Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session R14: Drops: Wetting and Spreading II |
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Chair: Omar K. Matar, Imperial College, London Room: 3009/3011 |
Tuesday, November 25, 2014 1:05PM - 1:18PM |
R14.00001: Laws of spreading: why Tanner, Hoffman, Voinov, Cox and de Gennes were wrong, generally speaking Pirouz Kavehpour, Alireza MohammadKarim For nearly 50 years, most of the researchers in the area of wetting and spreading have used a relationship between the dynamics contact angle and velocity, $\theta^{3}-\theta_{0}^{3} \sim U$, where $\theta $ is dynamics contact angle, $\theta_{0} $ is the equilibrium contact angle and U is the velocity of the wetting line. Different forms of this relationship are known as Tanner's law, Hoffman-Voinov-Tanner law or Cox model, all of them are derived based on hydrodynamics assumptions. In this talk, we will discuss several common situations that this relationship is not valid and we propose a new way to look at spreading problem and its underlying physics. Our experimental result agrees with this interpretation of spreading dynamics. [Preview Abstract] |
Tuesday, November 25, 2014 1:18PM - 1:31PM |
R14.00002: The superspreading mechanism unveiled via molecular dynamics simulations Panagiotis Theodorakis, Erich Muller, Richard Craster, Omar Matar Superspreading, by which aqueous droplets laden with specific surfactants wet hydrophobic substrates, is an unusual and dramatic phenomenon. This is attributed to various factors, e.g., a particular surfactant geometry, Marangoni flow, unique solid-fluid interactions, however, direct evidence for a plausible mechanism for superspreading has not yet been provided. Here, we use molecular dynamics simulations of a coarse-grained model with force fields obtained from the SAFT-$\gamma$ equation of state to capture the superspreading mechanism of water drops with surfactants on model surfaces. Our simulations highlight and monitor the main features of the molecular behavior that lead to the superspreading mechanism, and reproduce and explain the experimentally-observed characteristic maxima of the spreading rate of the droplet vs. surfactant concentration and wettability. We also present a comparison between superspreading and non-superspreading surfactants underlining the main morphological and energetic characteristics of superspreaders. We believe that this is the first time a plausible superspreading mechanism based on a microscopic description is proposed; this will enable the design of surfactants with enhanced spreading ability specifically tailored for applications. [Preview Abstract] |
Tuesday, November 25, 2014 1:31PM - 1:44PM |
R14.00003: Kinetics of spreading of surfactant solutions Victor Starov, Nina Kovalchuk, Anna Trybala, Omar Matar Wetting properties of surfactant solutions are determined by adsorption of surfactant at all interfaces involved. Adsorption on liquid/air and liquid/solid interface depends on surfactant chemistry. That is why the lower surface tension does not result automatically in better wetting properties. Spreading of surfactant solutions causes redistribution of surfactant at the interface and in the bulk. As a result surface concentration gradients appear and spreading kinetics is influenced by solutal Marangoni effect. Disjoining pressure, being the driving force of spreading also depends on the local surfactant concentration. Therefore spreading kinetics of surfactant solutions differ considerably from those of pure liquids. The results of experimental study on spreading kinetics of synergetic surfactant mixtures on hydrophobic substrates such as polyethylene and sylanised glass are presented for the two different regimes: complete and partial wetting and compared with the spreading kinetics of a pure liquid in those regimes. [Preview Abstract] |
Tuesday, November 25, 2014 1:44PM - 1:57PM |
R14.00004: Dynamic Wetting in a Non-Equilibrium Gas: The Effect of Gas Pressure on Air Entrainment James Sprittles Experimentally, it is now well established that lowering the pressure of an ambient gas can suppress wetting failures, or ``air entrainment,'' at a liquid-solid-gas moving contact-line in both coating processes and drop impact dynamics. In this work, we consider the possibility that non-equilibrium effects in the gas are responsible for such phenomena. These can be included into a continuum framework by allowing for slip at both the solid-gas and liquid-gas interfaces, caused by Knudsen layers attached to these boundaries, which is related to the mean free path in the gas, and hence the ambient pressure. This model has been incorporated into a computational framework developed for dynamic wetting phenomena, which resolves all scales in the problem, so that these new effects can be investigated. It is shown that reductions in gas pressure, and hence increases in slip, can dramatically modify the flow field in the gas-film in front of a moving contact-line so that air entrainment is prevented. Specifically, in a dip-coating setup it is shown that the new model (a) describes experimental results for the critical wetting speed at a given gas pressure and (b) allows us to identify new parameters associated with the non-equilibrium gas dynamics which govern the contact-line's motion. [Preview Abstract] |
