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 ER: Drops IV |
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Chair: Amrutur Anilkumar, Vanderbilt University Room: Long Beach Convention Center 203C |
Sunday, November 21, 2010 4:10PM - 4:23PM |
ER.00001: Tuning of the droplet motion in interconnected microfluidic devices Guoqing Hu, Kui Song, Li Zhang The problem of controlling the droplet motions in multiphase flows on the microscale has gained increasing attention because the droplet-based microfluidic devices provide great potentials for chemical/biological applications such as drug discovery, chemical kinetics study, material synthesis, and DNA/cell assays. It is critical to understand the relevant physics on droplet hydrodynamics and thus control the generation, motion, splitting, and coalescence of droplets in complex microfluidic networks. The operation of those applications sometimes requires the arrival of droplets from different branch microchannels at a designated location within a transit time. We propose a simple design for interconnected microfluidic devices that implement the feedback mechanism to synchronize the droplet motion via a passive way. Numerical simulations using the Volume of Fluid (VOF) algorithm are conducted to investigate the time-dependent dynamics of droplets in both gas-liquid and liquid-liquid systems. An analytical mode based on the electronic-hydraulic analogy is also developed to describe the transit behavior of the droplet traffic. Both the numerical and theoretical results agree well with the corresponding experimental results. Furthermore, we optimize the microfluidic networks to control the motion of a series of droplets. [Preview Abstract] |
Sunday, November 21, 2010 4:23PM - 4:36PM |
ER.00002: Experimental and numerical visualization of the recirculation flow inside a gas flow-focused liquid meniscus M. Torregrosa, C. Ferrera, A. Ganan-Calvo, M.A. Herrada, M. Marchand The liquid cone-jet mode, which can be produced upon stimulation by gas flow-focusing among other procedures, is explored by both numerical simulation and experimental visualization. The results for low viscosity liquids show that, like in previous computational simulations, a recirculation cell inside the meniscus appears when the injected liquid flow rate is reduced below a certain limit. The size of that cell increases as the flow rate decreases until a global instability is reached (minimum flow rate). The results were confirmed with experimental visualization of the flow inside the meniscus. However, when the viscosity of the liquid is increased over a threshold value, the recirculation cell disappears. In this case, the viscous diffusion of momentum from the meniscus surface tends to arrange the streamlines and direct the flow towards the meniscus tip, which prevents the recirculation cell from being formed even for very small injected flow rates and very large applied pressure drops. Besides, the recirculation cell exhibits a rich and beautiful collection of topological flow features when the capillary cone-jet configuration deviates from the pure axial symmetry. [Preview Abstract] |
Sunday, November 21, 2010 4:36PM - 4:49PM |
ER.00003: Paramagnetic Leidenfrost Drops Keyvan Piroird, Baptiste Darbois Texier, Christophe Clanet, David Quere Liquid oxygen drops have two remarkable properties: $1)$ they undergo Leidenfrost effect on a substrate at room temperature because of their low boiling point ($-183\,^{\circ}\mathrm{C}$): they levitate on a cushion of their own vapor which confers them extreme mobility and thermal insulation ; $2)$ they are paramagnetic hence subjected to a force in the presence of a magnetic field gradient. --- We study the effect of such a gradient on liquid oxygen drops using rare-earth permanent magnets. By changing the distance between the magnet and the drop, this additional force can be tuned at will between zero and roughly ten times the gravitational force. We define a magneto-capillary length that takes into account the modification of the shape of an oxygen drop in a magnetic field gradient. --- We show that oxygen drops, usually moving on a horizontal substrate with almost no friction, can be stopped and captured by a local magnetic field, if slow enough. For high velocities, the drop is not captured but loses a certain amount of energy while crossing the field. We study this special dissipation occurring in the presence of a magnetic field. [Preview Abstract] |
Sunday, November 21, 2010 4:49PM - 5:02PM |
ER.00004: Generation of Single, Monodisperse Compound Droplets James Black, 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. In an apparatus designed to produce single compound droplets, a piezoelectric diaphragm generates a pressure pulse from a voltage waveform input to eject a droplet. In the method presented, oil is allowed to flow into the water nozzle with the pressure pulse ejecting both fluids as a compound droplet. Experiments were performed to demonstrate how changes in water pressure affect compound droplet water/oil volume ratio. [Preview Abstract] |
Sunday, November 21, 2010 5:02PM - 5:15PM |
ER.00005: ABSTRACT WITHDRAWN |
Sunday, November 21, 2010 5:15PM - 5:28PM |
