The temperature-dependence of the dynamic contact angle

Temperature Dependence of the Dynamic Contact Angles of Water on a Smooth Stainless-Steel Surface under Elevated Pressures

The temperature dependence of the dynamic contact angles (DCAs) of water on a metallic surface remains unclear, especially under elevated pressures. Here in this work, the advancing and receding contact angles (RCAs), as well as the contact angle hysteresis (CAH), of water on stainless-steel 316 (SS316) surfaces were studied using the dynamic sessile drop method for temperatures up to 300 °C and pressures up to 10 MPa. It was found that the temperature dependence of the DCAs exhibits a different pattern as compared to the piecewise linear decline of static contact angles. The advancing contact angle (ACA) remains nearly constant and does not decrease until the temperature becomes close to the saturated temperature. The decrease in ACA is attributed to evaporation, which reduces the advancement of energy barrier. The RCA linearly declines below 120 °C and remains stable above 120 °C. The increasing temperature enhances the pinning effect and changes the droplet receding mode. Under all pressures tested, the CAH demonstrates a "increase-constant-decrease" trilinear relationship with temperature. Furthermore, the mean solid surface entropy and solid-gas interfacial tension of SS316 were estimated to be 0.1152 mJ/(m2·°C) and 61.49 mJ/m2, respectively.

Thermocapillary Migrating Odd Viscous Droplets

A droplet of a classical liquid surrounded by a cold gas placed on a hot substrate is accompanied by unremitting internal circulations, while the droplet remains immobile. Two identical cells with opposite sense of circulation form in the interior due to the thermocapillary effect induced by the gas and substrate temperature difference. Under the same conditions, a droplet composed of an odd viscous liquid exerts a compressive stress on the cell rotating in one sense and tensile on the cell rotating in the opposite sense resulting in a tilted droplet configuration. A sufficiently strong thermal gradient leads the contact angles to overcome hysteresis effects and induces droplet migration.

The receding contact line cools down during dynamic wetting

When a contact line (CL)-where a liquid-vapor interface meets a substrate-is put into motion, it is well known that the contact angle differs between advancing and receding CLs. Using non-equilibrium molecular dynamics simulations, we reveal another intriguing distinction between advancing and receding CLs: while temperature increases at an advancing CL-as expected from viscous dissipation, we show that temperature can drop at a receding CL. Detailed quantitative analysis based on the macroscopic energy balance around the dynamic CL showed that the internal energy change of the fluid due to the change of the potential field along the pathline out of the solid-liquid interface induced a remarkable temperature drop around the receding CL, in a manner similar to latent heat upon phase changes. This result provides new insights for modeling the dynamic CL, and the framework for heat transport analysis introduced here can be applied to a wide range of nanofluidic systems.

Early-Stage Liquid Infiltration in Nanoconfinements

Liquid infiltration is one of the commonly adapted flow mechanisms in microscale/nanoscale heat-transfer applications. The theoretical modeling of dynamic infiltration profile in the microscale/nanoscale requires a deep study, because the acting forces are entirely different from those of a large-scale system. Herein, a model equation is developed from the fundamental force balance at the microscale/nanoscale level, to capture the dynamic infiltration flow profile. Molecular kinetic theory (MKT) is used to predict the dynamic contact angle. Molecular dynamics (MD) simulations are performed to study the capillary infiltration in two different geometries. The infiltration length is computed from the simulation results. The model is also evaluated over surfaces having different surface wettability. The generated model provides a better estimation of the infiltration length, compared to the well-established models. The developed model is expected to aid in the designing of microscale/nanoscale devices where liquid infiltration plays a key role.

