Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session P12: Drops, Bubbles and Interfaces II |
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Sponsoring Units: DFD Chair: Alberto Fernandez-Nieves, Georgia Institute of Technology Room: 271 |
Wednesday, March 15, 2017 2:30PM - 2:42PM |
P12.00001: Deformations of soap bubbles in a uniform electric field Sebastien Mawet, Herve Caps, Stephane Dorbolo The study of the deformations undergone by a soap bubble submitted to an electrical force began with Taylor and Wilson \footnote{C.T.R Wilson \& G.I. Taylor,Math Proc Cambridge \textbf{22}, 728 (1925)} and the observation of so-called Taylor's cones. Beyond this particular structure, few studies analyzed the bubble deformations. For example, what is the link between the deformations and the electrical force or how do charges move in the thin soap film formed by the bubble ? To answer those questions, we characterize the shape variations of the surface of the bubble immersed in the uniform electric field of a plan capacitor. In particular, our study focuses on hemispherical bubbles lying on the bottom electrode of a plane capacitor. This study allows us to observe some interesting phenomena like the appearance of a hysteresis cycle in the deformation amplitude. [Preview Abstract] |
Wednesday, March 15, 2017 2:42PM - 2:54PM |
P12.00002: Emission modes in electro co-flow Josefa Guerrero Millan, Antonio J Hijano, Miguel A Lobato, Ignacio G Loscertales, Alberto Fernandez-Nieves We use glass-based microfluidic devices to study the emission regimes in electro co-flow. In contrast to classical electrospray, in electro-coflow a liquid is ejected through a nozzle into another co-flowing liquid. As a result, additional parameters provide control over the emission; these include the viscosity and flow rate of the outer, co-flowing liquid. These two new variables affect the parametric window where typical emission modes in electrospray are observed, and result in the observation of new modes that have not been reported before. [Preview Abstract] |
Wednesday, March 15, 2017 2:54PM - 3:06PM |
P12.00003: Wetting dynamics beneath fluid drops impacting on hot surfaces Kirsten Harth, Michiel A. J. van Limbeek, Minori Shirota, Chao Sun, Detlef Lohse Fluid droplets encountering a phase transition when they impact a target surface are involved in many applications, e.g., spray cooling or painting / coating, ink-jet and 3D printing, soldering, firefighting using sprinklers. Drop impact on hot plates is an emerging topic, involving a complex interplay of hydrodynamics, heat flux and the occurring phase transition, involving large spatial and temporal gradients. Whether and to what extent droplets touch the surface is of immense importance for the overall heat transfer. High-speed total internal reflection imaging allows us to discriminate wetted and vapor-covered regions of the substrate. We study the transient wetting behaviour of the plate by varying the latent heat of the droplet. The characteristic cooling time of the plate is not solely determined by the plate properties. In addition to current literature, we show that in those cases the wetting pattern is both spatially and temporally inhomogeneous. [Preview Abstract] |
Wednesday, March 15, 2017 3:06PM - 3:18PM |
P12.00004: A Microfluidic Route to Breaking Chiral Symmetry: Theory and Experiment Samuel Ocko, Laura Adams A robust route for the biased production of single handed chiral structures has been found in generating non-spherical, multi-component double emulsions using glass microfluidic devices. The specific type of handedness is determined by the final packing geometry of four different inner drops inside an ultra-thin sheath of oil. Before the three dimensional chiral structures are formed, the quasi-one dimensional chain of four inner drops re-arranges in two dimensions into either checkerboard or stripe patterns. We derive an analytical model predicting which pattern is more likely and assembles in the least amount of time. Moreover, our model accurately predicts our experimental results and is based on local bending dynamics, rather than global surface energy minimization. We gratefully acknowledge Professors D. Weitz and L. Mahadevan’s support. [Preview Abstract] |
Wednesday, March 15, 2017 3:18PM - 3:30PM |
