Wetting Phenomena Research Articles

Suppressing artificial equilibrium states caused by spurious currents in droplet spreading simulations with dynamic contact angle model

Accurate methods for numerical simulation of dynamic wetting and spreading phenomena are a valuable tool to support the advancement of related technological processes such as inkjet-printing. Here, it is demonstrated that numerical methods employing dynamic contact angle models are prone to artificial equilibrium states caused by spurious (parasitic) currents. The capability of different approaches in reducing spurious currents for sessile and spreading droplets with low equilibrium contact angle is evaluated. To minimise the influence of spurious currents on dynamic contact angle models, a smoothing step in the evaluation of the contact line velocity is introduced in this paper. The benefit and performance of this new approach is demonstrated by algebraic volume-of-fluid simulations of spreading and receding droplets with the Kistler dynamic contact angle model.

Wetting transitions in droplet drying on soft materials

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.

Nanostructured and oriented metal-organic framework films enabling extreme surface wetting properties.

We report on the synthesis of highly oriented and nanostructured metal–organic framework (MOF) films featuring extreme surface wetting properties. The Ni- and Co- derivatives of the metal–catecholate series (M-CAT-1) were synthesized as highly crystalline bulk materials and thin films. Oriented pillar-like nanostructured M-CAT-1 films exhibiting pronounced needle-like morphology on gold substrates were established by incorporating a crystallization promoter into the film synthesis. These nanostructured M-CAT-1 MOF films feature extreme wetting phenomena, specifically superhydrophilic and underwater superoleophobic properties with water and underwater oil-contact angles of 0° and up to 174°, respectively. The self-cleaning capability of the nanostructured, needle-like M-CAT-1 films was illustrated by measuring time-dependent oil droplet rolling-off a tilted surface. The deposition of the nanostructured Ni-CAT-1 film on a large glass substrate allowed for the realization of an efficient, transparent, antifog coating, enabling a clear view even at extreme temperature gaps up to ≈120 °C. This work illustrates the strong link between MOF film morphology and surface properties based on these framework materials.

Probing the interaction mechanism between oil droplets with asphaltenes and solid surfaces using AFM

Wetting phenomena of oil/water/solid systems are fundamentally governed by the stability of confined water film and interaction mechanism between oil droplet and solid surface in water. Herein, droplet probe AFM was used to quantify the surface forces of model oil droplets including toluene and heptol in presence of interfacial asphaltenes interacting with mica surfaces of varied hydrophobicity in different water environments. It was found that adsorption of asphalenes at oil/water interface could result in the enhanced electrical double layer (EDL) repulsion at low salinity while strengthen the steric repulsion at high salinity, both of which contributed to a more stable water film between oil droplets and mica surfaces, inhibiting oil droplet attachment. Addition of heptane strengthened the repulsive EDL force and steric hindrance since more asphaltenes were adsorbed onto the interface. For hydrophobized mica surface, the attractive hydrophobic interaction could overcome steric hindrance due to interfacially adsorbed asphaltenes, thereby inducing strong attachment and adhesion of oil droplet. Our results demonstrate the nanomechanical mechanism underlying the interactions between oil droplets and solid surfaces in presence of interfacial materials, which can further explain the wetting of oil/water/solid systems in many engineering applications such as oil fouling and corrosion, and oil/water separation.

Hybrid superhydrophilic\u2013superhydrophobic micro/nanostructures fabricated by femtosecond laser-induced forward transfer for sub-femtomolar Raman detection

Raman spectroscopy plays a crucial role in biochemical analysis. Recently, superhydrophobic surface-enhanced Raman scattering (SERS) substrates have enhanced detection limits by concentrating target molecules into small areas. However, due to the wet transition phenomenon, further reduction of the droplet contact area is prevented, and the detection limit is restricted. This paper proposes a simple method involving femtosecond laser-induced forward transfer for preparing a hybrid superhydrophilic–superhydrophobic SERS (HS-SERS) substrate by introducing a superhydrophilic pattern to promote the target molecules to concentrate on it for ultratrace detection. Furthermore, the HS-SERS substrate is heated to promote a smaller concentrated area. The water vapor film formed by the contact of the solution with the substrate overcomes droplet collapse, and the target molecules are completely concentrated into the superhydrophilic region without loss during evaporation. Finally, the concentrated region is successfully reduced, and the detection limit is enhanced. The HS-SERS substrate achieved a final contact area of 0.013 mm2, a 12.1-fold decrease from the unheated case. The reduction of the contact area led to a detection limit concentration as low as 10−16 M for a Rhodamine 6G solution. In addition, the HS-SERS substrate accurately controlled the size of the concentrated areas through the superhydrophilic pattern, which can be attributed to the favorable repeatability of the droplet concentration results. In addition, the preparation method is flexible and has the potential for fluid mixing, fluid transport, and biochemical sensors, etc.

