Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session HW: Surface Tension II |
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Chair: Iskander Akhatov, North Dakota State University Room: Hyatt Regency Long Beach Regency C |
Monday, November 22, 2010 10:30AM - 10:43AM |
HW.00001: Elastocapillary snapping Arnaud Antkowiak, Aurelie Fargette, Sebastien Neukirch An elastica buckled in the form of an arch is subjected to a transverse force. Above a critical load value, the buckling mode is switched and the elastica takes the form of a reversed arch. This is the well-known snap-through phenomenon which has been extensively studied in solid mechanics. Here, we revisit this phenomenon and show that capillary forces may promote snapping of a buckled polymer strip. We report detailed experiments of this new paradigm for elasto-capillary interactions, and the obtained results are in close agreement with a simple elastic stability theory. [Preview Abstract] |
Monday, November 22, 2010 10:43AM - 10:56AM |
HW.00002: Capillary bond between rod-like microparticles at interfaces Lorenzo Botto, Kathleen J. Stebe Elongated microparticles at a fluid interface create interface distortions; when deformations of neighboring particles overlap, the particles attract to minimize the interfacial energy. Two elongated microparticles have been studied -ellipsoids, which are predicted to assemble side-to-side, and cylinders, which chain end-to-end. The differences can be attributed to near field interactions, which we term the capillary bond. We simulate the capillary bond between two elongated particles as a function of inter-particle separation, relative orientation, and particle shape. The particle is represented as a super-ellipsoid, a parameterization which allows the study of a broad class of shapes, from ellipsoids to cylinders with rounded or sharp corners, upon varying a single parameter. The geometric details of the particles have a dramatic effect on the dependence of the capillary bond on the configuration, a finding with strong consequences for micro-structures formed by these particles. [Preview Abstract] |
Monday, November 22, 2010 10:56AM - 11:09AM |
HW.00003: Using Convective Flow to Reach the Kinetic Limit of Surfactant Transport to a Stationary Microbubble Nicolas Alvarez, Doug Vogus, Lynn Walker, Shelley Anna Surfactant transport is characterized near microscale interfaces in the presence of flow. We infer transport mechanisms by simultaneously measuring the radius of and the pressure jump across the interface of a micron size bubble. The surfactant is dissolved in the liquid surrounding the bubble. Flow is introduced to minimize concentration gradients caused by diffusion. The dynamic surface tension is monitored at initially clean interfaces for different flow rates. The kinetic limit is achieved as the flow rate increases. The observed dynamics are interpreted in the context of a scaling analysis and a one dimensional convective transport model. This device and theory will be instrumental in measuring and modeling kinetic exchange dynamics at fluid-fluid interfaces for more complex surface-active species, including mixed surfactants, polyelectrolytes, and biomolecules. [Preview Abstract] |
Monday, November 22, 2010 11:09AM - 11:22AM |
HW.00004: Wicking flow through microfluidic channels Hadi Mehrabian, Peng Gao, James J. Feng Diffuse-interface models can be used to simulate contact line motion on solid substrates by regularizing the singularity by diffusion. Using the Cahn-Hilliard model and a finite-element algorithm, we have computed wicking flows in microfluidic channels of three types of geometries. The first type features axisymmetric tubes with contractions and expansions of the cross section. Both drainage and imbibition dynamics are studied, and we define critical conditions for the contact line to negotiate sharp corners on the wall. The second type consists of bifurcations in micro-channels where the competition between capillary pressure in the branches and viscous loss in the feeding tube produces different flow patterns. Finally, we examine tortuous channels in Z and U-shaped turns, where the effect of streamline on the flow rate is analyzed as a prototype for tortuosity in porous medium. [Preview Abstract] |
Monday, November 22, 2010 11:22AM - 11:35AM |
HW.00005: Migration of droplets driven by thermocapillary stress Juan M. Gomba, George M. Homsy We study the effect of wettability on the flow of droplets driven by thermocapillary effects. An equation for the thickness profile of the droplet is derived by employing lubrication approximations. The model includes the effect of the contact angle introduced through a disjoining-conjoining pressure term. For complete wetting or low contact angles, the droplet spreads into a long film profile with a capillary ridge near the leading edge, a behaviour that resembles the experiments on Marangoni films reported by Ludviksson \& Lightfoot (1971). A self similar solution for the profile of the film and an expression for the non constant velocity of the leading edge are presented. For high contact angles, the droplet moves with a constant velocity as a single entity. Here, the effect of the disjoining pressure is strong enough to keep the droplets almost undistorted from its static shape. This regime is the usual one reported in experiments on thermocapillary migration of droplets. An expression for the velocity is derived. For intermediate values of the contact angle the Marangoni stress and the disjoining-conjoining pressure compete and, accordingly, the behaviour is transient and complex. The occurrence of these three regimes and their dependence on various parameters is analyzed. [Preview Abstract] |
