Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session CR: Drops II |
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Chair: Chuan-Hua Chen, Duke University Room: Long Beach Convention Center 203C |
Sunday, November 21, 2010 1:00PM - 1:13PM |
CR.00001: Effect of Viscosity and Size of a Droplet on Spreading Dynamics in Electrowetting Jiwoo Hong, Kwan Hyoung Kang Electrowetting-based devices require fast, stable and accurate positioning of the three-phase contact line (TCL). To meet this requirement, a concrete understanding on dynamics of electrowetting is necessary, which has been one of the main challenges in electrowetting research. In this work, we investigated the switching dynamics of a droplet in air actuated by electrowetting, for different applied voltages, drop sizes, and viscosities. We analyzed the spreading process for weakly viscous droplet and derived a relationship between the drop size and the switching time. The relationship is verified experimentally. We explored experimentally the effect of viscosity of a droplet on switching dynamics. During the initial spreading period, the spreading dynamics was hardly dependent on viscosity. The switching time is moderately dependent on fluid viscosity. [Preview Abstract] |
Sunday, November 21, 2010 1:13PM - 1:26PM |
CR.00002: Droplet jumping by resonant AC electrowetting envisioning three-dimensional digital microfluidics Seung Jun Lee, Sanghyun Lee, Kwan Hyoung Kang We introduce droplet jumping by resonant AC electrowetting (DJ-RACE) to transport droplets to vertical direction, envisioning three-dimensional digital microfluidics. Inphase oscillatory actuation by resonant AC electrowetting allows droplets to store sufficient energy for jumping on their stretched surfaces by conventional electrowetting methods. The detailed jumping mechanism is explained in comparison to experimental results, and the actual droplet transport from the superhydrophobic bottom surface to higher level surfaces is demonstrated by several electrode configurations and actuation methods. [Preview Abstract] |
Sunday, November 21, 2010 1:26PM - 1:39PM |
CR.00003: Jumping number in the droplet jumping by resonant AC electrowetting Sanghyun Lee, Seung Jun Lee, Kwang Hyoung Kang The droplet jumping by resonant AC electrowetting (DJ-RACE) is recently introduced to transport droplets to vertical direction, whereby three-dimensional digital microfluidics are envisioned. In DJ-RACE, the central mechanism of the droplet jumping is the conversion of the surface energy stored by resonant AC electrowetting to the kinetic energy for jumping. Here, we newly introduce the jumping number ($J_{u}$=\textit{$\gamma $/$\rho $gR}$^{2})$, measuring the energy conversion in the jumping process and, thus, the feasibility of droplet jumping. $J_{u}$ interprets that droplets having higher $J_{u}$ can make higher and easier jumping, and smaller and lighter droplets with higher surface tension can have higher $J_{u}$. Practically, $J_{u}$ should be greater than 1.5 for the droplet jumping, and active jumping was observed when $J_{u}$ is greater than 5. In addition, $J_{u}$ can predict the effect of diverse physicochemical changes in a system such as enzymatic additives or impurities on jumping, where it can also provide diverse strategies to compensate these changes. The newly introduced $J_{u}$ could be the fundamental and useful parameter in the three-dimensional digital microfluidic devices based on DJ-RACE. [Preview Abstract] |
Sunday, November 21, 2010 1:39PM - 1:52PM |
CR.00004: Electrophoresis of a charged electrolyte droplet Do Jin Im, Jihoon Noh, In Seok Kang We study the motion of a charged droplet in a dielectric fluid under electric field for the use as a microdroplet actuation method. The amount of electrical charging has been measured experimentally for a water droplet and an electrolyte droplet using the Stokes law. Comparison is made with theoretical value of a perfect conductor sphere. The effects of droplet size, electric field strength, and electrolyte concentration on the motion of aqueous droplet are investigated. A scaling law derived from experimental results shows that the amount of charging of de-ionized water droplet is proportional to the square of droplet radius and to electric field strength. This means de-ionized water droplet follows well the scaling law of perfect conductor. However, the electric charging characteristic of electrolyte droplet depends on electrolyte concentration and electric field strength. But, under sufficiently high electric field, electrolyte droplet behaves like perfect conductor regardless of its concentration. This fact implies that the same actuation speed can be obtained independent of electrolyte concentration under high electric field. This property may be utilized for stable actuation of electrolyte droplets. [Preview Abstract] |
Sunday, November 21, 2010 1:52PM - 2:05PM |
CR.00005: Translation and coalescence of freely floating droplets using electrophoresis Dong Woog Lee, Do Jin Im, In Seok Kang Aqueous droplets could be charged and transported between two DC electrodes in oil (Jung \textit{et al.} J. Colloid Interface Sci. 2008). And in that situation, two oppositely charged droplets coalesce under mild electric field but they do not coalesce under high electric field (Ristenpart \textit{et al.} Nature, 2009). In these previous experiments, the aqueous droplets were totally under oil. In present work, the aqueous droplets are floating on the heavy oil. The floating droplet system has several advantages comparing the submerged droplet system. First, the drag force on the moving droplet is lower than that of the submerged droplet. Second, post process could be simple, because the droplets are exist on the top of the system. Third, droplet coalescence is easy, because they are in same height. In this work, drag force and velocity field of the translating droplet are computed through numerical simulation. And the translating velocity and condition for coalescence of droplets are investigated through the experiments. [Preview Abstract] |
