Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session A11: Bubbles I |
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Chair: Francisco Pereira, CNR-INSEAN, The Italian Ship Model Basin Room: 26A |
Sunday, November 18, 2012 8:00AM - 8:13AM |
A11.00001: Formation of Micro-Scale Gas Pockets From Underwater Wall Orifices Francisco A. Pereira, Morteza Gharib Our experiments examine the formation of micro-scale gas pockets from orifices on walls with hydrophilic and hydrophobic wetting properties. Bubble injection is operated in a liquid at rest at constant flow rate and in a quasi-static regime, and the mechanism of bubble growth is investigated through high speed recordings. The growth dynamics is studied in terms of orifice size, surface wetting properties and buoyancy sign. The bubble formation is characterized by an explosive growth, with a pressure wave that causes the bubble to take highly transient shapes in its very initial stages, before stabilizing as a sphere and growing at a relatively slow rate. In case of positive buoyancy, the bubble elongates with the formation of a neck before detaching from the wall. When buoyancy acts towards the wall, the bubble attaches to the wall and expands laterally with a moving contact line. In presence of hydrophobic surfaces, the bubble attaches immediately to the wall irrespective of buoyancy direction and takes a hemispherical shape, expanding radially along the surface. A force balance is outlined to explain the different figures. [Preview Abstract] |
Sunday, November 18, 2012 8:13AM - 8:26AM |
A11.00002: Writing bubbles Sander Wildeman, Henri Lhuissier, Chao Sun, Andrea Prosperetti, Detlef Lohse We report on the nucleation of bubbles under a solid sphere immersed in a supersaturated liquid that is gently rubbed against a surface. For a fixed liquid supersaturation, bubbles are observed only above a certain rubbing velocity threshold. Above this threshold and provided that bubbles adhere better to the surface than to the sphere, a regularly spaced row of growing bubbles is left behind on the surface. Direct observation through a transparent sphere shows that each bubble in the row actually results from the early coalescence of several microscopic bubbles, which nucleate between the sphere and the surface. Together with the influence of the degree of supersaturation and the normal force between sphere and surface, we study the influence of the liquid itself (water or ethanol), the sphere material (glass, metal or Teflon) and of the surface roughness (polished or unpolished). Regardless of its precise origin, this method of ``writing bubbles'' also provides a simple way to spatially and temporally control the nucleation of bubbles on a surface and to study their interactions. [Preview Abstract] |
Sunday, November 18, 2012 8:26AM - 8:39AM |
A11.00003: The Influence of Injection Angle on Bubble Formation from a Micro-Pillar Farzad Houshmand, Daren Elcock, Yoav Peles Bubble formation in a microchannel in the presence of a 150 $\mu $m diameter micro-pillar was investigated. Nitrogen stream was injected into water flow in a 225 $\mu $m deep, 1500 $\mu $m wide, and 27.5 mm long horizontal microchannel through 20 $\mu $m slits cut on the micropillar located vertically in the centerline of the channel. Bubble formation in different devices with varying slit angles---with respect to liquid flow---of 0\r{ }, $\pm $30\r{ }, $\pm $80\r{ }, $\pm $110\r{ }, and 180\r{ } were studied for liquid flow rates of 13, 34 and 54 ml/min, and gas flow rates ranging from 0.5 to 7 ml/min. Based on high speed high magnification imaging, three distinct formation modes were observed depending on the slit angle and liquid and gas flow rates: discrete bubbling, attached ligament, and mixed modes. Micro-PIV technique was used to study the liquid flow in vicinity of the pillar to elucidate the phenomena controlling bubble formation. [Preview Abstract] |
Sunday, November 18, 2012 8:39AM - 8:52AM |
A11.00004: Highly time-resolved measurement for bubble nucleation induced by femtosecond laser pulses Yuki Mizushima, Takayuki Saito Femtosecond laser pulses (fs pulses) lead very interesting phenomena due to their extremely high energy density. The effects on substances are not thermal, but are multi-photon absorption. When this multi-photon absorption of fs pulses operates on water, extraordinary phenomena different from laser-induced cavitation by a usual laser such as nano-pulse laser are induced. In this study, fs pulses were focused on ultra-purified water in a glass cell through several types of lens. Some fs pulses split from original beams through a beam splitter were used as probing light of femtosecond order. Femtosecond-order time-resolved optical measurement was realized by adjusting a light path length of the probing light. We found out strange time-series process of refraction index changes of the water irradiated by the fs pulses, and the bubble nucleation and bubble growth, and the interesting bubble properties. Based on these results, we discuss a relation between those and fs-pulse peak intensity. Further, we discuss the nucleation and growth process from femtoseconds to picoseconds. [Preview Abstract] |
