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 LG: Drops VI |
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Chair: L. Gary Leal, University of California, Santa Barbara Room: 101A |
Monday, November 24, 2008 3:35PM - 3:48PM |
LG.00001: Numerical Studies of the Aspiration of Small Drops Using a Micropipette Gary Leal, Arun Ramchandran Aspiration of small drops, vesicles, or biological cells using a micropipette has been used as a means of characterizing the properties of the interface or membrane. The basic idea is that the shape of the deformable particle can be imaged, and this data can be translated into information about the interface or membrane properties by comparison with theoretical predictions. For the simple case of a drop, there is a critical condition of flow rate or pressure drop that depends on the interfacial tension (among other parameters) beyond which the drop is aspirated completely into the pipette. With a proper theory, this critical condition can then be used to determine the interfacial tension. Experiments to date when interpreted using a static force balance (neglecting all viscous forces) yield literature values for the interfacial tension. In this study, we use axisymmetric boundary integral methods for drops of different sizes relative to the pipette and different viscosities relative to the suspending fluid, in order to establish the range of validity of this simple method of data interpretation. [Preview Abstract] |
Monday, November 24, 2008 3:48PM - 4:01PM |
LG.00002: Micron-scale droplet deposition from a retreating syringe Bian Qian, Melissa Loureiro, Anubhav Tripathi, Kenneth Breuer Contact drop dispensing is initiated by the formation of a liquid bridge between the substrate and a dispensing syringe. As the syringe retreats, the liquid bridge stretches grows and breaks, leaving a drop on the substrate. The dynamics of liquid bridge breaking have broad interests in crystal growth, inkjet printing and micromanipulation, and contact drop dispensing has many applications in manufacturing and process control. The dynamics of the drop formation are surprisingly complex and the resulting droplet size can vary by orders of magnitude depending on the syringe diameter, $d$, fluid properties and syringe retraction speed, $u$. Experiments show that at low retraction speeds, arbitrarily large drops can be formed, their size scaling with $u^{-1/2}$. At a critical speed, the contact line on the substrate reverses direction and shrinks rather than expands as the needle retreats. Above this speed, droplets as small as 50 microns can be formed - much smaller than the characteristic needle dimension ($d\approx 500\mu$m). The limiting droplet size appears to be determined by dynamics of the contact line. We present experimental results and scaling based on measurements over a wide range of physical and geometric parameters. [Preview Abstract] |
Monday, November 24, 2008 4:01PM - 4:14PM |
LG.00003: The role of Peclet number in 3-D mixing inside drops Vinod Narayanan, George M. Homsy We consider the problem of mass transfer from a circulating liquid drop in which the flow produces 3-D chaotic trajectories of passive non-diffusing particles. The aim is to investigate effect of Peclet number, Pe, on the transport rates in such situations. The flow field is produced by applying an axial electric field, and by switching the field direction through an angle $\beta$ at constant time intervals. The 3-D advection-diffusion equation is solved numerically to obtain the concentration field, and the total mass in the drop follows an exponential decay. The enhancement factor, defined as the decay constant normalized by that for diffusion alone, shows a steep increase with Pe for large Pe, indicating large enhancement of the transport rate. An optimal switching period that maximizes the enhancement is obtained. Movies of the simulations shows the existence of periodically repeating spacial structures in the concentration field (``strange eigenmodes''). [Preview Abstract] |
Monday, November 24, 2008 4:14PM - 4:27PM |
LG.00004: ABSTRACT WITHDRAWN |
Monday, November 24, 2008 4:27PM - 4:40PM |
LG.00005: Electrospray Droplet Structures Imaged Using Digital Holographic PIV Alexandre de Chaumont Quitry, Jeffrey Guasto, Kenneth Breuer We present holographic measurements of an electrospray, illustrating the three-dimensional spatial structure of droplets after jet breakup including droplet divergence.~A conducting fluid (doped isopropanol) subjected to strong electric fields on the order of 300 kV m$^{-1}$ forms a Taylor cone, which emits a jet from the cone apex (tip streaming). Surface tension forces neck the jet into 2 $\mu $m diameter droplets, which subsequently diverge into a complex spray due to an instability involving Coulombic repulsion forces between the like-charged droplets. We use digital in-line holography with dual, pulsed Nd:YAG lasers to illuminate the droplet spray near the divergence region. Droplets are detected by reconstructing the three-dimensional intensity field from the recorded holograms. Due to the high-speed of the droplets (approximately 100 m/s), a unique imaging system is employed, which is described in the presentation. [Preview Abstract] |
Monday, November 24, 2008 4:40PM - 4:53PM |
