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 AR: Drops I |
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Chair: David Brutin, Universite de Provence Room: Long Beach Convention Center 203C |
Sunday, November 21, 2010 8:00AM - 8:13AM |
AR.00001: Non-clogging Coulter Counting with Electrohydrodynamic Jets Yuejun Zhao, Chuan-Hua Chen Coulter counting (resistive pulse sensing) is the standard technique for quantifying biological cells and colloidal particles. The solid-state sensing aperture in conventional Coulter counting is expensive and prone to clogging. Here, we report a new paradigm of Coulter counting using an electrohydrodynamic liquid jet as the sensing ``aperture.'' The stable electrohydrodynamic jets were produced between double cones at conditions similar to the well-known cone-jet mode, but at a much higher electric field previously thought to only produce unstable jets. Micron-sized particles were successfully detected through resistive pulse sensing in the liquid jet. Similar to conventional Coulter counting, the relative current change was proportional to the particle-to-jet volume ratio. However, the liquid sensing aperture can deform and accommodate impurities and agglomerates much larger than the aperture (jet) diameter, and therefore has a major advantage of being non-clogging. [Preview Abstract] |
Sunday, November 21, 2010 8:13AM - 8:26AM |
AR.00002: Structural Relaxation of Water-in-Oil Emulsion under Direct Current Electric Field Mehrnoosh Moradi, Vladimir Alvarado In this study, electrorheology of water-in-oil emulsion is explored in the presence of direct current (DC) electric fields. DC electrorheological (ER) experiments in dynamic mode are performed to study the structural evolution of emulsions from a random configuration to organized microstructures. Critical electric field (CEF), i.e. value of field at which the emulsion structure breakdown occurs, is measured. In this work, all the ER experiments are completed at electric field strength below CEF. Since ER emulsion behavior is controlled by electrical and hydrodynamic forces, structural relaxation of emulsions as a function of electric field strength, dynamic frequency and continuous phase viscosity is investigated. Time evolution of viscosity and electrical current are measured to reflect characteristics of the system microstructure. Also, hysteresis is measured at low electric fields to study the rheological properties restoration after the field has been turned off. The results on the relaxation process show that the electric field induces increase in viscosity as well as electrical current. [Preview Abstract] |
Sunday, November 21, 2010 8:26AM - 8:39AM |
AR.00003: Electrohydrostatics of Capillary Switches Krishnaraj Sambath, Osman Basaran A pair of supported droplets that are coupled via a liquid filled cylindrical hole of radius $R$ in a plate is referred to as a capillary switch (CS). A CS is known to exhibit two stable equilibrium states when the combined volume of the top and the bottom droplets is greater than $4/3 \pi R^3$. This fact is exploited in various applications, including optical lenses and adhesion, where the main challenge is to come up with a method to``toggle'' the CS between the two stable states that is reliable, is energy efficient, and has fast response. The use of an electric field to achieve this purpose is explored here through simulations in which the axisymmetric shapes and stability of a CS are determined as a function of applied field strength. In the simulations, the liquid is taken to be perfectly conducting and the ambient fluid on either side of the plate outside the CS to be a passive gas. An axial electric field is applied either on one or both sides of a grounded plate. The equilibrium shapes of the CS and the electric potential in the surrounding gas are governed by an augmented Young-Laplace equation and the Laplace equation, respectively. These equations are solved computationally using the Galerkin finite element method. Results are shown as plots of dimensionless volume difference between the two droplets against electrical Bond number (ratio of electric to surface tension force). These phase diagrams are used to infer whether an electric field represents an effective means for toggling a CS. [Preview Abstract] |
Sunday, November 21, 2010 8:39AM - 8:52AM |
AR.00004: Symmetry breaking and chaos in droplet electrohydrodynamics Paul Salipante, Petia Vlahovska A classic result due to G.I.Taylor is that a drop placed in a uniform electric field adopts a prolate or oblate spheroidal shape, the flow and shape being axisymmetrically aligned with the applied field. However, recent studies have revealed an instability and transition to a nonaxisymmetric rotational flow in strong fields, similar to the rotation of solid dielectric particles observed by Quincke in the 19th century. We present an experimental and theoretical study of this phenomenon in DC uniform fields, focusing on nonlinear behavior arising from electromechanial coupling at the fluid-fluid interface. Charge convection by the both rotational and straining flows is included in the our model to explain the dependence of critical electric field on viscosity ratio. Hysteresis in the transition is observed for large low-viscosity drops. At stronger fields, chaotic drop tumbling and sustained shape oscillations are observed. [Preview Abstract] |
Sunday, November 21, 2010 8:52AM - 9:05AM |
AR.00005: Visualization of fluid flow inside the Taylor-cone using micro-particle image velocimetry Doyoung Byun, Jihoon Kim, Si Bui Quang Tran Nowadays, an electrohydrodynamic jetting technology has received great attention to create micro-scale and nano-scale patterns due to lower fabrication cost and good layer-to-layer registration for flexible electronic and bio-chip devices. However the details of the jetting and bifurcation mechanism have not been fully understood yet. Understanding of flow field inside the Taylor-cone is required to comprehend the mechanism of electrohydrohynamic spraying phenomenon and effects of fluid properties such as viscosity, surface tension, and electrical conductivity. In this paper, we visualized the flow fields and quantitatively measured velocity inside the Taylor-cone using micro-particle image velocimetry. Due to the refraction of light at the cone-shaped liquid meniscus surface, the velocity mapping method is used to correct the image distortion. As electrical conductivity increases, circulation in the cone becomes larger due to strong tangential force at the meniscus and we observed high velocity near the apex of the cone. As liquid viscosity increases, pulsating jet is observed and size of the circulation is decreased. [Preview Abstract] |