Avoiding Starvation in Tribocontact Through Active Lubricant Transport in Laser Textured Surfaces

Laser texturing is a viable tool to enhance the tribological performance of surfaces. Especially textures created with Direct Laser Interference Patterning (DLIP) show outstanding improvement in terms of reduction of coefficient of friction (COF) as well as the extension of oil film lifetime. However, since DLIP textures have a limited depth, they can be quickly damaged, especially within the tribocontact area, where wear occurs. This study aims at elucidating the fluid dynamical behavior of the lubricant in the surroundings of the tribocontact where channel-like surface textures are left after the abrasion wear inside the tribocontact area. In a first step, numerical investigations of lubricant wetting phenomena are performed applying OpenFOAM®. The results show that narrow channels (width of 10 μ m ) allow higher spreading than wide channels (width of 30 μ m ). In a second step, fluid transport inside DLIP textures is investigated experimentally. The results show an anisotropic spreading with the spreading velocity dependent on the period and depth of the laser textures. A mechanism is introduced for how lubricant can be transported out of the channels into the tribocontact. The main conclusion of this study is that active lubricant transport in laser textured surfaces can avoid starvation in the tribocontact.

Molecular Dynamics of Solid State Spreading in a Pb (Nanoparticle)/Cu (Substrate) System

The phenomenon of solid state wetting is studied via molecular dynamics (MD) in a Pb (nanoparticle)/Cu (substrate) system. It is found to be associated with capillarity-induced surface diffusion (CISD). This conclusion is confirmed by the coincidence between the MD-estimated characteristic nanoparticle spreading time and the evaluations made in the context of CISD.

Revealing How Topography of Surface Microstructures Alters Capillary Spreading

Wetting phenomena, i.e. the spreading of a liquid over a dry solid surface, are important for understanding how plants and insects imbibe water and moisture and for miniaturization in chemistry and biotechnology, among other examples. They pose fundamental challenges and possibilities, especially in dynamic situations. The surface chemistry and micro-scale roughness may determine the macroscopic spreading flow. The question here is how dynamic wetting depends on the topography of the substrate, i.e. the actual geometry of the roughness elements. To this end, we have formulated a toy model that accounts for the roughness shape, which is tested against a series of spreading experiments made on asymmetric sawtooth surface structures. The spreading speed in different directions relative to the surface pattern is found to be well described by the toy model. The toy model also shows the mechanism by which the shape of the roughness together with the line friction determines the observed slowing down of the spreading.

Analytical Modeling of Two-Phase Water Transport in a PEMFC

While significant progress has been achieved in PEMFC modeling, simulating liquid water formation and transportation in a PEMFC remains a challenge. Under wet operating conditions, liquid water may condense in the channel, gas diffusion layer, or electrode, which introduces significant complexities in simulating fuel cell performance. There have been several approaches to model two-phase water transport using both macroscopic and microscopic methods. Two-phase macroscopic models are mostly based on volumetric averaged method and solve the governing equation using CFD techniques, which require intensive computational resources. In recent years, microscopic approaches based on Lattice Boltzmann Method (LBM) and pore-network approach have been applied to detailed morphological geometric domain of gas diffusion layer. While microscopic approach provide useful information, they cannot be coupled with a full fuel cell model. Therefore, constructing an effective analytical two-phase model is crucial in developing materials and design to optimizing fuel cell performance. In this work, we present an empirical-based, steady-state, 1-D, non-isothermal, and two-phase PEMFC model that includes full cell geometry and water balance. Limiting current experiments based on Caulk and Baker [1] is employed to study both dry and wet oxygen transport phenomena with wet-proofed Toray-H-060 carbon fiber paper with microporous layer. Specifically, the ratio of tortuosity and porosity under wet and dry conditions are characterized by the limiting current method and incorporated into the model. The two-phase model uses tendril length to represent the liquid water saturation and penetration in the diffusion layer. Figure 1 shows that the newly constructed two-phase model can capture the total transport resistance as a function of limiting current density in the dry, transition, and wet regions well for operating conditions at 300 kPa, 70°C, 80% and 90% relative humidity. This two-phase model also includes a two-phase electrode model, which correlates electrode relative humidity to available ECSA for fuel cell performance simulation. The details of the model and prediction results will be presented at the conference. Referece: 1. Caulk, D.A. and Baker, D.R., 2010. Heat and water transport in hydrophobic diffusion media of PEM fuel cells. Journal of The Electrochemical Society, 157(8), pp.B1237-B1244. Figure 1

Effect of the nonaxisymmetric endwall on wet steam condensation flow in a stator cascade

