structure on the temporal volume-averaged surfactants concentration.
Abstract: 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.