Showing papers on "Wetting transition published in 2018"

Recent advances in droplet wetting and evaporation

Abstract: The wetting of solid surfaces using liquid droplets has been studied since the early 1800s. Thomas Young and Pierre-Simon Laplace investigated the wetting properties, as well as the role of the contact angle and the coupling of a liquid and solid, on the contact angle formation. The geometry of a sessile droplet is relatively simple. However, it is sufficiently complex to be applied for solving and prediction of real-life situations (for example, metallic inks for inkjet printing, the spreading of pesticides on leaves, the dropping of whole blood, the spreading of blood serum, and drying for medical applications). Moreover, when taking into account both wetting and evaporation, a simple droplet becomes a very complex problem, and has been investigated by a number of researchers worldwide. The complexity is mainly due to the physics involved, the full coupling with the substrate upon which the drop is deposited, the atmosphere surrounding the droplet, and the nature of the fluid (pure fluid, bi- or multi-phase mixtures, or even fluids containing colloids and/or nano-particles). This review presents the physics involved during droplet wetting and evaporation by focusing on the evaporation dynamics, the flow motion, the vapour behaviour, the surface tension, and the wetting properties.

Forced Wetting Transition and Bubble Pinch-Off in a Capillary Tube.

Abstract: Immiscible fluid-fluid displacement in partial wetting continues to challenge our microscopic and macroscopic descriptions. Here, we study the displacement of a viscous fluid by a less viscous fluid in a circular capillary tube in the partial wetting regime. In contrast with the classic results for complete wetting, we show that the presence of a moving contact line induces a wetting transition at a critical capillary number that is contact angle dependent. At small displacement rates, the fluid-fluid interface deforms slightly from its equilibrium state and moves downstream at a constant velocity, without changing its shape. As the displacement rate increases, however, a wetting transition occurs: the interface becomes unstable and forms a finger that advances along the axis of the tube, leaving the contact line behind, separated from the meniscus by a macroscopic film of the viscous fluid on the tube wall. We describe the dewetting of the entrained film, and show that it universally leads to bubble pinch-off, therefore demonstrating that the hydrodynamics of contact line motion generate bubbles in microfluidic devices, even in the absence of geometric constraints.

Quasistatic fluid-fluid displacement in porous media: Invasion-percolation through a wetting transition

Abstract: We introduce an invasion-percolation model which reproduces quasistatic fluid-fluid displacement patterns in porous media under different wettability conditions. Depending on wettability, the fluid front advances through capillary invasion, cooperative filling, or corner flow.

Wetting Transition of Nonpolar Neutral Molecule System on a Neutral and Atomic Length Scale Roughness Substrate

Abstract: One recently proposed new method for accurately determining wetting temperature is applied to the wetting transition occurring in a single component nonpolar neutral molecule system near a neutral planar substrate with roughness produced by cosinusoidal modulation(s). New observations are summarized into five points: (i) for a planar substrate superimposed with one cosinusoidal modulation, with increasing of the periodicity length or the surface attraction force field, or decreasing of the amplitude, wetting temperature $$T_W$$ drops accordingly and the three parameters show multiplication effect; moreover, both the periodicity length and amplitude effect curves display pole phenomena and saturation phenomena, and the $$T_W$$ saturation occurs at small (for case of large amplitude) or large (for case of small amplitude) periodicity length side, respectively. (ii) In the case of the planar substrate superimposed with two cosinusoidal modulations with equal periodicity length, the initial phase difference is critical issue that influences the $$T_W$$ , which decreases with the initial phase difference. (iii) In the case of the planar substrate superimposed with two cosinusoidal modulations with zero phase difference, change of the $$T_W$$ with one periodicity length under the condition of another periodicity length unchanged is non-monotonous. (iv) When the parameters are chosen such that the $$T_W$$ draws ever closer to the bulk critical temperature, wetting transition on the roughness substrate eventually does not occur. (v) The present microscopic calculation challenges traditional macroscopic theory by confirming that the atomic length scale roughness always renders the surface less hydrophilic and whereas the mesoscopical roughness renders the surface more hydrophilic. All of these observations summarized can be reasonably explained by the relative strength of the attraction actually enjoyed by the surface gas molecules to the attraction the gas molecules can get when in bulk.