ER.00006: A scaling theory for the hydrodynamic interaction between a pair of vesicles or capsules L. Gary Leal, Arun Ramachandran We present a scaling theory based on the analysis of A. K. Chesters [Chem. Eng. Res. Des. 69, 259-270 (1991)] that describes the time required to drain the thin, suspending fluid film that forms between two deformable capsules or vesicles as they are pushed towards each other by a constant force. Capsules and vesicles show a decrease in the drainage time with the pushing force, which results in the prediction that in a shear flow, the number of doublet formation events increases with the shear rate.~ Both trends are exactly opposite to what is expected and observed for deformable drops.~ [Preview Abstract] |
Sunday, November 21, 2010 5:28PM - 5:41PM |
ER.00007: Sessile-drop oscillations fill a symmetry-breaking periodic table Joshua Bostwick, Paul Steen Oscillations of a sessile drop are of fundamental interest for the contact-line instabilities they can exhibit and of practical importance in a number of industrial applications. We consider the small oscillations of the inviscid sessile drop under a number of contact line conditions, including a contact-line modeled using a continuous contact-angle against speed relationship. The integro-differential equation, governing the motion of the interface, is formulated as a functional equation using inverse operators, which are parameterized by volume via the static contact angle of the drop base-state and by the mobility of the contact-line. In the symmetric limit, a hemispherical drop perturbed by a fixed contact-angle disturbance has characteristic oscillation frequencies, which are degenerate with respect to azimuthal wave-number much like the Bohr model of the atom is degenerate with respect to angular momentum quantum number. This degeneracy is broken by smoothly varying either i) the volume and/or ii) the contact line mobility. The analogy between the spectrum of these ``broken'' states and the filling order of the periodic table by energy levels both organizes and explains the hierarchy of frequencies. [Preview Abstract] |
Sunday, November 21, 2010 5:41PM - 5:54PM |
ER.00008: Surface tension propulsion of fungal spores by use of microdroplets Xavier Noblin, Sylvia Yang, Jacques Dumais Most basidiomycete fungi (such as edible mushrooms) actively eject their spores. The process begins with the condensation of a water droplet at the base of the spore. The fusion of the droplet onto the spore creates a momentum that propels the spore forward. The use of surface tension for spore ejection offers a new paradigm to perform work at small length scales. However, this mechanism of force generation remains poorly understood. To elucidate how fungal spores make effective use of surface tension, we performed high-speed video imaging of spore ejection in \textit{Auricularia auricula} and \textit{Sporobolomyces} yeast, along with a detailed mechanical analysis of the spore ejection. We developed an explicit relation for the conversion of surface energy into kinetic energy during the coalescence process. The relation was validated with a simple artificial system. [Preview Abstract] |
Sunday, November 21, 2010 5:54PM - 6:07PM |
ER.00009: Spreading Dynamics of a Droplet over a Superhydrophobic Surface Nikhil Bhole, Charles Maldarelli Aqueous droplet on a microtextured, superhydrophobic surface shows two distinct wetting behaviors, the Wenzel wetting and the Cassie-Baxter wetting. Most research efforts have focused on static energy arguments in which the overall surface energies of the Wenzel and Cassie-Baxter wetting states are compared to discern which is favored as a function of the surface topography and intrinsic surface energy. In this presentation we will construct a more relevant picture by examining the hydrodynamics of the wetting process on the scale of the topography. Our aim is to understand how the flow interacts with the topography to determine the wetting regime. We study the two dimensional spreading due to gravity of an aqueous drop over a well defined topographical pattern consisting of a periodic array of micron-sized posts. The flow in the droplet is assumed to be in the Stokes flow regime, and a boundary integral method is used for numerical solution with slip at the contact line and a velocity dependent relation for the dynamic wetting contact angle. The contact line shows a distinct slip-stick-jump (or slip-stick-penetration) motion over the topography determining the state of wetting. [Preview Abstract] |
Sunday, November 21, 2010 6:07PM - 6:20PM |
ER.00010: High speed droplet interactions with heated microtextured surfaces Micah Bergman, Tae Jin Kim, Carlos Hidrovo Liquid sprays are commonly used for cooling of heated surfaces. However, one issue associated with this method is the phase the liquid is in at the incident surface. At very high temperatures the liquid boils and a vapor layer is formed between the solid surface and the liquid droplet, thus significantly reducing the heat removal rate. In order to increase the cooling efficiency, a direct contact of the liquid droplets and the solid surface is desired. We study the liquid droplet interaction on surfaces with different roughness configurations, ranging from a flat surface to a very rough surface. Very rough surfaces may induce the Cassie state, which is known to be a superhydrophobic state with a very low surface energy. Conversely, and depending on surface properties, surface texturing can also lead to a superhydrophilic state, which would be a beneficial condition for heat removal. The rough surfaces are heated and droplets are sprayed at high velocities using a droplet generator assembly. Visual inspections of droplet interactions with the different surfaces are carried out using a goniometer and a high speed camera. [Preview Abstract] |
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