Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session EF: Surface Tension I |
Hide Abstracts |
Chair: Rachel Levy, Duke University Room: Hilton Chicago Continental C |
Sunday, November 20, 2005 4:10PM - 4:23PM |
EF.00001: An arbitrary Lagrangian-Eulerian method for interfacial flows with insoluble surfactants Xiaofeng Yang, Ashley J. James We present an arbitrary Lagrangian-Eulerian (ALE) method for interfacial flows with insoluble surfactants. The interface is captured using a coupled level set and volume of fluid method. By directly tracking the surfactant mass, the method conserves surfactant to machine accuracy, and prevents surfactant from diffusing off the interface. Interfacial area is also tracked. To accurately approximate the interfacial area, the fluid interface is reconstructed using piece-wise parabolas. The surfactant concentration, which determines the local surface tension through an equation of state, is then computed as surfactant mass per interfacial area. The evolution of the level set function, volume fraction, interfacial area, and surfactant mass are performed using a Lagrangian-Eulerian method. Adaptive triangular grids are used for illustration. However, the ALE method can be applied to any kind of grid. The surface tension force is included in the momentum equation. The fluid flow is simulated using a Stokes solver for illustration. Other flow solvers, such as a Navier-Stokes solver or a viscoelastic flow solver, can also be used. [Preview Abstract] |
Sunday, November 20, 2005 4:23PM - 4:36PM |
EF.00002: Wave Patterns in Surfactant-Driven Thin Liquid Films Rachel Levy Thin liquid films driven by surfactant develop complicated wave patterns that are solutions of a coupled system of PDE for the height and surfactant concentration. These solutions can be understood as a combination of similarity solutions, including traveling waves, and discontinuities in the height and surfactant concentration gradient. Thin films with surfactant occur in both biological and industrial applications, in which surfactant is employed to reduce surface tension. In the case of a thin film on an inclined plane, driven by both surfactant and gravity, the downstream height determines which of two distinct wave patterns emerges. The threshold between them is determined analytically and verified by PDE simulations. [Preview Abstract] |
Sunday, November 20, 2005 4:36PM - 4:49PM |
EF.00003: Reactive Spreading and Recoil of Oil on Water Ernst van Nierop, Howard Stone Droplets of oil containing oleic acid were observed to spread, then recoil, on an aqueous solution of sodium hydroxide. Surfactant is produced at the interface during spreading, and spreading is observed to be much faster (radius $R(t) \propto t^{\alpha}$ with $0.64 < \alpha < 0.89$) than what would be expected in the absence of a chemical reaction. It is observed that when using higher concentrations of reagents, spreading is faster and the maximum radius achieved is larger. Drops reach a maximum radius after $t\sim 10-60$~s with $R_{max}\sim 3.1-4.2$ times the initial radius, after which they are observed to recoil slowly with apparent power-law behavior ($-0.34 < \alpha < -0.14$). The fast spreading dynamics require reagent concentrations of order ${\cal O}$(1~mM). [Preview Abstract] |
Sunday, November 20, 2005 4:49PM - 5:02PM |
EF.00004: Liquid trampolines: droplets and spheres bouncing off soap films Laurent Courbin, Howard Stone, Jerome Dubail, Gautier Roux, Suzie Crotiere, David Quere We report on the impact of rigid spheres and liquid droplets on soap films. These systems exhibit a rich variety of dynamics including bouncing and adhesion. In the case of drop impact, at low speeds the drops bounce, while for intermediate speeds the drops can pass through the film without the film breaking. The bouncing can be analyzed using mechanical models. In the case of the impact of rigid spheres, by tuning the physical property of the surface of the impacting sphere, which may or may not be surrounded by a skin of oil, we experimentally observe two distinct regimes: the solid sphere can bounce off of the fluid film or get entrapped. In all cases the film can be considered an absorber of kinetic energy. Finally, the possibility of tuning the bounce of an object will be presented. [Preview Abstract] |
Sunday, November 20, 2005 5:02PM - 5:15PM |
