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 L2: Surface Tension Effects: Interfacial Phenomena |
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Chair: Valeria Garbin, Imperial College London Room: 3002 |
Monday, November 24, 2014 3:35PM - 3:48PM |
L2.00001: Modeling Interfacial Adsorption of Polymer-Grafted Nanoparticles Xin Yong Numerous natural and industrial processes demand advances in our fundamental understanding of colloidal adsorption at liquid interfaces. Using dissipative particle dynamics (DPD), we model the interfacial adsorption of core-shell nanoparticles at the water-oil interface. The solid core of the nanoparticle encompasses beads arranged in an fcc lattice structure and its surface is uniformly grafted with polymer chains. The nanoparticles bind to the interface from either phase to minimize total surface energy. With a single nanoparticle, we demonstrate detailed kinetics of different stages in the adsorption process. Prominent effect of grafted polymer chains is characterized by varying molecular weight and polydispersity of the chains. We also preload nanoparticles straddling the interface to reveal the influence of nanoparticle surface density on further adsorption. Importantly, these studies show how surface-grafted polymer chains can alter the interfacial behavior of colloidal particles and provide guidelines for designing on-demand Pickering emulsion. [Preview Abstract] |
Monday, November 24, 2014 3:48PM - 4:01PM |
L2.00002: Particle-laden microbubbles: forced oscillations, surface modes, jetting and particle ejection Vincent Poulichet, Valeria Garbin Self-assembly of microparticles at fluid-fluid interfaces is exploited for emulsion stabilization, tunable nanomaterials, and nanocomposites with complex morphologies. Disassembly of interfacial particle monolayers is equally important, for instance for green catalytic processes, but has been far less explored. We demonstrate controlled disassembly of monolayers of microparticles trapped at the interface of microbubbles. The bubbles are driven into oscillations by an applied ultrasound wave, triggering particle ejection. We visualize forced ejection events at the single particle level using high-speed video microscopy. Measurements of the local area density of particles and of the acceleration of the bubble interface reveal that the interplay of several mechanisms is responsible for particle expulsion: tangential stresses due to the fast compression of the bubble interface, interparticle interactions at the interface, and the body force acting on the particles due to the acceleration of the interface. Non-linear bubble dynamics can also be exploited to design complex particle expulsion scenarios, such as surface modes and jetting, with relevance to directed particle delivery in microreactors. [Preview Abstract] |
Monday, November 24, 2014 4:01PM - 4:14PM |
L2.00003: Viscous Marangoni migration of a drop in a Poiseuille flow at low surface Peclet numbers On Shun Pak, Jie Feng, Howard Stone The motion of a spherical drop with a bulk-insoluble surfactant immersed in a background flow in the low surface Peclet number and low Reynolds number limits is investigated. We develop a reciprocal theorem that applies to any prescribed background flow, and provide a specific example of an unbounded Poiseuille flow. Analytical formulas for the migration velocity of the drop are obtained perturbatively in powers of the surface Peclet number. We show that the redistribution of surfactant due to the background flow acts to retard the motion of the drop, with the magnitude of this slip velocity independent of the drop's position in the Poiseuille flow. Moreover, a surfactant-induced cross-streamline migration of the drop occurs towards the center of the Poiseuille flow, with its magnitude depending linearly with the distance of the drop from the center of the Poiseuille flow. [Preview Abstract] |
Monday, November 24, 2014 4:14PM - 4:27PM |
L2.00004: Volume of fluid simulations of liquefied metal nanofilms with Marangoni effects Ivana Seric, Kyle Mahady, Shahriar Afkhami, Lou Kondic We present a method for including temperature dependent surface tension in a volume of fluid based Navier Stokes solver. The tangential gradient of the surface tension is implemented using an extension of the classical continuum surface force model that has been previously used for constant surface tension simulations. We apply the developed method to consider metal films liquefied by a pulse laser and discuss the effects of the resulting Marangoni stresses on the film evolution. [Preview Abstract] |
Monday, November 24, 2014 4:27PM - 4:40PM |
L2.00005: Film deposition on a partially wetting plate withdrawn from a liquid reservoir Peng Gao, Lei Li A partially wetting plate withdrawn from a liquid reservoir causes the deposition of liquid films, which are characterized by trapezoidal or triangular shapes. Interesting issues include the critical condition of the film deposition, the film structures and the dependence on the plate speed of the contact-line inclination angle. In the first part of this work, we performed numerical simulations of the problem with a diffuse-interface method, and reproduced the coexistence of the thick and thin films observed in recent experiments (Phys. Rev. Lett, 2006, 96, 174504, and Phys. Rev. Lett, 2008, 100, 244502). We demonstrated that the apparent contact angle vanishes at the onset of wetting transition, consistent with the lubrication theory. The critical condition for the onset of thin films was also quantified. In the second part of this work, we presented a lubrication analysis of films with inclined contact lines. It is shown that the traditional model of constant normal speed of the contact line is only a leading-order approximation; the normal speed actually exhibits a weak decrease with the inclination angle. In addition, the inclination of the contact line results in a tangential flux of the liquid. Simple scaling relations are provided for both the normal velocity and the flux. [Preview Abstract] |
Monday, November 24, 2014 4:40PM - 4:53PM |