Self-Recovery Superhydrophobic Surfaces: Modular Design

Abstract: Superhydrophobicity, the enhanced hydrophobicity of surfaces decorated with textures of suitable size, is associated with a layer of gas trapped within surface roughness. The reduced liquid/solid contact makes superhydrophobicity attractive for many technological applications. This gas layer, however, can break down with the liquid completely wetting the surface. Experiments have shown that the recovery of the “suspended” superhydrophobic state from the wet one is difficult. Self-recovery—the spontaneous restoring of the gas layer at ambient conditions—is one of the dreams of research in superhydrophobicity as it would allow to overcome the fragility of superhydrophobicity. In this work we have performed a theoretical investigation of the wetting and recovery processes on a set of surfaces characterized by textures of different dimensions and morphology in order to elucidate the optimal parameters for avoiding wetting and achieving self-recovery. Results show that texture size in the nanometer range is a ...

On the equilibrium contact angle of sessile liquid drops from molecular dynamics simulations.

Abstract: We present a new methodology to estimate the contact angles of sessile drops from molecular simulations by using the Gaussian convolution method of Willard and Chandler [J. Phys. Chem. B 114, 1954–1958 (2010)] to calculate the coarse-grained density from atomic coordinates. The iso-density contour with average coarse-grained density value equal to half of the bulk liquid density is identified as the average liquid-vapor (LV) interface. Angles between the unit normal vectors to the average LV interface and unit normal vector to the solid surface, as a function of the distance normal to the solid surface, are calculated. The cosines of these angles are extrapolated to the three-phase contact line to estimate the sessile drop contact angle. The proposed methodology, which is relatively easy to implement, is systematically applied to three systems: (i) a Lennard-Jones (LJ) drop on a featureless LJ 9-3 surface; (ii) an SPC/E water drop on a featureless LJ 9-3 surface; and (iii) an SPC/E water drop on a graphite surface. The sessile drop contact angles estimated with our methodology for the first two systems are shown to be in good agreement with the angles predicted from Young’s equation. The interfacial tensions required for this equation are computed by employing the test-area perturbation method for the corresponding planar interfaces. Our findings suggest that the widely adopted spherical-cap approximation should be used with caution, as it could take a long time for a sessile drop to relax to a spherical shape, of the order of 100 ns, especially for water molecules initiated in a lattice configuration on a solid surface. But even though a water drop can take a long time to reach the spherical shape, we find that the contact angle is well established much faster and the drop evolves toward the spherical shape following a constant-contact-angle relaxation dynamics. Making use of this observation, our methodology allows a good estimation of the sessile drop contact angle values even for moderate system sizes (with, e.g., 4000 molecules), without the need for long simulation times to reach the spherical shape.

Reversible Hydrophobicity-Hydrophilicity Transition Modulated by Surface Curvature.

Abstract: Wettability (hydrophobicity and hydrophilicity) is of fundamental importance in physical, chemical, and biological behaviors, resulting in widespread interest. Herein, by modulating surface curvature, we observed a reversible hydrophobic-hydrophilic transition on a model referred to a platinum surface. The underlying mechanism is revealed to be the competition between strong water-solid attraction and interfacial water orderliness. On the basis of the competition, we further propose an equation of wetting transition in the presence of an ordered interfacial liquid. It quantitatively reveals the relation of solid wettability with interfacial water orderliness and solid surface curvature, which can be used for predicting the critical point of the wetting transition. Our findings thus provide an innovative perspective on the design of a functional device demonstrating a reversible wettability transition and even a molecular-level understanding of biological functions.

Dynamics of non-wetting drops confined in a Hele-Shaw cell

Abstract: We experimentally investigate the sedimentation of a non-wetting drop confined between two parallel walls. The whole system is immersed in a bath of liquid of low viscosity and a lubricating film may be dynamically formed between the drop and the walls of the cell. Depending on the thickness of the film and on the viscosity ratio between the droplet and the surrounding liquid, viscous dissipation localizes either in the lubrication layer or in the bulk of the drop. The velocity of the droplet is non-trivial as the thickness of the lubricating layer may depend on the interplay between interfacial tension and viscous dissipation. Interestingly, thin films whose nanometric thickness is set by long range intermolecular interactions may lubricate efficiently the motion of highly viscous droplets. We derive asymptotic models that successfully capture the settling velocity of the drop in the different regimes observed experimentally. The effect of partial wetting is finally illustrated by a sharp increase of the velocity of the drops that we attribute to a wetting transition.

Rates of cavity filling by liquids.

