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During oscillatory wetting, a phase retardation emerges between contact angle variation and contact line velocity, presenting as a hysteresis loop in their correlation – an effect we term dynamic hysteresis. This phenomenon is found to be tunable by modifying the surface with different molecular layers. A comparative analysis of dynamic hysteresis, static hysteresis and contact line friction coefficients across diverse substrates reveals that dynamic hysteresis is not a result of dissipative effects but is instead proportionally linked to the static hysteresis of the surface. In the quest for appropriate conditions to model oscillatory contact line motion, we identify the generalized Hocking's linear law and modified generalized Navier boundary condition as alternative options for predicting realistic dynamic hysteresis. [ABSTRACT FROM AUTHOR] more...

The motion of the three-phase contact line between two immiscible fluids and a solid surface arises in a variety of wetting phenomena and technological applications. One challenge in continuum theory is the effective representation of molecular motion close to the contact line. Here, we characterize the molecular processes of the moving contact line to assess the accuracy of two different continuum two-phase models. Specifically, molecular dynamics simulations of a two-dimensional droplet between two moving plates are used to create reference data for different capillary numbers and contact angles.We use a simple-point-charge/extended water model. This model provides a very small slip and a more realistic representation of the molecular physics than Lennard-Jones models. The Cahn–Hilliard phase-field model and the volume-of-fluid model are calibrated against the drop displacement from molecular dynamics reference data. It is shown that the calibrated continuum models can accurately capture droplet displacement and droplet break-up for different capillary numbers and contact angles. However, we also observe differences between continuum and atomistic simulations in describing the transient and unsteady droplet behaviour, in particular, close to dynamical wetting transitions. The molecular dynamics of the sheared droplet provide insight into the line friction experienced by the advancing and receding contact lines. The presented results will serve as a stepping stone towards developing accurate continuum models for nanoscale hydrodynamics. [ABSTRACT FROM AUTHOR] more...

A comprehensive three-dimensional, time-dependent model for simulating the adsorption kinetics and the redistribution of surfactants at the surface and in the bulk of a weld pool is presented. The multicomponent surfactant mass transfer model could be used to study the effect of the influence of sulfur and oxygen redistribution in welding of a super duplex stainless steel. more...

We investigate the behavior of a fluid near the critical point by using numerical simulations of weakly compressible three-dimensional isotropic turbulence. Much has been done for a turbulent flow with an ideal gas. The primary focus of this work is to analyze fluctuations of thermodynamic variables (pressure, density, and temperature) when a non-ideal Equation Of State (EOS) is considered. In order to do so, a hybrid lattice Boltzmann scheme is applied to solve the momentum and energy equations. Previously unreported phenomena are revealed as the temperature approaches the critical point. Fluctuations in pressure, density, and temperature increase, followed by changes in their respective probability density functions. Due to the non-linearity of the EOS, it is seen that variances of density and temperature and their respective covariance are equally important close to the critical point. Unlike the ideal EOS case, significant differences in the thermodynamic properties are also observed when the Reynolds number is increased. We also address issues related to the spectral behavior and scaling of density, pressure, temperature, and kinetic energy. [ABSTRACT FROM AUTHOR] more...

Non-Newtonian droplet dynamics commonly exist in microfluidic systems. We report simulations of viscoelastic (VE) droplets traveling in a two dimensional capillary bifurcation channel. A numerical system combining phase field method, VE rheology, and Stokes flow equations is built. As a generic microfluidic two-phase problem, we study how a non-Newtonian droplet that approaches a channel bifurcation will behave. We identify conditions for when a droplet will either split into two or be directed into one of the branches. In particular, we study the importance of the non-Newtonian properties. Our results reveal two different non-Newtonian mechanisms that can enhance splitting and non-splitting of droplets with respect to Newtonian droplets, depending on the size of droplet and capillary number. [ABSTRACT FROM AUTHOR] more...

Axisymmetric rapid expansion of supercritical carbon dioxide is investigated in this article. The extended generalized Bender equation of state is used to give a good description of the fluids over a wide range of pressure and temperature conditions. The locations of Mach disks are analyzed and compared with an experimental correlation for the case where there is no plate positioned in front of the nozzle exit. It is found that the disagreement between our numerical results and the experimental formula is very small when the pressure ratio is small, and increases as the pressure ratio increases. It is also found that with different equations of state, the predicted positions of Mach disks do not differ a lot, but the temperature profiles in the chamber differ a lot. The case where there is a plate positioned in front of the nozzle exit is also studied in this article. A universal similarity solution is obtained. © 2014 American Institute of Chemical Engineers AIChE J, 61: 317-332, 2015 [ABSTRACT FROM AUTHOR] more...