Dynamic wetting of various liquids: Theoretical models, experiments, simulations and applications

Dynamic wetting is a ubiquitous phenomenon and frequently observed in our daily life, as exemplified by the famous lotus effect. It is also an interfacial process of upmost importance involving many cutting-edge applications and has hence received significantly increasing academic and industrial attention for several decades. However, we are still far away to completely understand and predict wetting dynamics for a given system due to the complexity of this dynamic process. The physics of moving contact lines is mainly ascribed to the full coupling with the solid surface on which the liquids contact, the atmosphere surrounding the liquids, and the physico-chemical characteristics of the liquids involved (small-molecule liquids, metal liquids, polymer liquids, and simulated liquids). Therefore, to deepen the understanding and efficiently harness wetting dynamics, we propose to review the major advances in the available literature. After an introduction providing a concise and general background on dynamic wetting, the main theories are presented and critically compared. Next, the dynamic wetting of various liquids ranging from small-molecule liquids to simulated liquids are systematically summarized, in which the new physical concepts (such as surface segregation, contact line fluctuations, etc.) are particularly highlighted. Subsequently, the related emerging applications are briefly presented in this review. Finally, some tentative suggestions and challenges are proposed with the aim to guide future developments.

Velocity-Dependent Contact Angle and Energy Dissipations of Dynamic Wetting Nanodroplets on Nanopillared Surfaces

Dynamic wetting, described by a dynamic contact angle (DCA), is a fundamental behavior of fluid on surface. With the development of blue energy, the research of droplet nanogenerator is flourishing. There is a growing interest in the dynamic wetting behavior of nanodroplets on surfaces. Molecular dynamics simulations are performed to reveal the influence of the velocity of nanodroplets and the wetting state (Cassie and Wenzel) on the DCA and the energy dissipation on the contact line. The simulation results demonstrate a more complicated scenario of dynamic wetting than the static wetting: The increasing rate of advancing DCA is lower than the decreasing rate of the receding DCA with respect to the nanodroplet velocity. As for the Wenzel state, larger surface roughness increases the dynamic wetting hysteresis, while for Cassie nanodroplets, the larger surface roughness leads to smaller dynamic wetting hysteresis. It is found that a structural force exists on the rough surface. The energy dissipation of the dynamic wetting mainly comes from the motion of the contact line, which is positively correlated to the velocity and can be decomposed to the viscosity and friction dissipations, respectively. The Cassie state causes much lower energy dissipation than the Wenzel state. Furthermore, the quasi-static contact angle is proposed to describe the contact angle on a rough surface. These findings advance the understanding of dynamic wetting behavior and inspire theoretical guidance for the design of novel functional interfaces.

Surface Reconstruction of Fluoropolymers in Liquid Media

Surface reconstruction is the rearrangement of atoms or molecules at an interface in response to a stimulus, driven by lowering the overall free energy of the system. Perfluoroalkyl acrylate polymers with short side chains undergo reconstruction at room temperature when exposed to water. Here, we use contact angle aging to examine the liquid- and temperature- dependency of surface reconstruction of plasma polymerized perfluoroalkyl acrylates. We use a first order kinetic model to examine the dynamics of reconstructive processes. Our results show that, above the bulk melting point of the polymers, the contact angles of both polar and nonpolar (hydrocarbon) liquids show a time dependency well fit by the model. We conclude that surface reconstruction can be driven by the preferential segregation of hydrocarbon and fluorocarbon moieties as well as by polar interactions. This has implications in terms of using fluorocarbons to design oleophobic surfaces (and vice versa) and in terms of designing fluorocarbon and/or hydrocarbon surfaces with switchable wettability.

Slip transition in dynamic wetting for a generalized Navier boundary condition

HYPOTHESIS

Computer fluid dynamics simulations of dynamic wetting are often performed using a slip model on the substrate. In previous studies, the generalized Navier boundary condition (GNBC) has shown promising results and could help clear the gap between molecular and continuum scales, but lacks quantitative comparisons to experiments. We seek to investigate the dependence between the contact-line velocity and the slip length in a GNBC, by confronting numerical simulations to experimental data.

EXPERIMENTS

The physical properties of a molten polymer (polyethylene glycol) were assessed thoroughly. Its dynamic contact angle on a cellulosic substrate was measured carefully using the Wilhelmy method. The experiment was reproduced in a finite elements model using a GNBC. It was repeated for capillary numbers between 10^-6 and 10^-1, and slip lengths ranging from 1 μm to 1 mm.

