Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session G38: Focus Session: Modeling, Computations and Applications of Wetting/Dewetting Problems IIIFSI
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Chair: Satish Kumar, University of Minnesota Room: 304 |
Monday, November 20, 2017 10:35AM - 10:48AM |
G38.00001: Viscoelastic and poroelastic effects in the wetting dynamics of soft gels by liquids. Laurent Limat, Julien Dervaux, Matthieu Roche, Menghua Zhao, Tetsuharu Narita, Francois Lequeux We have developed experiments and modeling of elastowetting dynamics on soft gels. First, wetting is very sensitive to the thickness of the gel, when deposited on a rigid basis. We reconsidered Long et al approach, and extended it to finite depth. This yields a new scaling law, at low thickness, for dynamic contact angle, in very good agreement with experiment but not consistent with recent approachs assuming Neuman triangle to hold even in the dynamics. In a second step, we examined solvent migration in the bulk of the gel, and showed that poroelasticity is an essential ingredient to understand old unsolved issues (hysteresis on elastomers by Extrand and Kumagai), as well as recent puzzling measurements (long life footprints left by drops). Our calculations lead to ridges at the contact lines evolving logarithmically with time, with a very strong infuence on wetting properties of soft materials, and with possible applications to biophysics. [Preview Abstract] |
Monday, November 20, 2017 10:48AM - 11:01AM |
G38.00002: Modified lubrication theory including inertia effects Hyoungsoo Kim We study experimentally and theoretically a partially dewetting flow problem at a finite Reynolds number 1 $<$ $Re$ $<$ $O$(100), which is inspired by the industrial problem of immersion lithography machines. Based on shadowgraphy measurement and tomographic particle image velocimetry results, we develop a modified three-dimensional lubrication model to consider the inertia effects. The model describes observations that are somewhat similar to the results in the Stokes flow regime, such as the relationship between the contact angle and substrate speed, and the self-similar flow pattern near the de-wetting contact lines, although the current case is outside the Stokes flow regime. The theoretical model shows a good agreement with experimental results. The introduced model well predicts the critical condition for the droplet breakup, i.e. the relation between the corner opening angle and the dynamic receding contact angle. We will also discuss inertia effects on the dynamic contact angle as a function of the capillary number at a relatively high Reynolds number regime. [Preview Abstract] |
Monday, November 20, 2017 11:01AM - 11:14AM |
G38.00003: Wetting and Adhesion mediated by Nanoscale Capillary Bridges Szu-Pei Fu, Sijia Huang, Yuan-Nan Young, Howard Stone, Carlos Colosqui The formation of capillary bridges occurs in numerous natural and industrial processes involving the transfer of liquids between solid surfaces. This talk will discuss results from continuum-based models and fully atomistic molecular dynamics (MD) simulations of a nanoscale water bridge between two solid surfaces. For nanoscale separations between the solid surfaces, molecular interactions and thermal fluctuations significantly affect the capillary bridge shape, liquid-solid contact area, and equilibrium contact angles. For bridge heights below 10 nm, we observe significant differences between results from MD simulations and predictions from a conventional Young-Laplace equation considering solely the capillary pressure induced by free surface curvature. To account for results from MD simulations, we extended the Young-Laplace equation by including a disjoining pressure term due to DLVO interactions and steric effects. The proposed Young-Laplace equation is able to model nanoscale phenomena such as molecular layering that lead to strong structural forces and metastable configurations when the solid surfaces are separated by molecularly thin gaps. [Preview Abstract] |
Monday, November 20, 2017 11:14AM - 11:27AM |
G38.00004: Multi-scale strategies for dealing with moving contact lines Edward R. Smith, Panagiotis Theodorakis, Richard V. Craster, Omar K. Matar Molecular dynamics (MD) has great potential to elucidate the dynamics of the moving contact line. As a more fundamental model, it can provide {\it a priori} results for fluid-liquid interfaces, surface tension, viscosity, phase change, and near wall stick-slip behaviour which typically show very good agreement to experimental results. However, modelling contact line motion combines all this complexity in a single problem. In this talk, MD simulations of the contact line are compared to the experimental results obtained from studying the dynamics of a sheared liquid bridge. The static contact angles are correctly matched to the experimental data for a range of different electro-wetting results. The moving contact line results are then compared for each of these electro-wetting values. Despite qualitative agreement, there are notable differences between the simulation and experiments. Many MD simulation have studied contact lines, and the sheared liquid bridge, so it is of interest to review the limitations of this setup in light of this discrepancy. A number of factors are discussed, including the inter-molecular interaction model, molecular-scale surface roughness, model of electro-wetting and, perhaps most importantly, the limited system sizes possible using MD simulation. [Preview Abstract] |
Monday, November 20, 2017 11:27AM - 11:40AM |
G38.00005: Moving contact lines in partial wetting: bridging the gap across the scales Amir Pahlavan, Luis Cueto-Felgueroso, Gareth McKinley, Ruben Juanes The spreading and dewetting of liquid films on solid substrates is a common phenomenon in nature and industry from a snail secreting a mucosal film to printing and coating processes. A quantitative description of these phenomena, however, requires a detailed understanding of the flow physics at the nanoscale as the intermolecular interactions become important close to the contact line. Classical hydrodynamic theory describes wetting as an interplay between viscous and interfacial forces, neglecting the intermolecular interactions, leading to a paradox known as the moving contact line singularity. By contrast, molecular kinetic theory describes wetting as an activated process, neglecting the bulk hydrodynamics in the spreading viscous fluid film altogether. Here, we show that our recently developed model for thin liquid films in partial wetting, which properly incorporates the role of van der Waals interactions in a thin spreading fluid layer into a height-dependent surface tension, bridges the gap between these two approaches and leads to a unified framework for the description of wetting phenomena. We further use our model to investigate the instability and dewetting of nanometric liquid films, and show that it brings theoretical predictions closer to experimental observations. [Preview Abstract] |
