Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session MG: Drops VII |
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Chair: Eduardo Ramos, National Autonomous University of Mexico Room: 101A |
Tuesday, November 25, 2008 8:00AM - 8:13AM |
MG.00001: Anti-foaming without defoaming agents A. Bick, W.D. Ristenpart, E. van Nierop, H.A. Stone We study the entrainment of air bubbles as a result of multiple drop impacts on a liquid/air interface. Previous studies from the literature have focused almost exclusively on the mechanism by which a single drop impacting a flat liquid-air interface entrains an air bubble. For sufficiently small droplets at low velocities, the existing literature predicts that no air bubbles will be entrained, but we often observe air entrainment if two drops impact sequentially. We qualitatively identify different entrainment behaviors following the sequential impact of two drops, and we present experimental data quantifying the critical crater depth and the time interval between successive drops necessary to entrain bubbles. We apply this approach to 1 mm diameter drops impacting a liquid surface with speed $u\approx 1$ m/s (We $\approx 10$) and find that a critical separation time $t< 5$ ms is necessary for bubble entrainment. This critical time agrees with a dimensional estimate of the time necessary for an impact crater to close owing to capillary effects. Using these ideas we demonstrate a rotating-nozzle apparatus which prevents sequential drop impacts and consequently suppresses foam formation. The key implication of this technology is the development of liquid-into-liquid dispensers that suppress foam without requiring the use of chemical defoaming agents. [Preview Abstract] |
Tuesday, November 25, 2008 8:13AM - 8:26AM |
MG.00002: Experimental Study of Oscillatory Motion of Particles and Bubbles with Applications to Coriolis Flow Meters David R. Kassoy, Joel A. Weinstein, Mark J. Bell The experimental study is designed to measure the motion of a spherical particle in a non-inertial reference frame when the environment oscillates horizontally at a prescribed frequency and amplitude. Measurements are compared with equations of motion over a wide range of fluid to particle density ratios and amplitude ratios as well as inverse Stokes numbers, the three most critical non-dimensional parameters. The experimental configuration consists of a bubble or solid sphere rising or falling in a vertical column while vibration occurs in the horizontal direction. Motion is measured with a high speed video camera and contemporary image and signal processing techniques are used to evaluate the data. Excellent agreement for amplitude and phase shift is found between theory and experiment over the full range of testing, which is defined by small oscillatory Reynolds numbers, finite Strouhal numbers, widely varying density ratios, inverse Stokes numbers, and amplitude ratios. The setup closely resembles multiphase flow in a Coriolis flow meter, a device which measures mass flow rate and density by oscillating two tubes at resonance. Accurate predictions of the motion of the sphere may lead to estimates of measurement errors due to entrained gas or solid particles. [Preview Abstract] |
Tuesday, November 25, 2008 8:26AM - 8:39AM |
MG.00003: Microbubble Evolution due to Acoustic Droplet Vaporization: Observation via Ultra-High Speed Imaging Zheng Zheng Wong, Oliver Kripfgans, J. Brian Fowlkes, Joseph Bull A potential therapy for cancer, gas embolotherapy, is being researched. It involves selective, acoustic vaporization of liquid PFC droplets (encapsulated by albumin shells) into gas bubbles that can lodge in the nearby vasculature to achieve occlusion of blood flow and ``starvation'' of the tumor. The shape evolution of microbubbles due to acoustic droplet vaporization in a 120-micron flow tube at room temperature was imaged via a 8-channel, 16-frames ultra-high-speed camera, at rates of several million frames per second. Initial droplet size ranges from 0.05 to 0.95 times tube diameter D. The bubbles followed an elliptical evolution more closely than a circular evolution. Depending on the initial droplet size, bubbles of two- to six-fold diameters were produced within a few microseconds. Growth-collapse cycles were observed in cases where the albumin shell was broken completely. For large droplets, internal phase change events could be observed. When small droplets were lined up in close proximity, coalescence was achieved for dual as well as multiple droplets. The experimental results show general consensus with a computational model by Ye {\&} Bull (2004) and a detailed comparison would be worthwhile. [Preview Abstract] |
Tuesday, November 25, 2008 8:39AM - 8:52AM |
MG.00004: Hysteresis in the shape of a droplet under supergravity Minerva Vargas, Guillermo Hernandez-Cruz, Eduardo Ramos Using a centrifuge, we have made observations of the shape of a sessile water droplet with a volume of 0.1 ml, as the total body force (gravity plus centrifugal) on it is increased from 1g to up to 14g during 20 min in a stepwise fashion, and then decreased back to 1g in 90 s. At 1 g, the drop acquires the shape of an spherical cap and we observed that it flattens as the total acceleration gets larger and the capillary length gets smaller. The drop retains its flattened shape for few seconds even when the force is reduced to 10 g, indicating that process is hysteretic. We offer plausible interpretation of our observations in terms of the dominant physical effects. [Preview Abstract] |
Tuesday, November 25, 2008 8:52AM - 9:05AM |
MG.00005: An experimental study of evaporatively-driven flows inside an inkjet-printed colloidal drop Sailee Gawande, Vadim Bromberg, Ying Sun, Timothy Singler The evaporatively-driven fluid flow of an inkjet-printed colloidal drop has been investigated experimentally. A piezoelectric inkjet device (55 $\mu $m nozzle diameter) printed an aqueous suspension of fluorescent spheres ($\sim $1 $\mu $m) onto a solid substrate. Using a laser scanning confocal microscope and micro-particle image velocimetry, the motion of these colloidal spheres inside the liquid drop has been qualitatively and quantitatively assessed. During the initial stages of the evaporation process, a novel inward radial flow is observed to focus the particles into a single group towards the center of the drop. Once the particles have converged near the center, a flow reversal occurs and the spheres are driven radially towards the contact line. Observations reveal that the velocities associated with the initial inward motion of particles are an order of magnitude larger than the particle velocities associated with flow to the contact line. These flow regimes were further investigated as a function of the jetting frequency, ink formulation, substrate material, and processing conditions (temperature and humidity). An attempt has been made to explain the observed results using the framework of thermal Marangoni theory. [Preview Abstract] |
