Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session OF: Drops and Bubbles IX: Drop Coalescence and Breakup |
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Chair: L. Gary Leal, University of California, Santa Barbara Room: Tampa Marriott Waterside Hotel and Marina Florida Salon 4 |
Tuesday, November 21, 2006 12:15PM - 12:28PM |
OF.00001: Studies of Surfactant Effects in the Breakup and Coalescence of Viscous Drops L. Gary Leal, Bing Dai, Adam Hsu, Yosang Yoon In the present study, we utilize boundary integral solutions in an attempt to understand recent ``unexpected'' experimental results for surfactant effects on the breakup and coalescence of viscous drops in low Reynolds number flow. The experiments were carried out using low molecular weight polymers that acted as viscous Newtonian fluids, with block copolymers that act as insoluble surfactants. Model parameters were selected, so far as possible, to correspond to the experimental systems. For drop breakup, we explore the non-uniformity of the surfactant concentration on the interface, which does not always increase with increase of the shear rate, as expected. For coalescence, we examine a number of unexplained results including: the details of the inhibition of film rupture due to Marangoni effects; the fact that the scaling with capillary number remains the same as for a clean interface system; and the discontinuous transition in the coalescence process that occurs for low surface coverage as the capillary number is increased. [Preview Abstract] |
Tuesday, November 21, 2006 12:28PM - 12:41PM |
OF.00002: Coalescence in low viscosity liquids Sarah Case, Sidney Nagel The coalescence of two fluid drops is an extremely rapid process. To study very early stages of coalescence, we employ a modified electrical method[1]. A drop of aqueous NaCl solution is suspended in air above a flat surface of the same solution. A constant voltage is maintained across the system. The flat surface is raised until it touches the drop at which point a rapidly widening bridge forms between them. During coalescence, we measure the resistance of the system, thus obtaining the time dependence of the ratio $r^{2}/L$, where $r$ is the characteristic bridge radius and $L$ is its characteristic length. Previous photographic studies[2] have reported a $20-60 \ \mu s$ lag between the apparent initiation of coalescence and initiation of electrical contact used to trigger the camera. However, we observe a smooth evolution in the resistance after the initiation of electrical contact. By combining high-speed imaging with our electrical measurements, we are able to address the cause of this discrepancy. \newline [1] J. C. Burton, J. E. Rutledge, and P. Taborek, Phys. Rev. Lett., 92, 244505 (2004) \newline [2] S. T. Thoroddsen, K. Takehara, and T. G. Etoh, J. Fluid Mech., 527, 85-114 (2005) [Preview Abstract] |
Tuesday, November 21, 2006 12:41PM - 12:54PM |
OF.00003: Role of Dimensionality in the Pinch-off and Coalescence of Thin Liquid Alkane Lenses Floating on Water J.C. Burton, P. Taborek We present high-speed videos of the pinch-off and coalescence of thin liquid alkane lenses floating on water. Pinch-off in quasi-2D lenses is distinctly different from pinch-off in 3D drops, and involves a cascade of satellite droplets which extends to micron length scales. In contrast, coalescence of lenses is very similar to coalescence of 3D drops. Coalescence is predicted to involve entrainment of the exterior fluid as the droplets merge. This reentrant folding is obscured in 3D droplets, but is clearly visible in coalescence of thin lenses. [Preview Abstract] |
Tuesday, November 21, 2006 12:54PM - 1:07PM |
OF.00004: Effect of Collision Angle on Binary Droplet Coalescence Jungyong Kim, Ellen Longmire Drop pairs of water/glycerin solution were injected into silicone oil of lower density through opposing tubes at varying initial angles with the goal of controlling the eventual collision angles. Simultaneous dual-field PIV measurements were obtained in index-matched fluids to characterize coalescence and rebounding behavior. The larger field captured trajectories, and the smaller field captured the thin film region. Experiments were performed for Weber numbers [\textit{We}] in the range of 1-50 and collision angles of 15-80 degrees below the horizontal. Above \textit{We} $\sim $ 10, drops coalesced, with the rebounding/coalescence boundary shifting to higher \textit{We} with increasing collision angle. Also, the collision angle affected the eventual location of film rupture. The rupture location moved higher in the thin film region as the collision angle increased. Interactions of vortex rings within drops and strong deformation associated with shallow collision angles and sufficient \textit{We} encouraged coalescence. Details of these interactions will be discussed in the presentation. Supported by Petroleum Research Fund (42939-AC9) and NSF (CTS-0320327). [Preview Abstract] |
Tuesday, November 21, 2006 1:07PM - 1:20PM |
