Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session M8: Drops XI |
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Chair: Arun Ramchandran, University of Toronto Room: 25A |
Tuesday, November 20, 2012 8:00AM - 8:13AM |
M8.00001: Spontaneous Capillarity-Driven Droplet Ejection Drew Wollman, Trevor Snyder, Donald Pettit, Mark Weislogel The first large length-scale capillary rise experiments were conducted by R. Siegel fifty years ago using a drop tower at NASA LeRC. Siegel was curious if the wetting fluid would expel itself from the end of short capillary tubes in low-gravity. He observed that although the fluid partially left the tubes, it was always pulled back by surface tension, which caused it to remain pinned at the tubes' end. By exploiting tube geometry and fluid properties, we demonstrate that such capillary flows can in fact `auto-eject' a variety of jets and drops. Multiple and stationary drops, encapsulations, and a wide range of deployed drop diameters are demonstrated using a drop tower (diameters up to $\sim $10mm). Terrestrial gravity experiments are demonstrated as well as droplets ejected aboard the International Space Station---drops one million times larger than their 1-g counterparts. Scaling arguments reveal the single dimensionless group that best identifies the ejection criteria. The general auto-ejection approach provides a novel mechanism from which to investigate jets, droplets, bubbles, and other large length-scale capillary phenomena. [Preview Abstract] |
Tuesday, November 20, 2012 8:13AM - 8:26AM |
M8.00002: The effect of interfacial slip on the rheology of a dilute emulsion of drops for small capillary numbers Arun Ramchandran, L. Gary Leal We present the constitutive equation for the volume-averaged hydrodynamic stress for a dilute emulsion in a linear ambient flow, when there is slip at the liquid-liquid interface between the Newtonian drop and suspending fluids. Slip is modeled using the Navier slip boundary condition. We provide analytical solutions in the limit of small capillary numbers for the shape deformation, viscosity and normal stresses. Slip moderates these quantities, with changes from the no slip case being more pronounced for large drop viscosities relative to the suspending fluid. It has been suggested in the past that slip can explain the anomalously low viscosities of certain polymeric blends. Our analysis indicates that slip can only partially account for these deviations, and that other mechanisms should be explored to explain this discrepancy. [Preview Abstract] |
Tuesday, November 20, 2012 8:26AM - 8:39AM |
M8.00003: Droplet generation at the critical Weber number Laurent Tanguy, Dong Liang, Roland Zengerle, Peter Koltay The ejection of liquid droplets from a nozzle is highly important for physics of fluid. The Weber number describes how much kinetic energy is needed to overcome the surface tension and create a free-flying droplet. According to literature Weber numbers above 12 assure the creation and safe break up of a liquid droplet. However, even when this number goes down below 8, it is still possible to observe droplet break-up but with particular effects. We present here experimental results and CFD simulations for droplet break-up at low Weber number where the droplet is generated with negative kinetic energy. Such droplet generation is characterized by the droplet breaking up and then returning back into the nozzle. This is due to the fact that during the droplet formation the surface tension begins to slow down the flow velocity inside the droplet and then finally inverts the flow direction, while the droplet tail still breaks off from the nozzle. Thus after the break up the droplet momentum is oriented toward the nozzle. It is therefore possible to observe the droplet returning into the bulk fluid. High-speed images of this particular phenomenon are shown and simulation results are presented to illustrate the break up dynamics and the local velocities in the droplet. [Preview Abstract] |
Tuesday, November 20, 2012 8:39AM - 8:52AM |
M8.00004: How to freeze drop oscillations with powders Jeremy Marston, Ying Zhu, Ivan Vakarelski, Sigurdur Thoroddsen We present experiments that show when a water drop impacts onto a bed of fine, hydrophobic powder, the final form of the drop can be very different from the spherical form with which it impacts. For all drop impact speeds, the drop rebounds due to the hydrophobic nature of the powder. However, we observe that above a critical impact speed, the drop undergoes a permanent deformation to a highly non-spherical shape with a complete coverage of powder, thus creating a deformed liquid marble. This powder coating acts to freeze the drop oscillations during rebound. [Preview Abstract] |
Tuesday, November 20, 2012 8:52AM - 9:05AM |
M8.00005: Flow induced by membraneless osmosis between a droplet and a bath Matthieu Roche, Corentin Tr\'egouet, Bo Sun, Howard A. Stone Biological material can be sorted using aqueous solutions containing two polymers. These mixtures, known as aqueous two-phase systems, experience phase separation after production because of the incompatibility of one polymer with the other. Each phase of the final solution is rich in one of the polymers. However, the concentration in polymers will often be very different from the concentration of the two initial solutions. Indeed, the interaction of water with each of the polymers induces a gradient of osmotic pressure, which leads to the redistribution of water between the two phases. Here we study the consequences of this so-called membrane-less osmosis on the deposition of a droplet of a solution of dextran on the surface of a layer of solution of poly(ethylene glycol). A flow on a scale comparable to that of the layer sets in over timescales of a few hours. We use particle tracking to describe the flow pattern. We observed the growth of vortices. By changing the concentration in polymers, the depth of the layer and the size of the drop, we relate the properties of the velocity field to the osmotic pressure of water in the ternary system water/dextran/poly(ethylene glycol). Finally, we describe important consequences of these observations for applications. [Preview Abstract] |