Abstract: In this work, the effect of external loading on the martensitic transformation is analyzed using an elasto-plastic phase field model. The phase field microelasticity theory, incorporating a non-linear strain tensor and the effect of grain boundaries, is used to study the impact of applied stresses on an Fe–0.3%C polycrystalline alloy, both in two and three dimensions. The evolution of plasticity is computed using a time-dependent equation that solves for the minimization of the shear strain energy. Crystallographic orientation of the grains in the polycrystal is chosen randomly and it is verified that the said assumption does not have a significant effect on the final volume fraction of martensite. Two-dimensional (2-D) and three-dimensional (3-D) simulations are performed at a temperature significantly higher than the martensitic start temperature of the alloy with uniaxial tensile, compressive and shear loading, along with hydrostatic stresses. It is found that the 3-D simulations are necessary to investigate the effect of external loading on the martensitic transformation using the phase field method since the 2-D numerical simulations produce results that are physically incorrect, while the results obtained from the 3-D simulations are in good agreement with the empirical observations found in the literature. Finally, it is concluded that the given model can be used to predict the volume fraction of martensite in a material with any kind of external loading. [Copyright &y& Elsevier] more...

Presented in this paper is a finite-element-method-based model for phase change material actuators, modeling the active material as a fluid as opposed to a solid. This enables the model to better conform to localized loads and offering the opportunity to follow material movement in enclosed volumes. Modeling, simulation, and analysis of an electrothermally activated paraffin microactuator have been conducted. The paraffin microactuator used for the analysis in this study exploits the large volumetric expansion of paraffin upon melting, which, combined with its low compressibility in the liquid state, allows for high hydraulic pressures to be generated. The purpose of this study is to supply a geometry-independent model of such a microactuator through the implementation of a fluid model rather than a solid one, which has been utilized in previous studies. Numerical simulations are conducted at different frequencies of the heating source and for different geometries of the microactuator. The results are compared with the empirical data obtained on a close to identical paraffin microactuator, which clearly show the advantages of a fluid model instead of a solid-state approximation. \hfill[2012-0030] [ABSTRACT FROM AUTHOR] more...

In this paper we present simulations of dynamic wetting far from equilibrium based on phase field theory. In direct simulations of recent experiments [J. C. Bird, S. Mandre, and H. A. Stone, Phys. Rev. Lett. 100, 234501 (2008)], we show that in order to correctly capture the dynamics of rapid wetting, it is crucial to account for nonequilibrium at the contact line, where the gas, liquid, and solid meet. A term in the boundary condition at the solid surface that naturally arises in the phase field theory is interpreted as allowing for the establishment of a local structure in the immediate vicinity of the contact line. A direct qualitative and quantitative match with experimental data of spontaneously wetting liquid droplets is shown. [ABSTRACT FROM AUTHOR] more...

The impact of a solid sphere on a liquid surface has challenged researchers for centuries and remains of interest today. Recently, Duez et al. [Nat. Phys. 3, 180 (2007)] published experimental results of the splash generated when a solid sphere enters water. Interestingly, the microscopic properties of the solid surface control the nature of the macroscopic behavior of the splash. So by a change in the surface chemistry of the solid sphere, a big splash can be turned into an inconspicuous disappearance and vice versa. This problem was investigated by numerical simulations based on the Navier–Stokes equations coupled with the Cahn–Hilliard equations. This system allows us to simulate the motion of an air-water interface as a solid sphere impacts the liquid pond. The inclusion of the surface energies of the solid surface in the formulation gives a reasonably quantitative description of the dynamic wetting. Numerical results with different wetting properties and impact speed are presented and directly compared with the recent experimental results from Duez et al. [ABSTRACT FROM AUTHOR] more...

