Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session D32: Surface Tension Effects II: Interfacial Flows |
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Chair: Karen Daniels, North Carolina State University Room: 403 |
Sunday, November 24, 2013 2:15PM - 2:28PM |
D32.00001: The Elasto-capillary Landau-Levich Problem George Homsy, Harish Dixit We consider the dip-coating flow problem when the interface has both an elastic bending stiffness and a constant surface tension. In the case where interfacial tension is negligible, we assume the elasticity number $El$ - the ratio of surface elasticity to viscous forces - is small and develop the solution for the free boundary as a matched asymptotic expansion in powers of $El^{1/7}$, thus determining the film thickness as a function of $El$. A remarkable aspect of the problem is the occurrence of multiple solutions, and five of these are found numerically. In any event, the film thickness varies as $El^{4/7}$, or equivalently, $U^{4/7}$, where $U$ is the plate speed, in agreement with previous experiments. The solution for the elasto-capillary problem is formulated in a similar way, with an elasto-capillary number, $\epsilon$, (the ratio of elasticity to surface tension), as an additional parameter. It is possible to connect the problems of pure elasticity and elasto-capillarity respectively through the parameter $\epsilon$, but connecting one of the five elasto-capillary branches to the classical Landau-Levich result result remains an elusive goal. [Preview Abstract] |
Sunday, November 24, 2013 2:28PM - 2:41PM |
D32.00002: Capillarity-Driven Bubble Separations Andrew Wollman, Mark Weislogel, Michael Dreyer Techniques for phase separation in the absence of gravity continue to be sought after 5 decades of space flight. This work focuses on the fundamental problem of gas bubble separation in bubbly flows through open wedge-shaped channel in a microgravity environment. The bubbles appear to rise in the channel and coalesce with the free surface. Forces acting on the bubble are the combined effects of surface tension, wetting conditions, and geometry; not buoyancy. A single dimensionless group is identified that characterizes the bubble behavior and supportive experiments are conducted in a terrestrial laboratory, in a 2.1 second drop tower, and aboard the International Space Station as part of the Capillary Channel Flow (CCF) experiments. The data is organized into regime maps that provide insight on passive phase separations for applications ranging from liquid management aboard spacecraft to lab-on-chip technologies. [Preview Abstract] |
Sunday, November 24, 2013 2:41PM - 2:54PM |
D32.00003: Inertial and Washburn Regimes in Filling of Charged Capillaries Siddhartha Das, Sushanta K. Mitra, J.C.T. Eijkel, N.R. Tas, Suman Chakraborty We discuss the filling dynamics of charged capillaries. Presence of charge on the capillary walls leads to formation of an Electric Double Layer (EDL) at the liquid-capillary-wall interface. Migration of the charge density of the EDL during capillary filling leads to two distinct effects - on one hand it reduces the capillary drive by triggering an opposing electrical force, while on the other hand the induced electroosmotic transport causes a reduction in the net drag force. The ultimate result is a capillary filling process with reduced filling speed, with altered inertial and Washburn regimes, as well as the criteria that dictates the transition between these two regimes. [Preview Abstract] |
Sunday, November 24, 2013 2:54PM - 3:07PM |
D32.00004: Impact on Floating Membranes Nicolas Vandenberghe, Laurent Duchemin We report on an experiment focusing on the wave dynamics triggered by the impact of a sphere on a floating elastic membrane. The thin rubber elastic sheet floats on a pool of water. After impact two distinct waves propagate. First a tensile wave propagates at the speed of sound in the elastic material. Behind the longitudinal wavefront the elastic membrane is stretched. A transverse wave, accompanied by fluid motion, propagates in the stretched region. The transverse wave presents a dispersion relation similar to capillary waves but the equivalent ``surface tension'' is the tension in the membrane, which results from the impact. We investigate the coupling between the two waves, documenting the variation of the ``surface tension'' with impact speed. The deceleration dynamics of the impactor and the instability of the membrane giving rise to wrinkles will also be discussed. [Preview Abstract] |
Sunday, November 24, 2013 3:07PM - 3:20PM |
D32.00005: Marangoni forces in interfacial dilatational rheology Gwynn Elfring, Gary Leal, Todd Squires Many methods for measuring the mechanical properties of fluid interfaces involve generating a flow at the interface with both dilatation and shear, such as by translating a probe through a fluid interface. We examine here the force on a translating probe at an interface laden with a soluble surfactant, that exhibits Newtonian interfacial rheology. We assume that the interface is neither incompressible nor equilibrated. We look at the effects on the force measured by a probe due to small deviations in the concentration field which result from the dilatational flows induced by the probe. In particular we discuss the coupling of concentration relaxation and surface viscosity on Marangoni forces generated by the interfacial flows. [Preview Abstract] |