Sunday, November 18, 2012 8:52AM - 9:05AM |
A11.00005: Simulated Collapse of Small Bubble Clusters on a Wall Arpit Tiwari, Carlos Pantano, Jonathan B. Freund The dynamics of bubble clusters have largely been studied either via simplified continuum models for bubbly liquids that augment single-phase system with equations for spherical bubble dynamics (forgoing shape distortions and jetting) or by assuming the liquid to be incompressible. However, these simplifications are not fully applicable, even for a near-wall single-bubble collapse. It is expected that jetting and acoustic emissions potentially play an important role, for example, in damage to the wall. We present simulations in which up to 50 bubbles interact in a compressible liquid adjacent a wall. A new numerical scheme enables these simulations by using a diffuse-interface-based Eulerian approach with a multiphase mixture model that maintains interface integrity without requiring the geometric description demanded in typical interface tracking methods. The properties of this scheme and its integration in an adaptive mesh refinement framework facilitate simulation of three-dimensional clusters. We will discuss the cluster dynamics, including the collapse and re-expansion history, and particularly assess pressure on the wall as potential surrogate for damage. We see strong, though at times significantly asymmetric and transitory, focused high and low pressures on the wall. [Preview Abstract] |
Sunday, November 18, 2012 9:05AM - 9:18AM |
A11.00006: Simulations of surfactant effects on the coalescence of drops and bubbles David Martin, Francois Blanchette We present simulations of coalescence in the presence of surfactant. We assume axial symmetry, and consider a fluid-fluid interface on which surfactant concentration and mass are tracked as functions of arclength. Our model can account for two physically distinct setups: a soap bubble merging with a suspended soap film; and a surfactant covered liquid drop merging with a reservoir. In both cases, we describe the regime in which coalescence is only partial. Along with viscous effects, represented by the Ohnesorge number, the elasticity of the surface tension relative to the surfactant concentration is seen to play a key role, and exhibits a surprising nonmonotonic influence. Effects of gravity are also simulated, along with effects of differing initial conditions, including uneven initial surfactant concentration, as is likely to arise in physical applications. [Preview Abstract] |
Sunday, November 18, 2012 9:18AM - 9:31AM |
A11.00007: Large Bubble Rupture Sparks Fast Liquid Jet Thomas Seon, Arnaud Antkowiak The novel experimental observation of long and narrow jets shooting out in disconnecting large elongated bubbles is presented. We investigate this phenomenon by carrying out experiments varying the control parameters and we propose a universal scaling law for the jet velocity, which unexpectedly involves the bubble height to the power 3/2. This anomalous exponent suggests an energy focusing phenomenon. We demonstrate experimentally that this focusing is purely gravity-driven and independent of the pinch-off singularity. [Preview Abstract] |
Sunday, November 18, 2012 9:31AM - 9:44AM |
A11.00008: Coalescence cascade of bubbles Fenghua Zhang, Peter Taborek, Sigurdur Thoroddsen Coalescence of two bubbles produces satellite bubbles, known as partial coalescence. The coalescence of the satellite with the main bubble may produce even smaller satellites, so that the coalescence proceeds like a cascade. While the coalescence cascade of a drop has been well known, we show here for the first time that bubbles can do so too. By performing experiments using high-speed imaging and the non-dimensional analysis, the sizes and formation time of the generated satellites are characterized and explained. In addition, we have observed satellites formation at focusing of multiple waves for the first time, as well as formation of several sub-satellites. The conditions of their formation are identified. These findings are important for multi-phase fluid flows, like foams or emulsions, separation of immiscible fluids, printing and painting, etc. [Preview Abstract] |
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