Abstract: Understanding the fundamental wetting behavior of liquids on surfaces with pores or cavities provides insights into the wetting phenomena associated with rough or patterned surfaces, such as skin and fabrics, as well as the development of everyday products such as ointments and paints, and industrial applications such as enhanced oil recovery and pitting during chemical mechanical polishing. We have studied, both experimentally and theoretically, the dynamics of the transitions from the unfilled/partially filled (Cassie–Baxter) wetting state to the fully filled (Wenzel) wetting state on intrinsically hydrophilic surfaces (intrinsic water contact angle i ) the intrinsic contact angle, ( ii ) the concentration of dissolved air in the bulk water phase (i.e., aeration), ( iii ) the liquid volatility that determines the rate of capillary condensation inside the cavities, and ( iv ) the presence of surfactants.

In-situ ATR-FTIR for dynamic analysis of superhydrophobic breakdown on nanostructured silicon surfaces.

Abstract: Superhydrophobic surfaces are highly promising for self-cleaning, anti-fouling and anti-corrosion applications However, accurate assessment of the lifetime and sustainability of super-hydrophobic materials is hindered by the lack of large area characterization of superhydrophobic breakdown In this work, attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) is explored for a dynamic study of wetting transitions on immersed superhydrophobic arrays of silicon nanopillars Spontaneous breakdown of the superhydrophobic state is triggered by in-situ modulation of the liquid surface tension The high surface sensitivity of ATR-FTIR allows for accurate detection of local liquid infiltration Experimentally determined wetting transition criteria show significant deviations from predictions by classical wetting models Breakdown kinetics is found to slow down dramatically when the liquid surface tension approaches the transition criterion, which clearly underlines the importance of more accurate wetting analysis on large-area surfaces Precise actuation of the superhydrophobic breakdown process is demonstrated for the first time through careful modulation of the liquid surface tension around the transition criterion The developed ATR-FTIR method can be a promising technique to study wetting transitions and associated dynamics on various types of superhydrophobic surfaces

The effect of surface anisotropy on contact angles and the characterization of elliptical cap droplets

Abstract: In this paper, the variation of contact angles of a droplet on grooved surfaces was studied from microscale to macroscale experimentally and theoretically. The experimental results indicated that the contact angle changes nonlinearly with anisotropic factor. To get clear of the changing process of contact angle on grooved surfaces from microscale to macroscale, we carried out theoretical analysis with moment equilibrium method being adopted. In addition, the variation of contact angles in different directions was investigated and a mathematic model to calculate arbitrary contact angles around the elliptic contact line was suggested. For the convenience of potential applications, a symbolic contact angle was proposed to characterize the ellipsoidal cap droplet on grooved surfaces. Our results will offer help to the future design of patterned surfaces in practical applications, and deepen the understanding of wetting behavior on grooved surfaces.

Asymptotic theory of fluid entrainment in dip coating

Abstract: When a contact line moves with a sufficiently large speed, liquid or gas films can be entrained on a solid depending on the direction of contact-line movement. In this work, the contact-line dynamics in the situation of a generic two-fluid system is investigated. We demonstrate that the hydrodynamics of a contact line, no matter whether advancing or receding, can formally reduce to that of a receding one with small interfacial slopes. Since the latter can be well treated under the classical lubrication approximation, this analogy allows us to derive an asymptotic solution of the interfacial profiles for arbitrary values of contact angle and viscosity ratio. For the dip-coating geometry, we obtain, with no adjustable parameters, an analytical formula for the critical speed of wetting transition, which in particular predicts the onset of both liquid and gas entrainment. Moreover, the present analysis also builds a novel connection between the Cox–Voinov law and classical lubrication theory for moving contact lines.

Wetting states of two-dimensional drops under gravity

Abstract: In this study we give an analytical model for the Young-Laplace equation of two-dimensional (2D) drops under gravity. Inspired by the pioneering work of Landau and Lifshitz [Fluid Mechanics, 2nd ed. (Pergamon Press, Oxford, 1987), pp. 242--243], we derive general analytical expressions of the profile of drops on flat surfaces, which is available for arbitrary contact angles and drop volumes. We then extend the theoretical model to drops on inclined surfaces and reveal that the contact line plays an important role in determining the wetting state of the drops: (1) when the contact line is completely pinning, the rear and front contact angles and the shape of the drop can be uniquely determined by the drop volume, the slope of the inclined surface, and the contact area; (2) when the contact angle hysteresis is taken into consideration, various mathematical solutions of the wetting state exist for a drop of given volume on a given surface, but there is only one wetting state corresponding to a minimum free energy which results from the competition between the capillary force and gravity. Our theory is in excellent agreement with numerical and experimental results.