L2.00006: Dynamics of a solid sphere bouncing on or penetrating through a liquid-air interface Seong Jin Kim, Sunghwan Jung, Sungyon Lee In this study, we investigate the dynamics of a solid particle moving from liquid to air through a liquid-air interface. The experimental setup consists of an air-piston system that shoots a solid particle into water towards the free surface from below. Experimental results indicate that the particle either penetrates or bounces back depending on the particle size, impact speed, and surface tension. In particular, the particle needs to overcome the resistive interfacial forces in order to penetrate through the liquid-air interface. This transition from bouncing to penetration regimes is captured theoretically by conducting a simple force balance and is further compared with experiments. [Preview Abstract] |
Monday, November 24, 2014 4:53PM - 5:06PM |
L2.00007: Marangoni-buoyancy convection in binary fluids under varying noncondensable concentrations Yaofa Li, Minami Yoda Marangoni-buoyancy convection in binary fluids in the presence of phase change is a complex and poorly understood problem. Nevertheless, this flow is of interest in evaporative cooling because solutocapillary stresses could reduce film dryout. Convection was therefore studied in methanol-water (MeOH-H$_2$O) layers of depth $h \approx 1-3$~mm confined in a sealed rectangular cell driven by horizontal temperature differences of $\sim 6 ^\circ$C applied over $\sim5$~cm. Particle-image velocimetry (PIV) was used to study how varying the fraction of noncondensables ({\it i.e.}, air) $c_{\rm a}$ from $\sim7$~mol\% to ambient conditions in the vapor space affects soluto- and thermocapillary stresses in this flow. Although solutocapillary stresses can be used to drive the flow towards hot regions, solutocapillarity appears to have the greatest effect on the flow at small $c_{\rm a}$, because noncondensables suppress phase change and hence the gradient in the liquid-phase composition at the interface. Surprisingly, convection at $c_{\rm a} \approx 50$\% leads to a very weak flow and significant condensation in the central portion of the layer {\it i.e.}, away from the heated and cooled walls). [Preview Abstract] |
Monday, November 24, 2014 5:06PM - 5:19PM |
L2.00008: The Capillary Fluidics of Espresso Nathan Ott, Drew Wollman, John Graf, Mark Weislogel Espresso is enjoyed by tens of millions of people daily. The coffee is distinguished by a complex low density colloid of emulsified oils. Due to gravity, these oils rise to the surface forming a foam lid called the crema. In this work we present a variety of large length scale capillary fluidic effects for espresso in a gravity-free environment. Drop tower tests are performed to establish brief microgravity conditions under which spontaneous capillarity-driven behavior is observed. Because the variety of espresso drinks is extensive, specific property measurements are made to assess the effects of wetting and surface tension for `Italian' espresso, caffe latte, and caffe Americano. To some, the texture and aromatics of the crema play a critical role in the overall espresso experience. We show how in the low-g environment this may not be possible. We also suggest alternate methods for enjoying espresso aboard spacecraft. [Preview Abstract] |
Monday, November 24, 2014 5:19PM - 5:32PM |
L2.00009: Relaxation of an elastic filament on a viscous interface Joel Marthelot, S. Ganga Prasath, Narayanan Menon What is the shape of an elastic filament floating on a fluid interface? We observe the time-dependent relaxation of a bent filament reopening towards its straight, stress-free, configuration. We study a regime in which the dynamics are overdamped, but with the rod initially bent into a geometrically nonlinear regime. The dynamics of reopening are governed by a competition between the viscous drag of the liquid and the bending elastic force of the rod. We study the relaxation of shape as a function of the length, diameter and elasticity of the rod, and the viscosity of the fluid interface. The opening dynamics are governed by a time scale that is much smaller than a time constructed from these quantities, but scales in the same way. This simple system could provide an easy method to characterize interfacial properties of fluid interfaces. [Preview Abstract] |
Monday, November 24, 2014 5:32PM - 5:45PM |
L2.00010: Revised Capillary Breakup Rheometer Method Louise Lu, William Schultz, Michael Solomon Rather than integrate the one-dimensional equation of motion for a capillary breakup rheometer, we take the axial derivative of that equation. This avoids the determination of the axial force with all of its complications and correction factors. The resulting evolutionary equation that involves either two or four derivatives of the capillary radius as a function of the axial coordinate determines the ratio of elongational viscosity to surface tension coefficient. We examine several silicone and olive oils to show the accuracy of the method for Newtonian fluids. We will discuss our surface tension measurement techniques and briefly describe measurements of viscoelastic materials, including saliva. [Preview Abstract] |
Monday, November 24, 2014 5:45PM - 5:58PM |
L2.00011: Partial coalescence of sessile drops with different liquids Rodica Borcia, Michael Bestehorn We examine numerically the interaction between two deformable drops consisting of two perfectly miscible liquids sitting on a solid substrate under a given contact angle. Driven by solutal Marangoni forces, several distinct coalescence regimes are achieved after the droplets collision [1]. Phase diagrams for different control parameters are emphasized, which give predictions about drop behavior along the solid substrates, control of various interfacial effects, manipulations of tiny droplets in micro- and nano-fluidic devices without power supply, design of droplets or cleaning surfaces.\\[4pt] [1] R. Borcia, M. Bestehorn, {\it{Langmuir}} {\bf{29}} (2013) 4426; {\it{Fluid Dynamics Research}} {\bf{46}} (2014) 041405. [Preview Abstract] |
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