EF.00005: The ``fakir drop'': a (un)stable situation Mathilde Callies-Reyssat, David Qu\'{e}r\'{e} Hydrophobic surfaces can be made super-hydrophobic by creating a texture on them. This effect, sometimes referred to as the ``fakir effect,'' is due to air trapping in the structure, which provides a composite surface made of solid and air on which the deposited drop sits. Here we give evidence for this effect using forests of micro-pillars, made by photolithography and deep etching. This allows us to control the density of solid and air under the drop, and thus the degree of super-hydrophobicity. However, the ``fakir state'' is not always the most stable situation for a drop on a hydrophobic surface. The drop may instead fill the microstructure and this other state, called the ``Wenzel state,'' has very different properties. We show how to observe transitions between these two states. We also achieve materials with a density gradient of micropillars, and discuss the possibility of inducing spontaneous drop motion on such surfaces. [Preview Abstract] |
Sunday, November 20, 2005 5:15PM - 5:28PM |
EF.00006: Surfactant spreading on thin viscous films Angelica Aessopos, Jose Bico, Anette Hosoi A surfactant drop spreading on a thin viscous film may be accompanied by a fingering instability. The spreading is driven by Marangoni stresses and resisted by viscous dissipation in the sublayer. We present experimental results in a variety of regimes showing how the film thickness and viscosity affect the characteristics of the spreading droplet. Spreading data at short time scales, recorded with a high speed camera, are analysed as well as steady state phenomena. A number of new scaling laws are observed. [Preview Abstract] |
Sunday, November 20, 2005 5:28PM - 5:41PM |
EF.00007: The `beating heart': spontaneous oscillations of a sessile lens Roman Stocker, John Bush When an oil drop is emplaced on a water surface, it takes the form of a sessile lens. When the oil contains water-insoluble surfactant, the lens radius may oscillate in a quasi-periodic fashion; when viewed from above, the sessile lens resembles a beating heart. We here elucidate the subtle mechanism responsible for this striking flow structure by presenting the results of our combined experimental and theoretical investigation. [Preview Abstract] |
Sunday, November 20, 2005 5:41PM - 5:54PM |
EF.00008: Effects of surfactants on wakes of surface penetrating cylinders Robert Heitsenrether, Alan Brandt White water, bubble wakes are the most apparent signature of surface ships and their extent is indicative of the degree of mixing in the wake. The coastal ocean generally contains levels of surfactants, due to ocean biologics and anthropomorphic inputs that affect bubble persistence. To investigate the effects of surfactants a series of experimental studies have been conducted on the evolution of white water wakes (WWW) generated by surface penetrating cylinders. The cylinder was towed at speeds ranging from 0.7 -- 2.4 m/s (Froude numbers of 1.02 to 3.50) with varying levels of surfactants that that decreased the surface tension to values comparable to those observed in the coastal ocean. Digital analysis of images collected with a video camera system was used to determine the characteristics of the WWW. It was found that the presence of surfactants increased the WWW area by a factor of two and caused a coalescence of groups of bubbles on the surface that persisted for very long times. It was also found that there is an apparent saturation effect whereby additional surfactant did not further increase the WWW area. [Preview Abstract] |
Sunday, November 20, 2005 5:54PM - 6:07PM |
EF.00009: Surfactant effects on bubble pinch-off Yuan-Nan Young, Jie Li, Michael Siegel, Michael Booty, Demetrius Papageorgiou The effect of surfactant on the pinch-off of an inviscid bubble surrounded by a viscous fluid is studied theoretically and numerically. Equations governing the evolution of the interface and surfactant concentration in zero-Reynolds-number flow are derived using a long wavelength approximation. In the case of soluble surfactant the derivation assumes either zero bulk Peclet number Pe, or infinite Pe. Results of the long wavelength model are compared against numerical simulations of the full Navier-Stokes equations, performed using a highly accurate arbitrary Lagrangian-Eulerian method. The presence of insoluble surfactant significantly retards pinch-off: This is due to the development of a long, slender, quasi-stable cylindrical thread at the location of minimum radius, where the destabilizing influence of capillary pressure is balanced by the internal pressure. For soluble surfactant, depending on parameter values, a filament forms first but pinches off later due to the exchange between bulk and surface surfactants. We will show how the time from filament formation to pinch-off and the bubble shape at pinch-off depends on the parameters values. [Preview Abstract] |