Monday, November 22, 2010 11:35AM - 11:48AM |
HW.00006: On the structure of Marangoni-driven singularities Rouslan Krechetnikov This work presents an analytical study of the structure of steady Marangoni-driven singularities. While the results are applicable to a wide class of phenomena, the analysis is performed on the example of tip-streaming, which is driven by chemical-reaction producing a surfactant at the interface of a two-phase system. Due to the conical symmetry of the problem, there exist self-similar solutions of the Stokes equations, which are singular at the tip and thus provide no information on the thread structure which is responsible for tip-streaming. This cone-tip singularity is resolved with the help of asymptotic matching of the self-similar and thread solutions using thin layer (slender jet) approximation, which gives explicit asymptotic formulas for the scaling of the thread radius and thus of the emitted droplets as a function of physical parameters. [Preview Abstract] |
Monday, November 22, 2010 11:48AM - 12:01PM |
HW.00007: On the true nature of chemical reaction-driven tip-streaming Hans C. Mayer, Rouslan Krechetnikov In the course of recreating the experiments of J. Fernandez and G.M. Homsy [Phys. Fluids 16, 2548 (2004)] on chemical reaction-driven tip-streaming in a pendant drop, we identified the true source of this `amazing' phenomenon and thus provided physical interpretation to the unexplained observations in the aforementioned work. This finding added a new parameter dimension to the problem and led to its more complete parametric study. In particular, we studied the effect of the key physical parameters (chemical reaction rate, surfactant properties, etc.) on the emission frequency and size of drops, emitted in both the oscillating and steady tip-streaming modes. Altogether, the conducted study made the experiments more controllable and understanding of the phenomena less ``elusive.'' [Preview Abstract] |
Monday, November 22, 2010 12:01PM - 12:14PM |
HW.00008: A VOF-based method for the simulation of thermocapillary flow Chen Ma, Dieter Bothe This contribution concerns 3D direct numerical simulation of surface tension-driven two-phase flow with free deformable interface. The two-phase Navier-Stokes equations together with the energy balance in temperature form for incompressible, immiscible fluids are solved. We employ an extended VOF (volume of fluid) method, where the interface is kept sharp using the PLIC-method (piecewise linear interface construction). The surface tension, modeled as a body force via the interface delta-function, is assumed to be linearly dependent on temperature. The surface temperature gradient calculation is based on carefully computed interface temperatures. Numerical results on thermocapillary migration of droplets are obtained for a wide range of Marangoni numbers. Both the terminal and initial stage of the migration are studied and very good agreement with theoretical and experimental results is achieved. In addition, simulation of the B\'{e}nard-Marangoni instability in square containers with small aspect ratio and high-Prandtl-number fluids is discussed concerning the development and numbers of convection cells in relation to the aspect ratio. [Preview Abstract] |
Monday, November 22, 2010 12:14PM - 12:27PM |
HW.00009: Capillary interactions between elongated microparticles: A Pair Potential Kathleen Stebe, Eric Lewandowski, Lorenzo Botto Particles at fluid interfaces create distortions. When distortions from neighboring particles overlap, the area, and hence the energy, decreases if the particles migrate toward each other. Elongated particles orient as they approach, and have preferred orientations upon assembly. For elongated microparticles like ellipsoids or cylinders, the interface distortion resembles an elliptical quadrupole a few radii away from the particle surface. We present an anisotropic pair potential based on elliptical quadrupoles. This potential predicts an attractive force and a torque, which depend strongly on aspect ratio, in keeping with experiment on cylinders at interfaces. Particle trajectories and angular orientations recorded by video microscopy for cylinders agree with the predicted potential. In particular, the analysis predicts the rate of rotation, a feature lacking in prior analyses. Open issues associated with near field effects are briefly discussed. [Preview Abstract] |
Monday, November 22, 2010 12:27PM - 12:40PM |
HW.00010: Colloidal dynamics near an interface Madhav Mani, Vinothan Manoharan, Michael Brenner, David Kaz, Ryan Mcgorty Although the equilibrium state of a colloidal particle at an interface is well understood, the dynamics associated with the approach to equilibrium is not. Recent high-resolution experiments have shown that the dynamics are richer than expected. This part of the study focuses on the evolution of the system after the initiation of a contact-line. We model the dynamics associated with the three degrees of motion in this regime, the center of mass (c.o.m.) of the colloid, the location of the contact-line and the dynamic contact-angle. Following Nikolov et al. (Journal of Colloid and Interface Science - 112,1,1986), we derive the statements of force balance by taking variations of an energy functional. Appealing to a balance of power we are able to derive the dynamical laws. Associated with the degrees of motion are three modes of dissipation corresponding to a moving c.o.m., a moving contact-line and an evolving contact angle. We derive an asymptotically valid model for the system, which we integrate numerically and compare to experiments. [Preview Abstract] |
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