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 AH: Bubbles I |
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Chair: Michael A. Rother, University of Minnesota-Duluth Room: 101B |
Sunday, November 23, 2008 8:00AM - 8:13AM |
AH.00001: Role of Gas Pressure and Molecular Weight in Bubble Pinch-Off from an Underwater Nozzle Nathan C. Keim, Sidney R. Nagel We report on experiments that explore the role of gas pressure and molecular weight near the pinch-off of an air bubble from an nozzle submerged in water. We use high-speed video to image the dynamics close to the singularity occurring at pinch-off.\footnote{N.~C.\ Keim et al., \emph{PRL} 97, 144503 (2006).} As the neck collapses to a radius of several microns, the effects of the Bernoulli pressure associated with gas flow inside the neck begin to alter the bubble's shape and evolution, as was recently proposed.\footnote{J.~M.\ Gordillo, M.~A.Fontelos, \emph{PRL} 98, 144503 (2007).} We address the role that the gas plays in creating satellite bubbles during the pinch-off process, and its influence on the evolution of perturbations to axisymmetric collapse.\footnote{L.~E.\ Schmidt et al., in preparation.} [Preview Abstract] |
Sunday, November 23, 2008 8:13AM - 8:26AM |
AH.00002: Hourglass Bubble Plumes Michael Higley, Andrew Belmonte We present an experiment in which millimetric bubbles are injected into water from a small number of closely-spaced pipettes. These bubbles form a dilute plume with a repeated hourglass pattern of widening and narrowing diameter. We detail parameters affecting the formation and characteristics of this pattern. Two factors are shown to be particularly important: the initial radial spread of the bubbles from their sources, and their subsequent helical trajectories. A simple model which treats the bubbles as independent reproduces the hourglass pattern. [Preview Abstract] |
Sunday, November 23, 2008 8:26AM - 8:39AM |
AH.00003: Bubble chains in magnetic fluids Philip Yecko, Wah-Keat Lee Interactions between small numbers of bubbles of non-magnetic fluid immersed in a magnetic fluid (ferrofluid) are examined by direct numerical simulation using a volume of fluid (VOF) interface capturing method coupled to a magneto-quasistatic Maxwell solution. Constant magnetic susceptibility (linear magnetic material) is assumed and the Reynolds number is small, but does not vanish. For small gravitational and magnetic Bond numbers, the dynamics of multiple bubbles is controlled by the dipole fields induced by the bubbles, which for certain initial configurations leads naturally to the formation of linear chains of nearly spherical bubbles. The study of bubble chains using a VOF approach is facilitated by introducing multiple VOF phase functions, surpressing merger of bubbles. At larger Magnetic Bond numbers, the bubbles also elongate in the direction of the magnetic field, altering the coalescence process. Model results are shown to be in agreement with experiments performed using high resolution X-ray images of air bubbles in ferrofluid. The complementary problem of magnetic fluid droplets is also examined for its utility as a possible model for the microstructure of ferrofluids that can be used to predict their rheological properties, in particular the competition between shear and chain structures. [Preview Abstract] |
Sunday, November 23, 2008 8:39AM - 8:52AM |
AH.00004: Asymmetric bubble collapse Lipeng Lai, Konstantin S. Turitsyn, Wendy W. Zhang Recent studies reveal that an inertial implosion, analogous to the collapse of a large cavity in water, governs how a submerged air bubble disconnects from a nozzle. For the bubble, slight asymmetries in the initial neck shape give rise to vibrations that grow pronounced over time. These results motivate our study of the final stage of asymmetric cavity collapse. We are particularly interested in the generic situation where the initial condition is sufficiently well-focused that a cavity can implode inwards energetically. Yet, because the initial condition is not perfectly symmetric, the implosion fails to condense all the energy. We consider cavity shapes in the slender-body limit, for which the collapse dynamics is quasi two-dimensional. In this limit, each cross-section of the cavity evolves as if it were a distorted void immersed in an inviscid and irrotational fluid. Simulations of a circular void distorted by an elongation-compression vibrational mode reveal that a variety of outcomes are possible in the 2D problem. Opposing sides of the void surface can curve inwards and contact smoothly in a finite amount of time. Depending on the phase of the vibration excited, the contact can be either north-south or east-west. Phase values that lie in the transition zone from one orientation to the other give rise to final shapes with large lengthscale separation. We show also that the final outcome varies non-monotonically with the initial amplitude of the vibrational mode. [Preview Abstract] |
Sunday, November 23, 2008 8:52AM - 9:05AM |
AH.00005: Acoustic measurement of bubble size and position in a piezo driven inkjet printhead Arjan van der Bos, Roger Jeurissen, Jos de Jong, Richard Stevens, Michel Versluis, Hans Reinten, Marc van den Berg, Herman Wijshoff, Detlef Lohse A bubble can be entrained in the ink channel of a piezo-driven inkjet printhead, where it grows by rectified diffusion. If large enough, the bubble counteracts the pressure buildup at the nozzle, resulting in nozzle failure. Here an acoustic sizing method for the volume and position of the bubble is presented. The bubble response is detected by the piezo actuator itself, operating in a sensor mode. The method used to determine the volume and position of the bubble is based on a linear model in which the interaction between the bubble and the channel are included. This model predicts the acoustic signal for a given position and volume of the bubble. The inverse problem is to infer the position and volume of the bubble from the measured acoustic signal. By solving it, we can thus acoustically measure size and position of the bubble. The validity of the presented method is supported by time-resolved optical observations of the dynamics of the bubble within an optically accessible ink-jet channel. [Preview Abstract] |