P12.00005: Influence of Inertial, Visous and Capillary Effects on the Apical Behavior of Taylor Cone Formation in Liquid Metals Theodore Albertson, Sandra Troian Above a critical applied field strength, the surface of a liquid metal can deform into a conical shape whose apex can emit ions. The precursor shape and dynamics to that event have been debated for decades. In a landmark paper, Zubarev (2001) invoked potential flow theory to predict the existence of self-similar apical sharpening for the case of an ideal perfectly conducting liquid. He found that the Maxwell and capillary pressures at the cone tip scale in time as -2/3 upon approach to the singularity. In this talk, we examine the behavior of thin electrified microscale films placed in close proximity to a grounded planar counter electrode to probe how inertial and viscous forces, diminished or neglected in the original analysis, modify the power law exponents governing the apical self-similar regime. We employ finite element, moving mesh simulations to investigate these effects for low, intermediate and high electric Reynolds and capillary numbers. We confirm the robustness of the self-similar regime characterized by power law exponents despite the lack of potential flow - however, the power law exponents, no longer -2/3, assume values which depend on the choice of dimensionless numbers. [Preview Abstract] |
Wednesday, March 15, 2017 3:30PM - 3:42PM |
P12.00006: Droplet formation in Hele-Shaw T-junction. Joshua Ricouvier, Pavel Yazhgur, Alexander Leshansky, Patrick Tabeling The development of digital microfluidics has attracted considerable interest towards generation of highly monodisperse microdroplets. T-junction has become an essential element of most of microfluidic chips. Despite its importance, theoretical analysis of droplet formation at T-junction is still incomplete due to complexity of physics involved. We focused on droplet generation at the Hele-Shaw T-junction. The effect of various experimental parameters, such as channel geometry, flow rates, surface tension and fluid viscosities, was thoroughly investigated. Our results show that the experimental system exhibits three distinct regimes (squeezing, dripping and jetting regimes) and point out the effect of confinement on the transitions. We demonstrate that the size of the "plug" droplet depends not only on the flow rate ratio (as described in the literature), but also on the capillary number and the channel cross-section aspect ratio. Quasi-2D flow equations allow us to perform numerical simulations and to compare them with experimental results. [Preview Abstract] |
Wednesday, March 15, 2017 3:42PM - 3:54PM |
P12.00007: Modeling of Stable Emulsions using a Diffuse Interface Model with a Surfactant Phase and Interfacial Viscosity Sean Colbert-Kelly, Trevor Keller, Geoffrey McFadden, Frederick Phelan Jr. An outstanding problem in emulsion science is modeling of binary emulsions stabilized by surfactants. The presence of surfactants alters the interfacial tension and interfacial viscosity between the two phases leading to greater emulsion stability and fine control over drop size. The surfactants are very low in overall composition, but have a dominant effect at the interface which makes the modeling problem challenging. In this study we formulate a diffuse interface model to investigate binary emulsions with a ternary surfactant component. First, we compare a number of models for the Gibbs free energy for binary systems stabilized by a surfactant phase, including a Langmuir isotherm model and a modified ternary Ginzburg-Landau formulation. The properties of these various models are compared by means of phase diagrams to derive a model that best represents the phase behavior. Then, we examine a number of models for concentration dependent interfacial viscosity. This is accomplished by studying the effects of these different models on emulsion stability looking at growth rates and growth to stable drop size. Finally, we compare droplet dynamics in some simple flow fields, looking at the effect of the models on drop size distributions. [Preview Abstract] |
Wednesday, March 15, 2017 3:54PM - 4:06PM |
P12.00008: Role of surfactants in the number of secondary droplet generation during drop coalescence Krishnayan Haldar, Sudipto Chakraborty The current study focuses on the variation in secondary droplet generation number with surfactant types and concentrations while surfactant laden drop impinges on a water pool. Cationic, anionic and non-ionic surfactant solutions of different concentrations are used as liquid drop. We observe from high speed imaging technique that secondary droplet generation number increases with increasing concentration for cationic and nonionic whereas it decreases for anionic surfactants. The variation of dimensionless viscosity to surface tension ratio of each surfactant determines the droplet generation number. Also the empirical relations between dimensionless coalescence time and Reynolds, Ohnesorge Number for the impinging drops reveal the dominance of viscous force over inertial and surface forces during the cascade. High viscous force, low inertial force and low surface force reduces the coalescence time. Hence, partial coalescence is faster for drops which high viscosity, low surface tension and low impact velocity and consequently the number of secondary droplet generation in the cascade will also increase. [Preview Abstract] |