AbstractSteam turbines are critical pieces of power equipment in the electric power industry, and the study of wet steam condensation flow is important for improving the efficiency and safety of steam turbines. Wet steam nucleation, which is the main reason for thermodynamic losses, usually occurs downstream of the stator cascade. The mechanism of secondary flow loss control by the nonaxisymmetric endwall is the change of pressure distribution in the cascade channel. This mechanism also affects the wet steam condensation phenomenon. The positive/cosine function was proposed for endwall modification in the nucleation stage of a steam turbine. The design method presented in this paper can be used to produce endwall protrusions with different heights at different axial chords. To analyze the influence of the endwall protrusion design parameters on the condensation flow, nine endwall protrusion models were set up at varied axial positions. In comparison with the original cascade, the total pressure loss coefficient and outlet wetness were found to be ideal when the endwall protrusion maximum height was located at the 50% axial chord length of the stator and the protrusion was 3% of the blade height. The ideal incidence angle of this modified cascade was from −2° to + 10°.

Rationally Designed Nanostructure Features on Superhydrophobic Surfaces for Enhancing Self-Propelling Dynamics of Condensed Droplets

The self-propelling ability toward achieving more efficient dropwise condensation intensively appeals to researchers due to its academic significance to explain some basic wetting phenomena. Herein...

Physics of pre-wetted, lubricated and impregnated surfaces: a review.

Wetting phenomena occurring on pre-wetted flat and rough solid surfaces are reviewed. The wetting of lubricated flat surfaces is strongly correlated with the tribological properties of a solid/lubricant pair. The phenomena taking place on micro- and nano-rough oil-impregnated surfaces have attracted the attention of the scientific community due to their numerous promising applications as omniphobic, self-healing, anti-icing and anti-bacterial interfaces. On the other hand, these phenomena are rich in their physical content. The effects observed on natural and artificial, bioinspired oil-impregnated surfaces are discussed, including electrowetting of oil-impregnated surfaces, enabling low-voltage reversible control of the droplet shape. This article is part of the theme issue 'Bioinspired materials and surfaces for green science and technology'.

Wetting phenomena with ALE interface tracking

AbstractAn arbitrary Lagrangian Eulerian (ALE) interface tracking method is applied to multiphase flows involving wetting phenomena. The method is implemented in OpenFOAM and validated for the static shape of a drop deformed by gravity influence. Results for a constant contact angle model are compared to approximate analytic solutions for the limiting cases of vanishing or infinite Eötvös numbers. Results between the two limiting cases are compared to numerical results from literature.

Wetting, spreading and corrosion behavior of molten slag on dense MgO and MgO-C refractory

The wetting and spreading phenomena of molten slag were observed in situ on dense MgO and MgO-C refractory substrates. Parameters associated with wetting and spreading of molten slag, such as the contact angle, droplet height, diameter, and volume, were measured and calculated. The microstructure and chemical composition of the corroded dense MgO and MgO-C refractory were studied using SEM and EDS analysis. The droplet volume of molten slag on dense MgO declined faster than that on MgO-C refractory during the first 90 s of the testing period, whereas the droplet volume exhibited little difference across the two cases after 150 s. Molten slag penetrated the dense MgO and MgO-C refractory through grain boundaries and the channels which were formed by the dissolution of MgO. Besides, the slag also penetrated into the MgO-C refractory through the pores and channels formed by the redox reaction between slag and carbon, and a reaction product (Fe) was found at the interface. The dissolution of MgO and redox reactions changed the wetting process and increased corrosion of the MgO-C refractory.

Active wetting of epithelial tissues.

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.

Simple Experiment to Determine Surfactant Critical Micelle Concentrations Using Contact-Angle Measurements

A simple and reliable didactic laboratory has been developed to illustrate fundamental concepts of hydrophilicity, hydrophobicity, wetting, and spreading phenomena for undergraduate students. The d...

Anomalous Wetting of Underliquid Systems: Oil Drops in Water and Water Drops in Oil.