OF.00005: Hysteretic rheological response of a highly viscous drop in linear flows with rotation Yuan-Nan Young, Jerzy Blawzdziewicz, Vittorio Christini High-viscosity drops in flows with a small vorticity magnitude are known to possess two stable stationary shapes. One corresponds to nearly spherical drops stabilized primarily by rotation, and the other to elongated drops stabilized primarily by cpaillary forces. In this work we explore interesting quasi-static dynamics of high-viscosity drops in a linear flow with small rotation. For sufficiently high drop viscosity and an appropriate range of the vorticity component in the linear flow, the quasi-static drop states are found to be hysteretic. We explore in details such hysteretic drop dynamics using both small deformation equations and direct numerical simulations of the drop dynamics. Due to the multiplicity of the two steady drop states, we find novel chaotic drop dynamics in a linear flow with (temporal) sinusoidal variation in the vorticity. Based on these results we also suggest how to control size distribution of viscous drops in emulsion by forcing the emulsion in certain fashions. [Preview Abstract] |
Tuesday, November 21, 2006 1:20PM - 1:33PM |
OF.00006: Dynamics of colloidal consolidation process inside an emulsion droplet Danhong Wang, Patrick Spicer, Amy Shen Dense packing of small clusters of microspheres proposed by Manoharan, et al. involves removal of fluid from the droplets (dispersed phase) into the continuous phase, which is referred to as the consolidation process. The consolidation process can be characterized as a diffusion process with moving boundaries. Therefore its dynamics is largely controlled by the diffusivity between the dispersed and continuous phases. In addition to the diffusivity, surfactant concentration and the number of particles inside the droplets might also change the dynamics of the consolidation process. In this work, we study the effects of surfactant concentration and particle number on the consolidation process. We found that if normalized by the initial droplet size and the consolidation time, the consolidation process obeys the same power law with the power coefficient of 1/2, regardless of the significant change in droplet diameter, as well as surfactant concentration and particle numbers. We also examine the consolidation behavior of anisotropic particles and compare with that of spheres. [Preview Abstract] |
Tuesday, November 21, 2006 1:33PM - 1:46PM |
OF.00007: Emulsification of a very viscous liquid in water J.F. Hernandez-Sanchez, R. Zenit, G.M. Homsy Although emulsions are used widely, the process of emulsification is still largely based on empiricism. It is our interest to understand the basic mechanism that leads to breakage of a very viscous liquid in water. This particular case is of interest for the petroleum industry, as a means to transport and dispose of oil refining residues. Visualization experiments have been performed to investigate the mechanisms that lead to droplet formation in an ordinary mixing tank configuration. An impeller was immersed in a container with two unmixed immiscible liquids (water/silicon oil) that had a very large viscosity difference (1/30000). The rotational speed of the impeller was gradually increased up to Re $\approx$ 110,000 based on the properties of water, or Re $\approx$ 4, based on those of the oil. The dynamics of the system are, therefore, a combination of turbulent and creeping flows, a regime that has not been widely explored to date. As the rotational speed of the impeller increases the interface between the two liquids develops a curved cup-like shape. When the curved interface reaches the impeller blades, it becomes deformed, disrupted and, if the shear is strong enough, breaks. As a result of the breakage, long viscous filaments form which are stretched and further broken up by a combination of capillary instability and turbulent fluctuations. Visualization images and scaling arguments will be presented. This project is funded by the UC-MEXUS collaboration program. [Preview Abstract] |
Tuesday, November 21, 2006 1:46PM - 1:59PM |
OF.00008: The effects of confinement and inertia on the production of droplets Yuriko Renardy Recent experiments of Sibillo et al (2005 Society of Rheology Meeting) investigate the effect of walls on flow-induced drop deformation for Stokes flow. The drop and the fluid in which it is suspended have the same viscosities. The capillary numbers vary from 0.4 to 0.46. They find that complex start-up transients are observed with overshoots and undershoots in drop deformation. Drop breakup is inhibited by lowering the gap. The ratio of drop radius to wall separation is 0.34. We show that inertia can enhance elongation to break the drop by examining Reynolds numbers in the range 1 to 10. The volumes of the daughter drops can be larger than in the unbounded case, and even result in the production of monodisperse droplets. [Preview Abstract] |
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