LG.00006: Effect of AC and DC electric fields on the residence time of coalescing drops Aleksandra Lukyanets, H. Pirouz Kavehpour The residence time of de-ionized water droplets undergoing coalescence at a planar silicon oil/water interface under AC and DC electric fields is investigated with the aid of high-speed digital photography. We show that the residence time is composed of two sequential stages, which are dominated by deformation and drainage, respectively. Previously developed models predict that residence time occurs in a single stage that is dependent singularly on the magnitude of the electric field; our experiments, however, show that the frequency is also important. In the first stage, the underside of the droplet begins to deform as soon as the electric field in the gap between the droplet and fluid bulk reaches a critical value; although this value is constant for any set of experimental parameters, the first stage residence time should be frequency dependent, as is shown in our experiments. In the second stage, the residence time is dependent only on the drainage of the interfacial fluid film, and thus is inversely proportional to the strength of the electric field and independent of frequency; our results for the second stage are in good agreement with the previously developed model. [Preview Abstract] |
Monday, November 24, 2008 4:53PM - 5:06PM |
LG.00007: Stability determination of crude oil emulsions by electrorheological measurement Vladimir Alvarado, Xiuyu Wang, Henry Plancher Emulsion stability is paramount to the success of many industrial applications and the remediation of naturally undesirable occurring fluid-fluid dispersions. Bottle tests and critical electric field (E$_{c})$ measurements are two commonly used techniques to interpret emulsion stability. In the former, the amount of water resolved after gravitational settling test or centrifugation as a function of time is used as an indicator of stability. Generally, the lower the total water fraction resolved, the higher the emulsion stability. In the second method, the value of E$_{c}$ leading to drop coalescence is used as an indicator of stability. A larger value of E$_{c}$ is a reflection of a more stable dispersion. The value of E$_{c}$ is usually determined by measuring a sudden increase in electrical conductivity in water-crude oil emulsions as the field value is increased. In this work, an electrorheological test is used to establish the value of E$_{c}$ and hence the stability criterion. Results of electrorheological measurements are compared to results of bottle tests for water-crude oil emulsions with or without stabilizing solid micro-particles. Results will show the consistency among the different measuring techniques, for a wide range of ionic strength and composition of the water phase and two crude oils. [Preview Abstract] |
Monday, November 24, 2008 5:06PM - 5:19PM |
LG.00008: Effects of Electric Fields on Coalescence of Drops at Planar Interfaces Pirouz Kavehpour, Hamarz Aryafar Although electro-coalescence has applications in such fields as oil purification, lab on a chip, and mass spectroscopy, the dynamics involved within it are not fully understood. In series of experiments, we investigate the effects of an electric field on coalescing fluid bodies. A neutrally-charged droplet is deposited inside a layer of silicone oil slightly above a planar silicone oil-drop fluid interface. By introducing a DC electric field, we apply additional forces to the interface and droplet. In most cases, the presence of the electric field causes the droplet to initiate coalescence. The effect of this additional field in conjunction with the effect of other physical properties of liquids such as viscosity and interfacial tension are studied by utilizing a digital high-speed camera. The characteristics of this phenomenon are compared with those of equivalent systems in absence of the electric field. [Preview Abstract] |
Monday, November 24, 2008 5:19PM - 5:32PM |
LG.00009: Destabilization of Pickering emulsions using external electric fields Kyuho Hwang, Pushpendra Singh, Nadine Aubry It is known that emulsions can be stabilized by the presence of particles which get trapped at fluid-fluid interfaces and prevent adjacent drops from coalescing with each other. We show here that such emulsions, or Pickering emulsions, can be destabilized by applying external electric fields. This is demonstrated experimentally by studying water drops in decane and using various types of particles, including micro and nanoparticles. It is conjectured that the destabilization occurs due to the motion of particles on the surface of drops in presence of a uniform electric field. Although there should be no electrostatic forces acting on neutral particles in a uniform electric field, the presence of the drop itself introduces some non-uniformity which is responsible for particle motions along the surface. Particles translate either to the poles or equator of the drop, depending on the relative dielectric constants of the particles, the surrounding fluid and the fluid within the drop. Such motions break the particle barrier, thus allowing for drops to merge into one another. [Preview Abstract] |
Monday, November 24, 2008 5:32PM - 5:45PM |
LG.00010: Spreading Process of a Drop in Electrowetting Kwan Hyoung Kang, Jung Min Oh, Sung Hee Ko Spreading process of a conducting drop by electric field is called the electrowetting, which is derived by the electrical force concentrated on the three-phase contact line (TCL). During the spreading process, the shape of the drop changes dynamically, and the transient behavior of the drop becomes more complicated due to the contact line friction and the capillary force acting on TCL. In the present work, the shape mode equations are developed to describe the dynamic evolution of the shape of the drop. The small deformation from spherical shape and the weak viscosity of the liquid are assumed to apply the domain perturbation method. The normal stress balance and the dynamical contact angle model are unified as single boundary condition, which distinguishes our method from others. The electrical, capillary, and contact line friction forces concentrated on the TCL are approximated by using the delta function. The derived shape mode equations show a relatively good agreement with experiments. [Preview Abstract] |
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