Tuesday, November 20, 2012 9:05AM - 9:18AM |
M8.00006: Universal deformation of soft substrates near contact line reveals solid surface stresses Robert Style, John Wettlaufer, Larry Wilen, Eric Dufresne We study how sessile droplets behave on soft substrates. Using confocal microscopy, we investigate how droplet surface tension (and Laplace pressure) deforms the substrate. We show that the near-tip shape of the wetting ridge is entirely determined by the surface tensions of the three contacting phases. In particular we can use this observation to (i) directly measure solid-vapour and solid-liquid surface tensions, (ii) resolve how out-of-plane force balance is ensured at the contact line. [Preview Abstract] |
Tuesday, November 20, 2012 9:18AM - 9:31AM |
M8.00007: The stability of viscous liquid filaments Theo Driessen, Roger Jeurissen, Herman Wijshoff, Detlef Lohse The stability of liquid filaments is relevant both in industrial applications, such as inkjet printing and atomization, and in nature, where the stability of filaments has a large influence on the final drop size distribution of rain droplets and waterfalls. The liquid filament may either stably collapse into a single droplet, or break up into multiple droplets. Which scenario is realized depends on the viscosity and the aspect ratio of the filament. Here we study the collapse of an axisymmetric liquid filament is analytically and with a numerical model. We find that a long, high viscous filament can only break up due to the Rayleigh-Plateau instability, whereas a low viscous filament can break up due to end-pinching. The theory shows quantitative agreement with recent experimental findings by Castr\'{e}jon-Pita et al., PRL 108, 074506 (2012). [Preview Abstract] |
Tuesday, November 20, 2012 9:31AM - 9:44AM |
M8.00008: Contraction dynamics of planar liquid filaments Nicole Devlin, Krishnaraj Sambath, Michael Harris, Osman Basaran Thin liquid sheets are ubiquitous in nature and urban landscapes, e.g. waterfalls, and industry, e.g. in various atomizers where sheets of liquid emanate from a nozzle or off a solid surface. These liquid sheets contract due to surface tension and may or may not break into smaller fragments depending on physical properties and flow conditions. The cross-section of a liquid sheet in a plane perpendicular to the main flow direction is a planar or 2D filament. Here, we study the contraction dynamics of an idealized 2D filament of an incompressible Newtonian fluid the initial shape of which is a rectangle terminated by two identical semi-circles. The dynamics are analyzed by solving the full 2D Navier-Stokes system and a1D, slender-jet approximation to it by a numerical technique based on the Galerkin finite element method. Simulation results are summarized by means of a phase diagram in the space of Reynolds number and initial filament aspect ratio. The talk will conclude with a discussion of the different modes of contraction and a critique of the capabilities and limitations of the 1D model. [Preview Abstract] |
Tuesday, November 20, 2012 9:44AM - 9:57AM |
M8.00009: Disk impact on an oil-water bilayer Devaraj van der Meer, Ivo Peters, Matteo Madonia, Detlef Lohse When a round disc impacts on a water surface, a cavity is created that collapses under the influence of hydrostatic pressure. This leads to the formation of two jets that originate from the pinch-off point, one shooting upwards, the other downwards. To investigate the mechanism by which the jets are formed, we add a thin layer of oil on top of the water surface and repeat the experiment. Initially, just after the pinch-off of the cavity, the jet consists exclusively of oil. At a later stage, we observe that water enters into the jet, but a stable core of oil remains that extends all the way down into the bulk of the liquid. We compare our findings to several theoretical models from the literature. [Preview Abstract] |
Tuesday, November 20, 2012 9:57AM - 10:10AM |
M8.00010: Mesler entrainment in alcohols J.R. Saylor, R.K. Sundberg When a drop impacts a flat surface of the same liquid at an intermediate velocity, the impact can result in the formation of a very large number of very small bubbles. At lower velocities, drops bounce or float, and at larger velocities a single bubble forms, or there is a splash. The formation of large numbers of small bubbles during intermediate velocity impacts is termed Mesler entrainment and its controlling mechanism is poorly understood. Existing research has shown that Mesler entrainment is highly irreproducible when water is the working fluid, and very reproducible when silicone oil is the working fluid. Whether this is because water is problematic, or silicone oil is uniquely well-suited, is unclear. To answer this question, experiments were conducted using three different alcohols. The results of these experiments were very reproducible for all alcohols tested, suggesting that there is something unique about water which accounts for its lack of reproducibility. The data from these experiments were also used to develop a dimensionless group that quantifies the conditions under which Mesler entrainment occurs. This dimensionless group is used to provide insight into the mechanism of this unique method of bubble formation. [Preview Abstract] |
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