Sunday, November 21, 2010 2:05PM - 2:18PM |
CR.00006: Electrically Modulated Partial Coalescence of Oppositely Charged Droplets J.C. Creasey, B.S. Hamlin, W.D. Ristenpart Oppositely charged drops fail to coalesce above a critical field strength, despite the attractive force between the opposite charges. Here we report a technique to externally control the extent to which a charged droplet is allowed to coalesce. For sufficiently low ionic conductivities, the degree of coalescence of water drops in oil can be tuned from complete coalescence at low field strengths to complete non-coalescence at high field strengths. Strikingly, in this regime the size and charge of the daughter droplet are both independent of the drop conductivity. We present evidence that the charge transfer is instead dominated by convective effects associated with the capillary-driven penetration of a vortex into the larger drop. Moreover, we demonstrate that measurements of the size of the daughter droplet are consistent with a model based on a balance between capillary forces and electrostatic repulsion. [Preview Abstract] |
Sunday, November 21, 2010 2:18PM - 2:31PM |
CR.00007: How Do Droplets Acquire Charge? W.D. Ristenpart, B.S. Hamlin Liquid droplets immersed in a poorly conductive medium are known to acquire charge upon contact with an electrified surface. Although there is some evidence that electrochemical reactions limit the degree of charge transfer, the details of the underlying mechanism are unclear. Previous work has relied on estimates of the drop charge obtained from a balance between the electrophoretic driving force and viscous drag, which necessitates an accurate description of the hydrodynamic resistance experienced by the drop. Here we establish a procedure to directly measure the droplet charge using a high resolution electrometer. Simultaneous high speed video and voltammetry provide a quantitative comparison of the drop charge obtained by the two methods. We find significant deviations between the measured charges and Maxwell's limiting case for the charge acquired by a perfectly conducting rigid sphere in contact with a planar electrode. We interpret the discrepancy in terms of electrochemical limitations, and we provide physical evidence that under appropriate conditions electrochemical reactions play a key role in the charge transfer to the drop. [Preview Abstract] |
Sunday, November 21, 2010 2:31PM - 2:44PM |
CR.00008: Formation and Electrically Induced Reversal of ``Dynamic'' Stagnant Caps B.S. Hamlin, W.D. Ristenpart Drops are commonly observed to move more slowly than predicted by the classic Hadamard-Rybczynski model for the drag force on an immiscible spherical drop. The discrepancy is commonly interpreted in terms of the presence of a ``stagnant cap'' of surfactant molecules at the trailing edge of the drop; the surfactants exert a Marangoni stress that impedes recirculation inside the drop. Here we present high-speed video of the formation and electrically induced reversal of ``dynamic'' stagnant caps. A charged water droplet is subjected to a sudden reversal in the direction of an applied electric field, and the overall motion of the drop and the relative motion of tracer particles on the droplet surface are observed. The droplet is shown to decelerate over a time period commensurate with the transient rearrangement of the tracer particles on the surface. We interpret the behavior in terms of the dynamic reversal of the stagnant cap, and we demonstrate that the observations are consistent with a scaling analysis of the transient cap rearrangement. [Preview Abstract] |
Sunday, November 21, 2010 2:44PM - 2:57PM |
CR.00009: Particle-particle Interactions on the Surface of a Drop Subjected to a Uniform Electric Field Sai Nudurupati, Mansoor Janjua, Pushpendra Singh, Nadine Aubry We recently proposed a technique in which an externally applied uniform electric field was used to alter the distribution of particles on the surface of a drop immersed in another immiscible liquid. Particles move along the drop surface to form a ring near the drop equator or collect at the poles depending on their dielectric constant relative to that of the two liquid involved. This motion is due to the dielectrophoretic force that acts upon particles because the electric field on the surface of the drop is non-uniform, despite the fact that the applied electric field is uniform. This technique could be useful to concentrate particles at a drop surface, and also separate two types of particles. In this talk we show that in addition to the dielectrophoretic force the particles also interact with each other via the dipole-dipole interactions to form chains or move away from each other depending on the local direction of the electric field. The regions in which the local electric field is normal to the drop surface (poles), particles move away from each other. On the other hand, where the local direction of electric field is tangential to the drops surface (equator), they form chains that are aligned parallel to the electric field direction. [Preview Abstract] |
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