Tuesday, November 25, 2008 9:05AM - 9:18AM |
MG.00006: Dynamics of transparent nanoparticulate liquid marbles Kenneth Gahan, Mahesh Panchagnula, Prasad Bhosale Liquid marbles is a term used to describe small quantities of liquids coated with a hydrophobic nanoparticulate substance. This study is motivated by the proposition that liquid marbles can provide for a rapid, contamination-free transport mechanism of bio-fluids. In this context, nanoparticulate liquid marbles are shown to be transparent and mechanically more robust than similar microparticulate marbles. This is shown to be a result of the formation of an elastic nanoparticulate thin film on the liquid free surface. The thin film not only keeps the marble from rupturing but also provides a barrier to diffusion of the material contained in the marble. The current research further seeks to understand the behavior of these liquid marbles under dynamical conditions. The motion of liquid marbles on an inclined plane is studied through high speed imaging. The relationship between nondimensional terminal velocity and the Bond number indicates that the dynamical behavior of these liquid marbles is well-modeled using an angular spring-mass-damper model. Finally, we experimentally validate the prospect of moving the liquid marbles using magnetic actuation. [Preview Abstract] |
Tuesday, November 25, 2008 9:18AM - 9:31AM |
MG.00007: Numerical analysis of drop-on-demand (DOD) drop formation: dynamics due to a square wave inflow boundary condition Haijing Gao, Qi Xu, Michael Harris, Osman Basaran Applications of drop-on-demand (DOD) ink-jet technology are legion and include fields as diverse as graphic arts, micro-arraying, and printing flexible electronics. Despite the importance of the subject, a fundamental understanding of the mechanisms of drop formation from a DOD nozzle is still in its infancy. Complicating matters is the fact that in piezoelectrically-driven nozzles, diverse waveforms are used to cause the formation of drops. Simplest among the waveforms is tantamount to imposing a transient inflow rate upstream of the nozzle that has the form of a square wave with or without rising/falling ramps. The dynamics are analyzed by solving the governing equations with a method of lines that uses the Galerkin/finite element method for spatial discretization and adaptive finite differences for time integration. Regions of the parameter space are identified over which drops can be successfully formed from DOD nozzles. Moreover, the dynamics are followed beyond the first breakup to determine whether primary drops can be produced without the formation of undesirable secondary or satellite droplets. [Preview Abstract] |
Tuesday, November 25, 2008 9:31AM - 9:44AM |
MG.00008: Microfluidics with compound ``bubble-drops'' Saif A. Khan, Suhanya Duraiswamy ``Bubble-drops'' are compound fluid particles comprising a gas bubble and liquid drop that flow as a single fluid object through another immiscible liquid in a microchannel network. These fluid particles represent discrete multiphase `quanta', and expand the sphere of application of droplet microfluidics to inter-phase phenomena. We present here a simple method to generate monodisperse bubble-drop trains in microfabricated channel networks. The difference in drag force exerted on flowing bubbles and drops by the immiscible carrier liquid implies different translational speeds, thus providing the driving force for bubble-drop formation. We outline the criteria for stable generation and analyze factors influencing bubble-drop dynamics. We will also highlight several applications in chemical and biological synthesis and screening. [Preview Abstract] |
Tuesday, November 25, 2008 9:44AM - 9:57AM |
MG.00009: Generation of fine droplets in a simple microchannel Sudong Kim, Young Won Kim, Jung Yul Yoo In the present study, we designed a microfluidic flatform that generates monodisperse droplets with diameters ranging from hundreds of nanometers to several micrometers. To generate fine droplets, T-junction and flow-focusing geometry are integrated into the microfluidic channel. Relatively large aqueous droplets are generated at the upstream T-junction and transported toward the flow-focusing geometry, where each droplet is broken up into the targeted size by the action of viscous stresses. Because the droplet prior to rupture blocks the straight channel that leads to the flow-focusing geometry, it moves very slowly by the pressure difference applied between the advancing and receding regions of the moving droplet. This configuration enables very low flow rate of inner fluid and higher flow rate ratio between inner and outer fluids at the flow-focusing region. It is shown that the present microfluidic device can generate droplets with diameters about 1 micrometer size and standard deviation less than 3{\%}. The present study is extended to synthesis of solid particles by using photopolymer mixed with fluorescence dye rhodamine and ultra violet light. [Preview Abstract] |
Tuesday, November 25, 2008 9:57AM - 10:10AM |
MG.00010: A synthetic jet produced by electrowetting-driven bubble/droplet oscillations Sung Hee Ko, Kwan Hyoung Kang Electrowetting is a technique to electrically control the wettability of a droplet and bubble which are submerged in another immiscible fluid. If an ac electric signal is applied in electrowetting, then a time-periodic electrical force is exerted on the three-phase contact line, and the interface of a bubble shows regular periodic motions. We present that a synthetic liquid jet is generated by electrowetting-actuated oscillation of a bubble which is submerged in water. The jet velocity is greatest at particle frequencies. Measurement of oscillation amplitudes by a high-speed camera reveals that those frequencies correspond to the natural frequency of oscillations. The jet velocity which is measured by a PIV technique is shown to be in general proportional to oscillation amplitude. A similar jet flow is also observed for an oil droplet. The jet is conjectured to be produced by the steady-streaming process of the oscillatory viscous boundary layer formed at the bubble surface. [Preview Abstract] |
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