Wednesday, March 15, 2017 4:06PM - 4:18PM |
P12.00009: Charge-Induced Viscous Fingers in Toroidal Droplets Alexandros Fragkopoulos, Aaron Aizenman, Alberto Fernandez-Nieves Toroidal droplets transform into spherical droplets to minimize their surface area. They do so either by breaking via the Rayleigh-Plateau instability or by shrinking; in this case, the handle collapses onto itself resulting in the formation of a single spherical droplet. Shrinking is always present for an uncharged toroidal droplet due to the variation of the Laplace pressure around circular cross-section of the torus. The presence of charge can qualitatively change this behavior and result the expansion of the torus; this happens as a result of the electric stress on the surface, which competes with the surface tension stress. Here, we will show that this expansion can result in the formation of fingers that are reminiscent of those formed via Saffman-Taylor instabilities. [Preview Abstract] |
Wednesday, March 15, 2017 4:18PM - 4:30PM |
P12.00010: Universal entrainment of oil in water by an impulsively started disk Ivo Peters, Matteo Madonia, Detlef Lohse, Devaraj van der Meer We experimentally investigate the oil entrainment in the wake of a disk, started impulsively from an oil-water interface. The experimental setup consists of a thick layer of oil floating on a deep layer of water, with the initial position of the disk exactly at the oil-water interface. As the disk is pulled down, part of the oil is entrained into the water bath and forms a smooth funnel shape. The shape and its temporal evolution depends on the relative contribution of gravity, surface tension, and inertia. We first show that there is a regime where both gravity and surface tension can be neglected and all shapes collapse. However, boundary integral simulations under these same conditions show a systematic difference in the shape compared to the experiments. We explain this difference by taking into account the influence of a disk-bounded starting vortex on the funnel shape. The surprising conservation of universal behavior in the experiments is explained by the growth-rate of the starting vortex, which follows the same scaling as the unperturbed funnel shape. [Preview Abstract] |
Wednesday, March 15, 2017 4:30PM - 4:42PM |
P12.00011: Drainage of a water film squeezed between oil and solid towards stabilization by charges Laure Bluteau, Maurice Bourrel, Nicolas Passade-Boupat, Laurence Talini, Emilie Verneuil, François Lequeux When a drop immersed in a liquid bath is put into contact with a solid, it loses its spherical shape and a liquid film is squeezed. The film dimples: it is thicker at the center. This trapped liquid drains out until an equilibrium film of uniform thickness is reached. We experimentally study the drainage dynamics and the equilibrium state of an oil droplet surrounded by brine pressed against a glass substrate. We evidence that this equilibrium state relies on the disjoining pressure and thus, depends strongly onto the salt concentration. Drainage experiments evidence three dynamical regimes. We successfully model those regimes in the lubrication approximation. In particular, we evidence that the first one is capillary dominated, the second is a mixed capillary and disjoining pressure regime, and the third is a disjoining pressure dominated regime. The role of the disjoining pressure is precisely investigated in the limit of thicknesses smaller than the range of electrostatic interactions. The originality of our results relies on the derivation of simple analytical laws quantitatively describing the drainage dynamics and providing tools to uncouple the effect of the film geometry from the effects of disjoining or capillary pressures. [Preview Abstract] |
Wednesday, March 15, 2017 4:42PM - 4:54PM |
