Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session BH: Drops II |
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Chair: Alberto Aliseda, University of Washington Room: 101H |
Sunday, November 22, 2009 10:30AM - 10:43AM |
BH.00001: Role of Gas Composition in Viscous Drop Splashing Cacey S. Stevens, Sidney R. Nagel Splashing occurs when a liquid drop impacts on a smooth, dry surface at high velocity. It has been discovered that the pressure of the surrounding air is important in causing a splash.\footnote{L. Xu, S. Nagel, and W. Zhang. Phys. Rev. Lett. 94, 184505 (2005).} Moreover, viscous splashing occurs at a much later time than the splashing of an inviscid liquid.\footnote{ L. Xu, Phys. Rev. E 75, 056316 (2007).} Here we investigate the relationship between the threshold pressure, $P_T$, and the impact velocity, $V_0$, for viscous splashes. $P_T$ demonstrates only a weak dependence on impact velocity above 2.0 m/s. Also, the composition of the gas in which splashing occurs is varied to gain insight on the role of gas molecular weight on $P_T$ in the viscous regime. [Preview Abstract] |
Sunday, November 22, 2009 10:43AM - 10:56AM |
BH.00002: Drop Impact of Viscous Suspensions on Solid Surfaces Daniel Bolleddula, Alberto Aliseda Droplet impact is a well studied subject with over a century of progress. Most studies are motivated by applications such as inkjet printing, agriculture spraying, or printed circuit boards. Pharmaceutically relevant fluids provide an experimental set that has received little attention. Medicinal tablets are coated by the impaction of micron sized droplets of aqueous suspensions and subsequently dried for various purposes such as brand recognition, mask unpleasant taste, or functionality. We will present a systematic study of micron sized drop impact of Newtonian and Non-Newtonian fluids used in pharmaceutical coating processes. In our experiments we extend the range of Ohnesorge numbers, $O(1)$, of previous studies on surfaces of varying wettability and roughness. [Preview Abstract] |
Sunday, November 22, 2009 10:56AM - 11:09AM |
BH.00003: Effect of parameters on redistribution and removal of particles from drop surfaces Sai Nudurupati, Muhammad Janjua, Pushpendra Singh, Nadine Aubry It was recently shown by us that particles distributed on the surface of a drop can be concentrated at the poles or the equator of the drop by subjecting it to a uniform electric field and that such concentrated particles can then be removed from the drop by increasing the electric field intensity. In this talk, we present experimental results for the dependence of the dielectrophoretic force on the parameters of the system such as the particles' and drop's radii and the dielectric properties of the fluids and particles, and define a dimensionless parameter regime in which the technique works. In particular, we show that the technique is guaranteed to work if the drop radius is smaller than a critical value that depends on the physical properties of the drop and ambient fluids and those of the particles. [Preview Abstract] |
Sunday, November 22, 2009 11:09AM - 11:22AM |
BH.00004: Particle deposition during evaporation of colloidal sessile drops James D. Felske, Hassan Masoud Deposition patterns of particles suspended in evaporating colloidal drops are determined by the flow fields within the drops. Using analytically determined velocities, particle motions are then tracked in a Lagrangian sense. It is found that the majority of particles intersect the free surface as it recedes. Such ``capture'' of particles by the free surface is found to be the major mechanism in establishing the deposition pattern. Patterns are calculated for wetting and non-wetting drops whose contact lines are either pinned or freely moving during evaporation. The distribution of evaporative flux which drives the flows is taken to be that engendered by gas-phase diffusion. The theoretical results are found to agree favorably with available experimental data. [Preview Abstract] |
Sunday, November 22, 2009 11:22AM - 11:35AM |
BH.00005: Generation of Single, Monodisperse Compound Droplets James Black, Prasad Bhave, Oumar Tour\'e, G. Paul Neitzel Compound, nanoliter-scale droplets consisting of an aqueous inner phase surrounded by an oil encapsulant are of interest in a lab-on-a-chip process that levitates the droplets between a pair solid surfaces using thermocapillarity. The application requires a droplet with an oil layer of sufficient thickness to permit the use of the levitation method, although not so thick as to impede effective combining and mixing of the contents of merged droplets. An apparatus was designed to produce single compound droplets of variable water/oil volume ratio and uniform size. Experiments were performed to characterize the effect of apparatus and fluid parameters on the volume ratio in the generation of droplets of silicone-oil-encapsulated water. [Preview Abstract] |
Sunday, November 22, 2009 11:35AM - 11:48AM |