Sunday, November 23, 2008 9:05AM - 9:18AM |
AH.00006: Airflow driven bubble pinch-off Raymond Bergmann, Anders Andersen, Tomas Bohr, Devaraj van der Meer We create air bubbles at the tip of a ``bathtub vortex'' which reaches to a finite depth. The bathtub vortex is created by letting water drain through a small hole at the bottom of a rotating cylindrical container. For sufficiently large rotation rates the tip of this needle-like surface depression becomes unstable and emits bubbles. The collapse follows a $R(t)\propto \tau^{1/3}$ power law for the minimal neck radius which is indicative of the balance between liquid inertia and the under pressure due to the airflow in the neck. In a variety of systems it is the under pressure created by airflow that induces and/or propagates the pinch-off of a bubble. In a co-focused jet, and the equivalent flow-focusing devices, it is the externally induced airflow that breaks up the bubbles. In other systems the collapse itself induces an airflow which becomes dominant in the final stages of bubble pinch-off (Phys. Rev. Lett. \textbf{98}, 144503 (2007)). Our system illustrates the importance of both contributions to the airflow, i.e., the external airflow induced by surface oscillations of the tip and the airflow induced in the neck by the collapse itself. Both of these contributions are of the same order and in Bernoulli's law the unsteadiness gives rise to terms of similar order. Surprisingly enough, all of these terms contribute with the same scaling exponent to the under pressure. [Preview Abstract] |
Sunday, November 23, 2008 9:18AM - 9:31AM |
AH.00007: Spherical cap bubbles with a bubbly crown C. Cossu, J.R. Landel, C.P. Caulfield Single large bubbles typically have a spherical cap shape with bubbles of larger volume rising faster than ones of smaller volume. However, except in well-controlled experiments, the released gas splits into a leading cap bubble followed by a crown of satellite bubbles that can contain up to 50\% of the total volume of gas. We find that in this case the satellite bubbles rearrange in a characteristic toroidal crown and the leading bubble takes a lenticular shape. The ratio of the torus radii to the leading cap curvature radius and the rise speed of these multipart bubbles are quite constant and predictable in the mean and are furthermore independent of the gas partitioning between the leading and the crown of satellite bubbles. We also show that this multi-part bubble rises slightly faster than a single cap bubble with the same total injected volume of gas. [Preview Abstract] |
Sunday, November 23, 2008 9:31AM - 9:44AM |
AH.00008: Bubble interactions with a traveling vortex tube Justin Finn, Ehsan Shams, Sourabh Apte We simulate the interaction of large bubbles with a traveling vortex tube using an Eulerian-Lagrangian discrete bubble model. The cases presented are 2D simplifications of a vortex ring studied experimentally by Sridhar \& Katz [JFM vol 397, 1999]. A plane jet is pulsed into a rectangular domain. After roll up into a vortex tube, eight bubbles are injected into its path to study the subsequent entrainment and vortex distortion. Three modeling approaches are considered: (a) one-way coupling; where the bubbles travel passively in the fluid, (b) two-way coupling; where the momentum exchange between the fluid and the bubbles is modeled, and (c) volumetric coupling; where the volume displacement of the fluid by the bubble motion and the momentum-exchange are modeled. It is found that a volumetric coupling model is critical to obtain any vortex distortion due to entrained bubbles. Parametric studies varying buoyancy effects relative to the vortex strength indicate that the greatest distortion of the vortex results from bubbles which continue to circle the vortex core after entrainment. Despite the two-dimensional approximation, bubble settling locations agree well with the experimental data. [Preview Abstract] |
Sunday, November 23, 2008 9:44AM - 9:57AM |
AH.00009: History force effects on contrast agent microbubbles in an ultrasound field Valeria Garbin, Benjamin Dollet, Leen van Wijngaarden, Nico de Jong, Detlef Lohse, Michel Versluis We study experimentally the radial and translational dynamics of an ultrasound contrast agent microbubble pair pulsating in an ultrasound field. The two bubbles attract each other through the so-called secondary Bjerknes force; quantifying these bubble-bubble interactions is therefore crucial for optimized medical imaging protocols. Using optical tweezers, we trap and control the distance between two microbubbles (BR-14, Bracco Research S.A., Geneva). We position the bubble pair away from the sample chamber wall, to prevent wall effects and quantify purely the acoustic bubble-bubble interaction and the dissipation due to viscosity in the fluid. The ultra-high speed Brandaris camera recorded the bubble dynamics at 15 million frames per second; from the optical measurements we track the instantaneous bubble radii and positions. We write a force balance for each bubble, assuming a no-slip boundary condition since the bubble interface is coated with a lipid monolayer to prevent dissolution. By comparison with the experimental results, we find that history effects are crucial to correctly account for the viscous forces. [Preview Abstract] |
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