Tuesday, November 25, 2014 1:57PM - 2:10PM |
R14.00005: Different Shades of Oxide: Wetting Mechanisms of Gallium-based Liquid Metal Drops Kyle Doudrick, Shanliangzui Liu, Eva M. Mutunga, Kate L. Klein, Viraj Damle, Kripa K. Varanasi, Konrad Rykaczewski Gallium-based liquid metals are of interest for a number of applications including biomedical devices, flexible electronics, and soft robotics. Yet, device fabrication with these materials is challenging because they adhere strongly to majority of common substrates. This unusually high adhesion is attributed to the formation of a thin gallium oxide shell, however, its role in the adhesion process has not yet been determined. Here, we show that, dependent on formation process and resulting morphology of the liquid metal-substrate interface, Galinstan adhesion can occur in two modes. The first mode occurs when the oxide shell is not broken as it comes in contact with the surface. Because of the nanoscale topology of the oxide, this mode results in minimal adhesion between the liquid metal and most solids, regardless of substrate's surface energy or texture. In the second mode, the formation of the Galinstan-substrate interface involves breaking of the original oxide skin and formation of a composite interface that includes contact between the substrate and pieces of old oxide, bare liquid metal, and new oxide. We show that in this mode Galinstan adhesion is dominated by the new oxide-substrate contact. [Preview Abstract] |
Tuesday, November 25, 2014 2:10PM - 2:23PM |
R14.00006: Surface structure determines dynamic wetting Junichiro Shiomi, Jiayu Wang, Minh Do-Quang, James Cannon, Feng Yue, Yuji Suzuki, Gustav Amberg Dynamic wetting, the spontaneous spreading process after droplet contacts a solid surface, is important in various engineering processes, such as in printing, coating, and lubrication. In the recent years, experiments and numerical simulations have greatly progressed the understanding in the dynamic wetting particularly on ``flat'' substrates. To gain further insight into the governing physics of the dynamic wetting, we perform droplet-wetting experiments on microstructured surfaces, just a few micrometers in size, with complementary numerical simulations, and investigate the dependence of the spreading rate on the microstructure geometries and fluid properties. We reveal that the influence of microstructures 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. The systematic study is also of practical importance since structures and roughness are omnipresent and their influence on spreading rate would give us additional degrees of freedom to control the dynamic wetting. [Preview Abstract] |
Tuesday, November 25, 2014 2:23PM - 2:36PM |
R14.00007: Different regimes of dynamic wetting Amberg Gustav, Jiayu Wang, Minh Do-Quang, Junichiro Shiomi Dynamic wetting, as observed when a droplet contacts a dry solid surface, is important in various engineering processes, such as printing, coating, and lubrication. Our overall aim is to investigate if and how the detailed properties of the solid surface influence the dynamics of wetting. Here we discuss 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. This is complemented by matching numerical simulations. We present a parameter map, based on the properties of the liquid and the solid surface, which identifies qualitatively different spreading regimes, where the spreading speed is limited by either the liquid viscosity, the surface properties, or the liquid inertia. The peculiarities of the different spreading regimes are studied by detailed numerical simulations, in conjuction with experiments. [Preview Abstract] |
Tuesday, November 25, 2014 2:36PM - 2:49PM |
R14.00008: Dynamic wetting on anisotropic patterned surfaces Minh Do-Quang, Jiayu Wang, Satoshi Nita, Junichiro Shiomi, Gustav Amberg Dynamic wetting, as occurs when a droplet of a wetting liquid is brought in contact with a dry solid, is important in various engineering processes, such as printing, coating, and lubrication. Our overall aim is to investigate if and how the detailed properties of the solid surface influence the dynamics of wetting. We have recently quantified the hindering effect of fairly isotropic micron-sized patterns on the substrate. Here we will study highly anisotropic surfaces, such as parallel grooves, either perpendicular or parallel to an advancing contact line. This is done by detailed phase field simulations and experiments on structured silicon surfaces. The dynamic wetting behavior of drops on the grooved surfaces is governed by the combined interplay of the wetting line friction and the internal viscous dissipation. Influence of roughness is quantified in terms of the energy dissipation rate at the contact line using the experiment-simulation combined analysis. The energy dissipation of the contact line at the different part of the groove will be discussed. The performance of the model is assessed by comparing its predictions with the experimental data. [Preview Abstract] |