Control of oscillatory thermocapillary convection in an annular geometry with a horizontal free surface is investigated by means of a numerical simulation. The objective is to suppress oscillations using a feedback opposition control. The temperature is measured at certain positions on the interface, this signal is amplified and used to apply local heating on the free surface. Many features of the controlled system observed in previous experiments could be reproduced by the simulation. The numerical simulation allows us to clarify the picture of the spatial structures of the controlled oscillation, which was not accessible in the experiments. In addition to what was found in the previous experiments, the present simulations also permit us to investigate the importance of the positioning of sensors and heaters, and the influence of the properties of the heaters. [ABSTRACT FROM AUTHOR] more...

Active control was applied to oscillatory thermocapillary flow in an open cylindrical container filled with Silicone oil. Thermocapillary convection was driven by imposing a radial temperature gradient on a flat free surface. This is an extension of the previous work by Shiomi et al. who applied proportional feedback control by locally heating the surface at a single position using the local temperature signal at a different position fed back through a simple algorithm. Although significant attenuation of the oscillation was detected, an uncertainty remained if global stabilization was achieved. In the present paper, two sensor/heater pairs were installed to achieve the global suppression of the oscillation. Successful global stabilization of the oscillation was achieved in a range of Marangoni number, with the best performance in the weakly nonlinear regime. Using a reliable temperature measurement method, quantitative analysis is carried out to quantify the performance of the control. The optimal values of gain and relative position of sensor/heater pairs were identified. [ABSTRACT FROM AUTHOR] more...

The stability of thermocapillary convection inside a cylindrical liquid bridge is studied using both a direct numerical simulation of the three-dimensional problem and linear stability analysis of the axisymmetric basic state. Previously this has been studied extensively for low and high Prandtl numbers. However, the intermediate range of Prandtl numbers between approximately 0.07 and 0.8 which joins the low and high ranges is quite complicated and has not been studied to the same extent. One striking feature is that the axisymmetric base state is much more stable in this intermediate range than at high or low Prandtl numbers. We identify four different oscillatory modes in this range, which have different qualitative features. Direct numerical simulations have been carried out for representative parameter values, and show that the bifurcations are supercritical. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR] more...

Multicomponent, multiphase, compressible flows are very important in real life, as well as in scientific research, while their modeling is in an early stage. In this paper, we propose a diffuse interface model for compressible binary mixtures, based on the balance of mass, momentum, energy, and the second law of thermodynamics. We show both analytically and numerically that this model is able to describe the phase equilibrium for a real binary mixture (CO2 + ethanol is considered in this paper) very well by adjusting the parameter which measures the attraction force between molecules of the two components in the model. We also show that the calculated surface tension of the CO2 + ethanol mixture at different concentrations match measurements in the literature when the mixing capillary coefficient is taken to be the geometric mean of the capillary coefficient of each component. Three different cases of two droplets in a shear flow, with the same or different concentration, are simulated, showing that the higher concentration of CO2 the smaller the surface tension and the easier the drop deforms. [ABSTRACT FROM AUTHOR] more...

Liquid wetting of a surface is omnipresent in nature and the advance of micro-fabrication and assembly techniques in recent years offers increasing ability to control this phenomenon. Here, we identify how surface roughness influences the initial dynamic spreading of a partially wetting droplet by studying the spreading on a solid substrate patterned with microstructures just a few micrometers in size. We reveal that the roughness influence can be quantified in terms of a line friction coefficient for the energy dissipation rate at the contact line, and that this can be described in a simple formula in terms of the geometrical parameters of the roughness and the line-friction coefficient of the planar surface. We further identify a criterion to predict if the spreading will be controlled by this surface roughness or by liquid inertia. Our results point to the possibility of selectively controlling the wetting behavior by engineering the surface structure. [ABSTRACT FROM AUTHOR] more...

We conduct numerical experiments on viscoelastic droplets hitting a flat solid surface. The results present time-resolved non-Newtonian stresses acting in the droplet. Comparing with the simulation of the impact of a Newtonian droplet, the effects of viscoelasticity on droplet behaviors such as splashing, the maximum spreading diameter and deformation are analyzed. With detailed information on the contact line region, we demonstrate how the contact line behaves according to the transition of the fluid property from elasticity dominated to shear-thinning dominated when a droplet expands and contracts on the substrate. The propose of this work is to discuss whether and how the elasticity in an impinging droplet takes effect. [ABSTRACT FROM AUTHOR] more...