Sunday, November 21, 2010 9:05AM - 9:18AM |
AR.00006: Electrodeless electro-hydrodynamic gentle printing of personalized medicines Boris Khusid, Ezinwa Elele, Yueyang Shen Drop-on-demand (DOD) principle appears to be a particular promising approach for manufacturing personalized treatments carefully tailored to a patient's genetic background. The authors have recently developed a DOD method for gentle printing of personalized medicines. A fluid is infused into an electrically insulating nozzle to form a pendant drop. A sufficiently strong voltage pulse is applied to external electrodes to stretch the pendant drop until it touches an electrically insulating film and forms a liquid bridge. As the liquid bridge is intentionally formed in an unstable configuration, it breaks up, creating two drops, one on the film and the other hanging from the nozzle. To prove the validity and versatility of the method, experiments are conducted on fluids whose viscosity, conductivity, dielectric constant, and surface tension vary over a broad range, respectively: 1-1045 cP, 0.02-290 $\mu $S/cm, 9-78, and 41-72 dyn/cm. We present a scaling analysis that captures the essential physics of drop evolution and provides the critical design guidelines. The work was supported by NSF Engineering Research Center on Structured Organic Particulate Systems. [Preview Abstract] |
Sunday, November 21, 2010 9:18AM - 9:31AM |
AR.00007: Effect of Nozzle shape on droplet generation stability in EHD Inkjet Jiyoung Kim, Vu Dat Nguyen, Jihoon Kim, Si Bui Quang Tran, Doyoung Byun This study reports an effect of nozzle shape that offers better uniformity and stable operation in jetting performance. The stability of a liquid meniscus is important for the ability to eject a small liquid droplet. We investigated jetting performance in term of uniformity of patterns for different nozzle shapes in DC based EHD inkjet. To generate droplet with nozzles which have different shape, two types of glass capillaries are used in this study; one is a circular capillary and the other is a square capillary. The square edges are shown to keep the liquid inbound better. When the liquid is supplied to the circular nozzle, it grows and limits by the outer edge of the nozzle. However, in the case of the square nozzle, the meniscus can be sustained stably by the square edges. The ejections were recorded with a high speed camera and analysed to examine the difference in dynamic movement of meniscus. The repeatability of jetting is more periodic in the case of the square nozzle. [Preview Abstract] |
Sunday, November 21, 2010 9:31AM - 9:44AM |
AR.00008: A Hybrid Inkjet Printer Utilizing Electrohydrodynamic Jetting and Piezoelectric Actuation Vu Dat Nguyen, Doyoung Byun This research demonstrates a hybrid electrohydrodynamic (EHD) inkjet printing technique that offers better uniformity and stable operation in drop-on-demand (DOD) patterns compared to the conventional methods. This hybrid technique takes advantage of both electrohydrodynamic and piezoelectric methods where a piezoelectric actuator is used to supply a fixed volume of ink to the nozzle's exit for every jetting period, and the electrohydrodynamic technique is used to form ink droplets. Experimental results show that the pattern uniformity improves significantly when ink was supplied to the nozzle exit at a controlled rate using piezoelectric actuation. This hybrid technique can be applied to small scale nozzle to obtain high resolution printing. [Preview Abstract] |
Sunday, November 21, 2010 9:44AM - 9:57AM |
AR.00009: Flight Behavior of a Charged Droplet in Electrohydrodynamics (EHD) Inkjet printing using DC and AC signal Hadi Teguh Yudistira, Vu Dat Nguyen, DoYoung Byun Flight behaviors of charged droplets such as reflection, deflection, and retreat, are presented for electrohydrodynamic (EHD) inkjet printing. Experimental results show that the flight paths of charged droplets may deviate from their regular straight route, i.e., directly from the nozzle to the substrate. Depending on the droplet charge and applied electric field, droplets may deflect and reflect on a substrate, or retreat back to the meniscus. The retreat phenomenon is one of the behaviors of the charged droplet due to interactions between a droplet, meniscus, and a substrate. The droplet reversely moves back to the meniscus due to loss of charges after the second fission. To estimate the amount of charge on both the droplet and the meniscus, the Rayleigh limit was used. [Preview Abstract] |
Sunday, November 21, 2010 9:57AM - 10:10AM |
AR.00010: Oscillations of an asymmetric double droplet system Santhosh Ramalingam, Osman Basaran When a small cylindrical hole in a plate is overfilled with a liquid, a double droplet system (DDS) is created, consisting of a sessile (top) drop and a pendant (bottom) drop. For small hole radii, R, equilibrium shapes of both drops are sections of spheres. Due to the drops' spherical surfaces and its miniature size, a DDS serves well as a micro lens and a DDS oscillating about its equilibrium shape can be used as a fast focusing lens. In this talk, we consider a DDS consisting of an isothermal, incompressible Newtonian liquid of constant density and constant viscosity that is surrounded by a gas and where the air-liquid surface tension is constant. Exciting the DDS by oscillating in time (a) the pressure in the gas surrounding either drop (pressure excitation), (b) the plate perpendicular to its plane (axial excitation), and (c) the hole radius (radial excitation), the natural modes of oscillation are identified from resonances during frequency sweeps. Here, we study numerically, using the Galerkin finite element method, the oscillation modes of a DDS in which the combined volume of the pendant and sessile drops is greater than the critical volume corresponding to that of a sphere of radius R. The frequencies are shown to accord well with experimental observations and, in the limit of vanishing plate thickness and negligible viscous effects, the mode shapes and frequencies are also shown to agree with theoretical predictions. [Preview Abstract] |
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