BH.00006: The mayonnaise droplet Denis Terwagne, Tristan Gilet, Nicolas Vandewalle, St\'ephane Dorbolo A compound drop is made of a millimetric water drop encapsulated by an oil shell. They are obtained by merging one drop of each component (water and oil). Afterwards, they are laid on a high viscosity oil bath which is vertically vibrated. When the forcing acceleration is higher than a given threshold, compound drops can bounce on the surface. We show that above a second threshold some oil contained in the shell enters in the inner water droplet. In a second experiment, we drop the compound droplet on the oil bath at rest. We can determine the range of impact speed in which capillary waves developed on the surface are able to generate an oil drop (coming from the shell) in the water drop. When the bouncing trajectories of the droplets are analyzed a correlation between the emulsion threshold and the static analysis can be made. [Preview Abstract] |
Sunday, November 22, 2009 11:48AM - 12:01PM |
BH.00007: Impact and Spreading of a Compound Droplet: A Model for Single Cell Epitaxi Savas Tasoglu, Gozde Kaynak, Metin Muradoglu In recent years, there has been a growing interest in generating compound droplets mainly due to their potential commercial value [1] and applications in emerging technologies such as single cell epitaxi [2]. Ejecting encapsulated cells on a rigid surface is a promising way to produce 2D/3D tissues [2]. However, this gained experimental capability requires a true understanding of the impact dynamics of the encapsulated cells on solid surfaces for further development. In the present study, a finite-volume/front-tracking method is used to model the impact and spreading of a viscous compound droplet on a flat solid surface as a first step in developing a model for the single cell epitaxi. The cell, the encapsulating droplet and ambient fluid are all assumed to be Newtonian. Simulations are performed for a range of dimensionless parameters and their effects on deformation of inner droplet are investigated. These results provide initial insight about the optimum parameter ranges for highest viability of cells. [1] Utada, Lorenceau, Link, et al., Science, 308(5721), (2005). [2] Demirci and Montesano, Lab Chip, 7, (2007). [Preview Abstract] |
Sunday, November 22, 2009 12:01PM - 12:14PM |
BH.00008: Dynamics of the electro-osmotically toggled droplet switch Dominik Barz, Michael Vogel, Paul Steen A capillary switch is a bi-stable system of liquid/gas interfaces with a trigger to toggle back and forth between the energy wells. The electro-osmotic droplet switch consists of a pair of coupled droplets whose shape-change is triggered by volume transfer using an electro-osmotic pump placed between the droplets. With the pump off, the stable states are a pair of identical sub-hemispherical drops for low total-volume or a large-small droplet configuration(two mirror-symmetric states) for large enough total-volume. With the pump on, these stationary states are shifted or there are no stationary states at all, depending on the pump strength. We report the pump-on behavior as a modification of the pump-off behavior. A dynamic model of the system which adopts an Ergun-equation characterization of the pump is introduced. Model predictions are compared to observation. [Preview Abstract] |
Sunday, November 22, 2009 12:14PM - 12:27PM |
BH.00009: Digital microfluidics by using Electric Charging of Droplet Yong-Mi Jung, In Seok Kang We evaluate the novel actuation of droplets by exploiting Coulombic forces acting on conductive droplets immersed in a dielectric fluid. The droplet under a DC voltage can be charged by direct contact, by charge transfer along an electrical conducting path, or by both mechanisms. A droplet is deformed to a shape with a sharper tip near electrode. The droplet acquires charges through it and may then be transported rapidly by exploiting Coulombic forces. For actuation by this phenomenon, we construct a platform for handling droplets on the device by using electrode dots arrayed on substrate. A programmable voltage supply is connected through an electrical switch to the electrodes. In the experimental results, droplets are actuated along the electric field signal. The droplet never adhere surface of chip when it moves. We also perform electrocoalescence and chemical reaction in a merged droplet as an essential step for being used as droplet-based microreactors. When two droplets approach each other due to Coulombic force, a liquid bridge is formed between them. The chemical reaction simultaneously occurs by coalescence of droplets. [Preview Abstract] |
Sunday, November 22, 2009 12:27PM - 12:40PM |
BH.00010: Bouncing of a Droplet on Superhydrophobic Surface in AC Electrowetting Kwan Hyoung Kang, Seung Jun Lee, Jiwoo Hong Oscillation of a droplet is induced in ac electrowetting by time-dependent electrical wetting tension. A droplet placed on a superhydrophobic surface bounces up like a rubber ball when an ac signal is applied. The bouncing pattern is highly frequency dependent. We investigated how the shape deformation and bouncing of a droplet are affected by applied frequency. The droplet motion is analyzed with the spectral method. The droplet is modeled as a simple linear oscillator, and the mass and spring constants are determined based on analytical results. We found that bouncing occurs periodically at a resonance frequency of the droplet. The motion of a sessile droplet released from a fixed shape is analyzed based on the phase field method. The numerical results show qualitative agreement with the experimental results for a bouncing droplet. Details on the flow field inside a bouncing droplet will be discussed based on numerical results. [Preview Abstract] |
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