Sunday, November 24, 2013 3:20PM - 3:33PM |
D32.00006: ABSTRACT WITHDRAWN |
Sunday, November 24, 2013 3:33PM - 3:46PM |
D32.00007: Surfactant on a Thin Liquid Layer: Outward Spreading Ellen Swanson, Stephen Strickland, Michael Shearer, Karen Daniels The spreading of insoluble surfactant molecules on a thin liquid layer can be modeled by a coupled system of two fourth order partial differential equations. The equations for the surfactant concentration and the height of the liquid layer are derived from the Stokes equations using the lubrication approximation, and have been accepted as an accurate prediction of the spreading behavior for over two decades. In experiments, we measure the surfactant concentration profile through fluorescence imaging of tagged lipids, while simultaneously measuring the height profile with laser profilometry. We compare the outward spreading observed in the experiment to that predicted by the numerical simulations of the model, and find inconsistencies for initial surfactant distributions both above and below the critical monolayer concentration. Similar inconsistencies are also found when the surfactant is spreading inward; these are described in the companion talk that follows. [Preview Abstract] |
Sunday, November 24, 2013 3:46PM - 3:59PM |
D32.00008: Surfactant on a Thin Liquid Layer: Self-Healing Dynamics Stephen Strickland, Cameron Conti, Matt Hin, Richard Sayanagi, Karen Daniels, Rachel Levy As in the previous talk, we explore the dynamics of an insoluble surfactant spreading on a thin viscous Newtonian layer. Here, a central disk-shaped surfactant-free region heals, with the fluid layer ideally becoming entirely coated with surfactant. In the initial dynamics of this self-healing, Marangoni forces drive an axisymmetric annular ridge inward to coalesce into a growing central fluid distension, unlike outward spreading in which the ridge decays. In later dynamics, this distension slowly relaxes and the surfactant concentration equilibrates. We measure the surfactant concentration profile through fluorescence imaging of tagged lipids, while simultaneously measuring the height profile with laser profilometry. For surfactant concentrations close to but below the critical monolayer concentration, we observe agreement between the height profiles in the numerical simulations and the experiment, but disagreement in the surfactant distribution. In experiments at lower concentrations, the surfactant spreading and formation of a Marangoni ridge are not present, leaving a hole which is essentially surfactant-free. This observation, not captured at all in simulations, may have undesirable implications for applications such as drug delivery. [Preview Abstract] |
Sunday, November 24, 2013 3:59PM - 4:12PM |
D32.00009: On the Measurement of Longitudinal Interfacial Waves and Surfactant Dynamic Properties N. Washuta, X. Liu, G.M. Korenowski, J.H. Duncan The behavior of longitudinal interfacial waves at a surfactant-laden air-water interface is studied experimentally. The waves are generated in a glass tank (4.5~cm deep, 17.8~cm wide, and 75~cm long) by horizontal oscillation of a thin Teflon barrier, which spans the width of the tank. Local surfactant concentration is measured instantaneously and non-intrusively by using a nonlinear optical method called Second Harmonic Generation (SHG). In this method, a laser pulse with a wavelength of 532 nm is reflected off of the free surface at a 60-degree angle. The reflected beam contains both the original wavelength of 532 nm and its second harmonic, 266 nm. The ratio of the intensity of the reflected 266-nm signal to the 532-nm signal is proportional to the square of the local surfactant concentration at the interface. This SHG signal is recorded at a range of distances from the barrier for several barrier-oscillation frequencies and the resulting data is used to determine the wavelengths, phase speeds and amplitude decay rates of the longitudinal waves. Applying the linear theory of longitudinal waves, these wave propagation characteristics are then used to determine the dynamic properties of the surfactant. [Preview Abstract] |
Sunday, November 24, 2013 4:12PM - 4:25PM |
D32.00010: Marangoni flow induced by alcohol deposition on a water film Federico Hernandez-Sanchez, Antonin Eddi, Jacco Snoeijer Bringing the interfaces of two miscible fluids into contact naturally generates strong gradients in surface tension. Here we investigate such a Marangoni-driven flow by continuously supplying isopropyl alcohol (IPA) on a film of water, using microdrops of IPA-water mixtures. These droplets create a localized depression in surface tension that leads to the opening of a circular hole in the water film, with water being collected in a growing rim at the edge of the hole. The dynamics of the thin film is monitored experimentally using high-speed imaging. We find that the radius of the hole opens as $r \sim t^{1/2}$. This result can be explained from a balance between Marangoni and viscous stresses, assuming that the gradients in surface tension are smoothened out over the entire size of the hole. We derive a scaling law that accurately predicts the influence of the IPA flux as well as the thickness of the thin film at the interior of the hole. [Preview Abstract] |
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