Wetting Transition of a Cylindrical Cavity Engraved on a Hydrophobic Surface

Abstract: This study theoretically examines the wetting of a cylindrical cavity engraved on a hydrophobic surface, in the context of the Cassie–Baxter-to-Wenzel transition of a water drop resting on such a s...

Controlling Octagon-to-Square Wetting Interface Transition of Evaporating Sessile Droplet through Surfactant on Microtextured Surface

Abstract: Producing and maintaining specific liquid patterns during evaporation holds great potential for techniques of printing and coating. Here we report the control over the evolution of surfactant solution droplets on the micropyramid substrates during evaporation. The polygonal droplet shape is achieved during the drying rather than solely at the beginning. As the initial surfactant concentration is 0.04 mM, the droplet maintains its initial octagonal shape throughout the lifetime. Interestingly, the initial octagonal shape transforms into a square during the evaporation as the initial surfactant concentration reaches 0.8 mM. These findings can shed light on wetting pattern control for complex solutions required in various applications.

Theory of Liquid Film Growth and Wetting Instabilities on Graphene.

Abstract: We investigate wetting phenomena near graphene within the Dzyaloshinskii-Lifshitz-Pitaevskii theory for light gases of hydrogen, helium, and nitrogen in three different geometries where graphene is either affixed to an insulating substrate, submerged or suspended. We find that the presence of graphene has a significant effect in all configurations. When placed on a substrate, the polarizability of graphene can increase the strength of the total van der Waals force by a factor of 2 near the surface, enhancing the propensity towards wetting. In a suspended geometry unique to two-dimensional materials, where graphene is able to wet on only one side, liquid film growth becomes arrested at a critical thickness, which may trigger surface instabilities and pattern formation analogous to spinodal dewetting. The existence of a mesoscopic critical film with a tunable thickness provides a platform for the study of a continuous wetting transition, as well as the engineering of custom liquid coatings. These phenomena are robust to some mechanical deformations and are also universally present in doped graphene and other two-dimensional materials, such as monolayer dichalcogenides.

Wetting Transitions of Liquid Gallium Film on Nanopillar-Decorated Graphene Surfaces.

Abstract: Molecular dynamics (MD) simulation has been employed to study the wetting transitions of liquid gallium droplet on the graphene surfaces, which are decorated with three types of carbon nanopillars, and to explore the effect of the surface roughness and morphology on the wettability of liquid Ga. The simulation results showed that, at the beginning, the Ga film looks like an upside-down dish on the rough surface, different from that on the smooth graphene surface, and its size is crucial to the final state of liquid. Ga droplets exhibit a Cassie–Baxter (CB) state, a Wenzel state, a Mixed Wetting state, and a dewetting state on the patterned surfaces by changing distribution and the morphology of nanopillars. Top morphology of nanopillars has a direct impact on the wetting transition of liquid Ga. There are three transition states for the two types of carbon nanotube (CNT) substrates and two for the carbon nanocone (CNC) one. Furthermore, we have found that the substrates show high or low adhesion to the Ga droplet with the variation of their roughness and top morphology. With the roughness decreasing, the adhesion energy of the substrate decreases. With the same roughness, the CNC/graphene surface has the lowest adhesion energy, followed by CNT/graphene and capped CNT/graphene surfaces. Our findings provide not only valid support to previous works but also reveal new theories on the wetting model of the metal droplet on the rough substrates.

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

Abstract: 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 Aun 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.

Highly non-additive symmetric mixtures at a wall

Abstract: This paper discusses the results of the grand canonical ensemble Monte Carlo simulation of the wetting behavior of non-additive symmetric mixtures at non-selective walls. We have focused on the mixtures that exhibit closed immiscibility loops in the bulk, and the results obtained have demonstrated that such systems show a rather complex wetting behavior. In particular, such mixtures may exhibit a complete wetting at temperatures below the bulk demixing point, and an incomplete wetting at higher temperatures. Such a situation occurs when the adsorbed film remains mixed up to the bulk coexistence. However, close to the bulk tricritical point, being the onset of the continuous demixing transition (λ-line), a second wetting transition takes place. On the other hand, in the systems in which the adsorbed films undergo the demixing transition, a complete wetting may occur below as well as at and above the bulk demixing transition temperature. It has also been demonstrated that the wetting behavior depends strongly on the strength of the surface–fluid interaction.