We have investigated the wetting phenomena of two underliquid systems, i.e., oil (drop) in water medium and water (drop) in oil medium for two different substrates, poly(methyl methacrylate) (PMMA) and glass. We have conducted detailed static (equilibrium) and dynamic contact angle measurements of drops on substrates kept in air, water, and oils of varying densities, viscosities, and surface tensions. We compared the experimentally observed contact angles with those predicted by the conventional wetting theories, namely, Young's equation and the Owens and Wendt approach. The results reported herein showed that experimental values vary in the range of 8-20% with the conventional theoretical model for water (drop) in oil (viscous surrounding medium) on PMMA substrate. However, oil (drop) in water medium on PMMA does not show such an anomaly. By taking into consideration a thin oil film between a water drop and PMMA originating from the surrounding oil medium, the modified Young's equation is proposed here. We found that the percentage difference between experimentally observed contact angles with modified Young's equation is in the range of 0.88-5.88%, which is very less compared to percentage difference with classic Young's equation. For glass substrates, the standard Young's equation does not translate to the underliquid systems whereas the Owens and Wendt theory could not correctly predict the underliquid contact angles. However, the modified Young's equation with thin-film consideration agrees very well with the experimental values and thereby demonstrated the presence of a thin film between a drop and glass substrate originating from the surrounding viscous medium. This present experimental study coupled with detailed theoretical analyses demonstrates the anomalous wetting signature of drops on substrates submerged in surrounding viscous medium, which is very different from the reported studies for drops on substrates kept in air (inviscid medium).

Growth mechanism of polycrystalline CsI(Tl) films on glass and single crystal Si substrates

The microstructure morphology and crystal quality of CsI(Tl) films are influenced by the crystal properties of substrates. In this work, CsI(Tl) films on the glass and single crystal silicon substrates are fabricated by vacuum thermal evaporation at a low deposition rate. The microstructure morphology, crystalline quality and scintillation properties of the films were examined by using scanning electron microscopy, X-ray diffraction, photoluminescence and radioluminescent spectrum. To study the growth mechanism of CsI(Tl) films on amorphous and monocrystalline substrates, a growth model based on the classical Volmer-Weber (VW) growth mode are proposed. During the film growing, the wetting and not-wetting phenomena of the clusters appear on the glass and Si(1 1 1) substrates respectively. In addition, the microcrystalline columns with different orientations are modeled to explain the surface morphology deteriorate of the CsI(Tl) film with 3 μm thickness on Si(1 1 1) substrate.

Multiscale Simulation Method for Quantitative Prediction of Surface Wettability at the Atomistic Level.

The solid-liquid interface is of great interest because of its highly heterogeneous character and its ubiquity in various applications. The most fundamental physical variable determining the strength of the solid-liquid interface is the solid-liquid interfacial tension, which is usually measured according to the contact angle. However, an accurate experimental measurement and a reliable theoretical prediction of the contact angle remain lacking because of many practical issues. Here, we propose a first-principles-based simulation approach to quantitatively predict the contact angle of an ideally clean surface using our recently developed multiscale simulation method of density functional theory in classical explicit solvents (DFT-CES). Using this approach, we simulate the surface wettability of a graphene and graphite surface, resulting in a reliable contact angle value that is comparable to the experimental data. From our simulation results, we find that the surface wettability is dominantly affected by the strength of the solid-liquid van der Waal's interaction. However, we further elucidate that there exists a secondary contribution from the change of water-water interaction, which is manifested by the change of liquid structure and dynamics of interfacial water layer. We expect that our proposed method can be used to quantitatively predict and understand the intriguing wetting phenomena at an atomistic level and can eventually be utilized to design a surface with a controlled hydrophobic(philic)ity.

Comparative study of two lattice Boltzmann multiphase models for simulating wetting phenomena: implementing static contact angles based on the geometric formulation

Wetting phenomena are widespread in nature and industrial applications. In general, systems concerning wetting phenomena are typical multicomponent/multiphase complex fluid systems. Simulating the behavior of such systems is important to both scientific research and practical applications. It is challenging due to the complexity of the phenomena and difficulties in choosing an appropriate numerical method. To provide some detailed guidelines for selecting a suitable multiphase lattice Boltzmann model, two kinds of lattice Boltzmann multiphase models, the modified S-C model and the H-C-Z model, are used in this paper to investigate the static contact angle on solid surfaces with different wettability combined with the geometric formulation (Ding, H. and Spelt, P. D. M. Wetting condition in diffuse interface simulations of contact line motion. Physical Review E, 75(4), 046708 (2007)). The specific characteristics and computational performance of these two lattice Boltzmann method (LBM) multiphase models are analyzed including relationship between surface tension and the control parameters, the achievable range of the static contact angle, the maximum magnitude of the spurious currents (MMSC), and most importantly, the convergence rate of the two models on simulating the static contact angle. The results show that a wide range of static contact angles from wetting to non-wetting can be realized for both models. MMSC mainly depends on the surface tension. With the numerical parameters used in this work, the maximum magnitudes of the spurious currents of the two models are on the same order of magnitude. MMSC of the S-C model is universally larger than that of the H-C-Z model. The convergence rate of the S-C model is much faster than that of the H-C-Z model. The major foci in this work are the frequently-omitted important details in simulating wetting phenomena. Thus, the major findings in this work can provide suggestions for simulating wetting phenomena with LBM multiphase models along with the geometric formulation.