P12.00012: Effects of elasticity and surface tension on the spreading dynamics of a thin film under the influence of intermolecular forces Yuan-Nan Young, Howard Stone The spreading dynamics of a thin layer of viscous Newtonian fluid between an elastic sheet and a wetting solid substrate is examined using lubrication theory. On the wetting substrate an ultra thin film (precursor film) develops as a result of an intermolecular force between the fluid and the solid substrate. Such a precursor film prevents the stress singularity associated with a moving contact line in the lubrication framework. Following the methodology by Glasner (2013, Phys. Fluids), the elasticity effects on the macroscopic contact line in the quasistatic limit are elucidated by an ordinary differential equation derived from an energetic analysis. Similar to the case of a fluid interface with surface tension (capillary spreading), the elasto-capillary thin film profile also consists of a core at the center, an ultra thin film in the far field, and a contact line region where the core film profile connects smoothly to the precursor film. For capillarity-dominated spreading, the precursor film transitions monotonically to the core film. For elasticity-dominated spreading, a spatial oscillation of film height in the contact line region is found instead. In addition, elasticity causes a sliding motion of the thin film: the contact angle is close to zero due to elasticity. [Preview Abstract] |
Wednesday, March 15, 2017 4:54PM - 5:06PM |
P12.00013: Pinning transition in shrinking nanobubbles Beng Hau Tan, Hongjie An, Claus-Dieter Ohl Surface nanobubbles are unusually long-lived gaseous domains that form on immersed substrates. Although liquid droplets are known to grow or shrink in either an unpinned (constant contact angle) or a pinned (constant footprint radius) mode, surface nanobubbles have only ever been observed in the pinned state. Theory suggests that, provided the nanobubbles are sustained by supersaturated liquid, they are indefinitely stable in the pinned mode, but rapidly dissolve into bulk liquid if not. Yet many basic aspects of the line pinning are not yet clarified, such as its magnitude or the conditions in which it becomes dominant. In this talk we present experiments with total internal fluorescence microscopy in which nanobubbles nucleated with a temperature difference method initially shrink in an unpinned mode, before transitioning to a pinned state. Using a simple energy balance we recover an estimate for the pinning force on each nanobubble. [Preview Abstract] |
Wednesday, March 15, 2017 5:06PM - 5:18PM |
P12.00014: Tears of Wine Prerana Rathore, Vivek Sharma `Tears of wine' refer to the rows of wine-drops that spontaneously emerge within a glass of strong wine. Evaporation-driven Marangoni flows near the meniscus of water-alcohol mixtures drive liquid upward forming a thin liquid film, and a rim or ridge forms near the moving contact line. Eventually the rim undergoes an instability forming drops, that roll back into bulk reservoir forming so called tears or legs of wine. Most studies in literature argue the evaporation of more volatile, lower surface tension component (alcohol) results in a concentration-dependent surface tension gradient that drives the climbing flow within the thin film. Though it is well-known that evaporative cooling can create temperature gradients that could provide additional contribution to the climbing flows, the role of thermocapillary flows is less well-understood. Furthermore, the patterns, flows and instabilities that occur near the rim, and determine the size and periodicity of tears, are not well-studied. Using experiments and theory, we visualize and analyze the formation and growth of tears of wine. The sliding drops, released from the rim towards the bulk reservoir, show oscillations and a cascade of fascinating flows that are analyzed for the first time. [Preview Abstract] |
Wednesday, March 15, 2017 5:18PM - 5:30PM |
P12.00015: Distribution of soap molecules in flowing soap films ILDOO KIM, Aakash Sane, Shreyas Mandre Flowing soap films are useful tools to simulate two-dimensional flows. The Marangoni elasticity due to the presence of soap molecules not only stabilizes the soap film but also imparts it compressibility to the two-dimensional flow in the soap film. Therefore, it is desirable to measure the surface concentration $c_s$ of soap molecules to understand the physics flowing soap films. In this study, we present an indirect measurement of $c_s$, by making a direct measurement of the surface tension and the Marangoni elasticity. Using a two-stage model for soap distribution in the flows, the range of $c_s$ is calculated for different thickness and the soap solution concentration. Our model shows that the soap film will have the same $c_s$ for the range of parameters in popular use and in agreements with experimental data. [Preview Abstract] |
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