Monday, November 20, 2017 11:40AM - 11:53AM |
G38.00006: Wetting of heterogeneous substrates. A classical density-functional-theory approach Peter Yatsyshin, Andrew O. Parry, Carlos Rasc\'on, Miguel A. Duran-Olivencia, Serafim Kalliadasis Wetting is a nucleation of a third phase (liquid) on the interface between two different phases (solid and gas). In many experimentally accessible cases of wetting, the interplay between the substrate structure, and the fluid-fluid and fluid-substrate intermolecular interactions leads to the appearance of a whole ``zoo'' of exciting interface phase transitions, associated with the formation of nano-droplets/bubbles, and thin films. Practical applications of wetting at small scales are numerous and include the design of lab-on-a-chip devices and superhydrophobic surfaces. In this talk, we will use a fully microscopic approach to explore the phase space of a planar wall, decorated with patches of different hydrophobicity, and demonstrate the highly non-trivial behaviour of the liquid-gas interface near the substrate. We will present fluid density profiles, adsorption isotherms and wetting phase diagrams. Our analysis is based on a formulation of statistical mechanics, commonly known as classical density-functional theory. It provides a computationally-friendly and rigorous framework, suitable for probing small-scale physics of classical fluids and other soft-matter systems. [Preview Abstract] |
Monday, November 20, 2017 11:53AM - 12:06PM |
G38.00007: Instability, rupture and fluctuations in thin liquid films: Theory and computations Rishabh Gvalani, Miguel Duran-Olivencia, Serafim Kalliadasis, Grigorios Pavliotis Thin liquid films are ubiquitous in natural phenomena and technological applications. They are commonly studied via deterministic hydrodynamic equations, but thermal fluctuations often play a crucial role that still needs to be understood. An example of this is dewetting, which involves the rupture of a thin liquid film and the formation of droplets. Such a process is thermally activated and requires fluctuations to be taken into account self-consistently. Here we present an analytical and numerical study of a stochastic thin-film equation derived from first principles. We scrutinise the behaviour of the stochastic thin film equation in the limit of perfectly correlated noise along the wall-normal direction. We also perform Monte Carlo simulations of the stochastic equation by adopting a numerical scheme based on a spectral collocation method. The numerical scheme allows us to explore the fluctuating dynamics of the thin film and the behaviour of the system's free energy close to rupture. Finally, we also study the effect of the noise intensity on the rupture time, which is in good agreement with previous works. [Preview Abstract] |
Monday, November 20, 2017 12:06PM - 12:19PM |
G38.00008: Separated rupture and retraction of a bi-layer free film Peter Stewart, Jie Feng, Ian Griffiths We investigate the dynamics of a rising air bubble in an aqueous phase coated with a layer of oil. Recent experiments have shown that bubble rupture at the compound air/oil/aqueous interface can effectively disperse submicrometre oil droplets into the aqueous phase, suggesting a possible mechanism for clean-up of oil spillages on the surface of the ocean. Using a theoretical model we consider the stability of the long liquid free film formed as the bubble reaches the free surface, composed of two immiscible layers of differing viscosities, where each layer experiences a van der Waals force between its interfaces. For an excess of surfactant on one gas--liquid interface we show that the instability manifests as distinct rupture events, with the oil layer rupturing first and retracting over the in-tact water layer beneath, consistent with the experimental observations. We use our model to examine the dynamics of oil retraction, showing that it follows a power-law for short times, and examine the influence of retraction on the stability of the water layer. [Preview Abstract] |
Monday, November 20, 2017 12:19PM - 12:32PM |
G38.00009: Contact line motion over substrates with spatially non-uniform properties Vladimir Ajaev, Elizaveta Gatapova, Oleg Kabov We develop mathematical models of moving contact lines over flat solid surfaces with spatial variation of temperature and wetting properties under the conditions when evaporation is significant. The gas phase is assumed to be pure vapor and a lubrication-type framework is employed for describing viscous flow in the liquid. Marangoni stresses at the liquid surface arise as a result of temperature variation in the vapor phase, non-equilibrium effects during evaporation at the interface, and Kelvin effect. The relative importance of these three factors is determined. Variation of wetting properties is modeled through a two-component disjoining pressure, with the main focus on spatially periodic patterns leading to time-periodic variation of the contact line speed. [Preview Abstract] |
Monday, November 20, 2017 12:32PM - 12:45PM |
G38.00010: Wettability dynamics of liquid filaments on horizontal substrates Javier Diez, Pablo Ravazzoli, Ingrith Cuellar, Alejandro Gonzalez We study the hydrodynamic mechanisms involved in the motion of the contact line formed at the end region of a liquid filament laying on a planar and horizontal substrate. Since the flow develops under partially wetting conditions, the tip of the filament recedes and forms a bulged region (head) that subsequently develops a neck region behind it. Later the neck breaks up leading to a separated drop, while the rest of the filament restarts the sequence. One main feature of this flow is that the whole dynamics and final drop shapes are strongly influenced by the hysteresis of the contact angle typical in most of the liquid-substrate systems. The time evolution till breakup is studied experimentally and pictured in terms of a hybrid wettability theory which involves the Cox-Voinov hydrodynamic approach combined with the molecular kinetic theory developed by Blake. The parameters of this theory are determined for our liquid-substrate system (silicone oil–coated glass). The experimental results of the retracting filament are described in terms of a simple heuristic model and compared with numerical simulations of the full Navier-Stokes equations. This study is of special interest in the context of pulsed laser-induced dewetting. [Preview Abstract] |
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