FINDINGS

A realistic value of the slip length was selected by matching the dynamic contact angles issued from numerical simulations and their experimental counterparts. The slip length behavior as a function of contact line velocity displayed a clear transition. The model also reproduced a dynamic wetting transition between frictional and viscous dissipations, which seems to be linked to an increasing difference between microscopic and macroscopic contact angles.

Taking a closer look: A molecular-dynamics investigation of microscopic and apparent dynamic contact angles

HYPOTHESIS

Molecular dynamics (MD) may be used to investigate the velocity dependence of both the microscopic and apparent dynamic contact angles (θ_m and θ_app).

METHODS

We use large-scale MD to explore the steady displacement of a water-like liquid bridge between two molecularly-smooth solid plates under the influence of an external force F⁰. A coarse-grained model of water reduces the computational demand and the solid-liquid affinity is varied to adjust the equilibrium contact angle θ⁰. Protocols are devised to measure θ_m and θ_app as a function of contact-line velocity U_cl.

FINDINGS

For all θ⁰, θ_m is velocity-dependent and consistent with the molecular-kinetic theory of dynamic wetting (MKT). However, θ_app diverges from θ_m as F⁰ is increased, especially at the receding meniscus. The behavior of θ_app follows that predicted by Voinov: (θ_app)³ = (θ_m)³ + 9Ca·ln(L/L_m), where Ca is the capillary number and L and L_m are suitably-chosen macroscopic and microscopic length scales. For each θ⁰, there is a critical velocity U_crit and contact angle θ_crit at which θ_app→0 and the receding meniscus deposits a liquid film. Setting θ_app=0, θ_m=θ_crit and U_cl=U_crit in the Voinov equation yields the value of L/L_m. The predicted values of θ_app then agree well with those measured from the simulations. Since θ_m obeys the MKT, we have, therefore, demonstrated the utility of the combined model of dynamic wetting proposed by Petrov and Petrov.

Dynamic wetting of solid-liquid-liquid system by molecular kinetic theory

HYPOTHESIS

The mechanisms of dynamic wetting of solid-liquid-liquid (SLL) system, especially the viscosity effects of two liquids, can be investigated by the molecular kinetic theory (MKT).

METHODS

The molecular kinetic theory combined with published data was used to study the roles of a fluid viscosity and a solid surface in dynamic wetting.

FINDINGS

First, the MKT on dynamic wetting was introduced and its limitation was analyzed. Second, a viscosity effect and a solid surface effect were considered. The viscosity effect was divided into three parts for the first time, including two pure liquid zones and a mixing zone. Third, a coefficient activation free energy model was proposed, considering the effects of mixing liquids and a solid surface. Finally, the key parameters in the MKT and the application and validation of the coefficient activation free energy model were discussed in detail. This model can explain the energy dissipation in a vicinity of a three-phase contact-line successfully in a SLL wetting system. This work sheds light on the physical mechanisms of fluid and solid surface properties on the dynamic wetting in a SLL system.

Molecular Investigation of Contact Line Movement in Electrowetted Nanodroplets

Molecular dynamics (MD) simulation of an electrowetted nanodroplet is performed to understand the fundamental origin of the involved parameters resulted from the molecular movement in the vicinity of the three-phase contact line (TPCL). During the spreading of the droplet, contact line friction (CLF) force is found to be the controlling one among all other resistive forces. Being molecular in nature, MD study is required to unveil the CLF, which is manifested by the TPCL friction coefficient ζ. The combined effect of temperature, electric field, and surface wettability, manifested by the solid-liquid Lennard-Jones interaction parameter, is studied to explore the droplet spreading. The entire droplet wetting dynamics is divided into two different regimes, namely, spreading regime and equilibrium regime. The molecular frequency during the TPCL movement in the equilibrium regime is affected by the presence of any external perturbation and results in an alteration of ζ. The predetermined knowledge of the alteration of CLF due to the coupling effect of electric field and temperature will have a potential application towards designing electric field-inspired droplet movement devices.