Sunday, November 20, 2005 6:07PM - 6:20PM |
EF.00010: Wetting by surfactant solutions: The role of mass transfer through the contact line E. Ram\'e, K. Varanasi, B. Luokkala, S. Conroy, S. Garoff Surfactants affect wetting dynamics dramatically compared to pure fluids. Theory for insoluble surfactants shows that the rate of surfactant transfer between the free surface and the solid through the contact line can have important consequences for the dynamic contact angle. Simple observation of the wetting behavior of soluble surfactants makes clear the dramatic impact this transfer has on wetting. For systems with strongly attractive surfactant-solid interaction, receding surfactant solutions leave behind surfactant adsorbed at the solid-vapor interface. By contrast, when this interaction is weak, the solid emerges with much less surfactant. To get to the solid-vapor interface, a very complex mass transfer and surfactant rearrangement must occur as the surfactant exchanges among the solid/liquid, solid/vapor and liquid/vapor interfaces, as well as the bulk. We show that all four of these channels play a significant role in the mass transfer and that altering this mass transfer by changing the surfactant-solid interaction has a major impact on the flow fields and mass balance near moving contact lines. We also show that $Ca_c$, the critical capillary number for transition from receding contact line to film pulling, exhibits a discontinuous jump around 0.45 cmc. This jump cannot be explained by the concentration dependence of fluid properties or the static contact angle. [Preview Abstract] |
Sunday, November 20, 2005 6:20PM - 6:33PM |
EF.00011: Observations of hysteresis and mode coupling in capillary bridge oscillations. Philip L. Marston, Wei Wei, David B. Thiessen Large amplitude axisymmetric oscillations of a liquid bridge in a Plateau tank were excited by applying oscillating Maxwell stresses. The liquids were selected to have unusually small kinematic viscosities. The modal frequency response was measured by incrementing the drive frequency. In a narrow range of frequencies the response depended on the direction (downward or upward) of the increments in a way consistent with a lumped-parameter model of hysteresis for a weakly-damped oscillator having a mode-softening nonlinearity. The driven mode was the (3,0) mode having three-halves of an axial wavelength. The slenderness of the bridge was selected so that the third harmonic was the natural frequency of a higher-order mode, the (5,0) mode that has five-halves of a wavelength. The response of that mode at the third harmonic also exhibited hysteresis. The observations are strongly suggestive of a mode-coupling term in the potential energy of the surface deformation. [Supported by NASA.] [Preview Abstract] |
Sunday, November 20, 2005 6:33PM - 6:46PM |
EF.00012: Theory of slope-dependent disjoining pressure with application to Lennard-Jones liquid films. Taeil Yi, Harris Wong A molecule in a thin liquid film may experience additional intermolecular forces if the film thickness h is less than roughly 100 nm. The effect of these intermolecular forces at the continuum level is captured by disjoining pressure P. Since P dominates at small film thicknesses, it determines the stability and wettability of thin films. To leading order, P = P(h) because thin films are generally uniform. This form, however, cannot be applied to films that end at the substrate with non-zero contact angles. We have developed a new theory of slope-dependent disjoining-pressure.[Wu {\&} Wong, J. Fluid Mech. \underline {506}, 157 (2004)] In this theory, the total energy of a drop on a solid substrate is minimized to yield an equilibrium equation that relates P to an excess interaction energy E = E(h, hx). By considering a fluid wedge on a solid substrate, E(h, hx) is found by pairwise summation of intermolecular potentials. This work applies the new theory to Lennard-Jones liquid films. We find a large class of equilibrium drop and meniscus shapes, including a drop with a finite precursor film. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700