Tuesday, November 25, 2014 2:49PM - 3:02PM |
R14.00009: Stick-slip motion and hysteresis behaviour of droplets with dynamic volume variation Marc Pradas, Nikos Savva, Jay B. Benziger, Ioannis G. Kevrekidis, Serafim Kalliadasis We investigate the dynamics of a droplet on planar substrate as its volume increases or decreases. We adopt a diffuse-interface model that incorporates an inflow/outflow boundary condition at the bottom-center of the droplet, hence allowing to dynamically control its volume, and we consider a topographically smooth substrate with a periodic chemical pattern. We observe that the droplet undergoes a stick-slip motion as the volume is increased (inflow conditions) which can be monitored by e.g. looking at the contact points. When we switch over to outflow conditions (i.e. the volume decreases) the droplet follows a different path giving rise to a hysteresis behaviour. By means of geometrical arguments we are able to theoretically predict the full bifurcation diagram of the equilibrium points of the system as the droplet volume is changed, finding excellent agreement with time-dependent computations. [Preview Abstract] |
Tuesday, November 25, 2014 3:02PM - 3:15PM |
R14.00010: Analogy between coalescence and spreading for water Su Jin Lim, E. Grace Kim, Kamel Fezzaa, Jung Ho Je, Byung Mook Weon A water drop gently placed on a flat solid surface rapidly spreads, while on a flat water surface, the drop rapidly merges into the water surface. Here we utilize high-speed X-ray microscopy to explore the initial coalescence of a water drop into a flat water surface: there clearly exists analogy in initial dynamics of coalescence and spreading. By comparing experimental and numerical results taken by Lattice Boltzmann simulations, we attribute the analogy to the hydrodynamic nature of water. The coalescence-spreading analogy for water would be important with respect to the universality of coalescence and spreading between liquids and solids (S.J.L. and E.G.K. equally contributed). [Preview Abstract] |
Tuesday, November 25, 2014 3:15PM - 3:28PM |
R14.00011: A Cahn-Hilliard framework for spreading in the partial-wetting regime Amir Pahlavan, Michael Chen, Luis Cueto-Felgueroso, Gareth McKinley, Ruben Juanes When a liquid puddle spreads on a solid surface in the complete-wetting regime, gravity is the dominant driving force and it has been shown that the dynamics close to the contact line has no influence on the rate of spreading. In the partial-wetting regime however, the spreading puddle transitions away from the gravity-dominated regime, slows down, and finally comes to an stop when it reaches the compactly-supported equilibrium state. Therefore, the contact-line dynamics cannot be neglected in the partial-wetting regime. The existing models (i.e. Cox-Voinov) for contact line dynamics compare well with experimental observations in the capillary-dominated regime, but when gravity is the main driving force, the spreading dynamics deviates from that in the capillary-dominated regime. In this work, we develop an energetic description for the system that leads to the Cahn-Hilliard framework and a generalized thin-film equation. The contact line dynamics emerges naturally as part of the solution in our model and is therefore coupled with the bulk flow. We further show that the developed model compares well with our large-scale puddle spreading experiments in the partial-wetting regime. [Preview Abstract] |
Tuesday, November 25, 2014 3:28PM - 3:41PM |
R14.00012: Slip effects in a dewetting polymer microdroplets T.S. Chan, J.D. McGraw, S. Maurer, T. Salez, M. Benzaquen, {\'E} Rapha{\"e}l, K. Jacobs, M. Brinkmann A non-equilibrium liquid drop sitting on a smooth substrate will contract or spread depending on the equilibrium contact angle and the initial shape of the drop. Previous studies assume a huge separation of length scales between the drop contact size $R$ and the slip length $b$ (typically $b/R$ = 10$^{-6}$-10$^{-5}$). One well known example is that of a drop spreading over a completely wetting surface, which follows Tanner's law. In this study, we experimentally and theoretically investigate contractions of microscopic droplets in regimes where these two length scales are not widely separated ($b/R$ = 10$^{-2}$-1). These regimes become relevant in micro- and nano-fluidic systems. Instead of a quasi-static spherical shape during the evolution, the profiles display more complex shapes in these regimes. We find that: 1) the interface profile near the contact line evolves in a self-similar way in the early stage; 2) depending on $b/R$, the profile can develop a characteristic bump shape in the intermediate stage of the evolution. 3) at late times, the radius saturates exponentially with a certain time scale, which depends on the slip length. [Preview Abstract] |
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