Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session S5: Drops XII: Coalescence |
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Chair: Jacco Snoeijer, University of Twente Room: 308 |
Tuesday, November 22, 2011 3:05PM - 3:18PM |
S5.00001: Measurements of liquid film thickness for a droplet at a two-fluid interface Gosse Oldenziel, Jerry Westerweel, Rene Delfos Coalescence of a droplet at a two-fluid interface is studied at Bond numbers larger than one and at three different values of the viscosity ratio. Both the thickness of the liquid film between the rising droplet and the two-fluid interface, and the location of film rupture are measured using Laser Induced Fluorescence. Particle Image Velocimetry was applied to the flow in the film. It is found that the film thins asymetrically, and that the time interval between collision and film rupture is shorter than predicted by commonly used models. The film ruptures at an off-center location. It can be concluded that asymmetric film drainage speeds up coalescence. [Preview Abstract] |
Tuesday, November 22, 2011 3:18PM - 3:31PM |
S5.00002: Coalescence of liquid drops Santosh Appathurai, Michael Harris, Osman Basaran, Joseph Paulsen, Juston Burton, Sidney Nagel Drop coalescence plays a central role in industrial contexts, e.g. emulsions, sintering processes, and inkjet printing, as well as in everyday phenomena ranging from dripping faucets to raindrops in clouds. During coalescence, two drops touch each other and then merge as a liquid bridge grows from microscopic scales to a size comparable to the drop diameter. This process has been thought to have just two regimes: a highly viscous one during the initial stages, pulling the drops together, and an inertial one later on, dominated by interface deformations near the neck. We use high-speed imaging, electrical measurements and full Navier-Stokes simulations to reveal a new regime that governs the asymptotic dynamics of coalescence for any finite viscosity in three dimensions. [Preview Abstract] |
Tuesday, November 22, 2011 3:31PM - 3:44PM |
S5.00003: Effects of Marangoni Stresses on Drop Coalescence Sofya Kabachek, H. Pirouz Kavehpour In many applications involving drop-drop interactions, such as alcohol-based fuel cells and many microfluid devices, miscible fluids are not necessarily of the same kind. To better understand these phenomena, further insight into coalescence of different fluids is essential. When miscible drops of different liquids come in contact, coalescence often occurs and a surface tension difference may arise at the interface. This difference plays an important role in the process due to the Marangoni effect. To quantify the extent of this effect we conducted a parametric study on the effects of surface tension variation on drop coalescence (partial and full); in particular coalescence time and drop diameter ratios were investigated. A high speed digital camera with high resolution was utilized to study the evolution of drops during the coalescence process. We showed that coalescing drops with higher surface tension than the reservoir fluid, behave similarly to drops coalescing in a viscous environment. A new physical model was developed in good agreement with the experimental data. [Preview Abstract] |
Tuesday, November 22, 2011 3:44PM - 3:57PM |
S5.00004: Mixing in Sessile Drops Merging on a Surface Shelley Anna, Ying Zhang, Samuel Oberdick, Stephen Garoff We investigate the mixing of two sessile drops that merge on a surface. The drops consist of low viscosity glycerol-water mixtures deposited on a silicone elastomer surface with contact angle near 90$^{\circ}$. We observe the shape of the drops and the location of their intersection by placing a fluorescent dye in one drop and using a laser light sheet to image a plane perpendicular to the surface. The initial healing of the meniscus bridge between the merging drops, and the damping of capillary waves appearing on their surfaces occur on timescales comparable to the inertio-capillary relaxation time. However, the interface between the two fluids remains sharp, broadening diffusively over several minutes. The shape of the merged drops and the boundary between them also continues to evolve on a timescale of minutes. This later motion is controlled by gravity, capillary pressure, and viscous stresses. Images of the 3D drop shape indicate that small contact line motions are correlated to the slow relaxation. Although the two drops contain identical liquids except for the presence of the dye, the shape of the interface consistently evolves asymmetrically, assuming a characteristic crescent shape. We note that very tiny surface tension gradients can produce an asymmetric flow like the one observed here. We characterize the long timescale flow as a function of the drop sizes, and we use numerical simulations to aid in elucidating the essential physics. [Preview Abstract] |
Tuesday, November 22, 2011 3:57PM - 4:10PM |
S5.00005: Vortex-driven charge transfer between partially coalescing droplets William Ristenpart, J.C. Creasey, B.S. Hamlin Oppositely charged drops fail to coalesce above a critical field strength, despite the attractive force between the opposite charges. Here we report the existence of a critical ionic conductivity below which oppositely charged drops only partially coalesce. The degree of coalescence of water drops in oil can be tuned from complete coalescence at low field strengths to complete non-coalescence at high field strengths, thus providing external control over the size of the resulting daughter droplet. Strikingly, in this regime the size and charge of the daughter droplet are both independent of the conductivity. We present evidence suggesting the charge transfer is instead dominated by convection associated with the capillary-driven penetration of a vortex into the larger drop, and we demonstrate that the size of the daughter droplet is consistent with a scaling model based on a balance between capillary-driven inertia and electrostatic repulsion. [Preview Abstract] |
Tuesday, November 22, 2011 4:10PM - 4:23PM |
S5.00006: ABSTRACT WITHDRAWN |
Tuesday, November 22, 2011 4:23PM - 4:36PM |
S5.00007: Drop impacting on a hydrophobic elastic beam Daniel Chique Canache, Sunghwan Jung Plant suraces found in nature often exhibit hydrophobic wetting properties; in particular, the surface of leaves are an example. When a water drop impacts a leaf a unique system of coupled solid and fluid mechanics is observed. By replacing the leaf as as simple thin polycarbonate cantilever beam it is possible to create a workable model for the system. A high-speed camera allows detailed observation of the dynamics of the beam and drop at the moment of impact. Through image analysis, the position and shape of the beam and drop are analyzed to calculate bending energy and kinetic energy. Experiments show that the available energy in the system is close to 0.1 mJ. The results of this experiment provide insight into energy harvesting from raindrops using a piezo cantilever. [Preview Abstract] |
Tuesday, November 22, 2011 4:36PM - 4:49PM |
S5.00008: Drop interaction with the ejecta sheet M.-J. Thoraval, S.T. Thoroddsen, K. Takehara, T.G. Etoh, P. Ray, C. Josserand, S. Zaleski We studied experimentally and numerically the dynamics of the ejecta sheet produced by a drop impacting on a deep pool of the same liquid at high Reynolds and Weber numbers. Ultra-high speed imaging revealed a diversity of evolutions by using different mixtures of water and glycerine, and different impact velocities. We observed a transition from a smooth ejecta sheet to a more irregular splashing for a Reynolds number of 3500. In this transition regime, the ejecta sheet interacts with the impacting drop. This interaction can pull the ejecta sheet towards the centre of the drop at lower splash parameters, or generate a bumping on the ejecta sheet moving outwards at higher splash parameters. The volume of fluid (VOF) code Gerris was used to reproduce numerically this peculiar dynamics in axisymmetric conditions. Very good agreement with the experiments was reached by using adaptive refinement and parallelization of the calculations. [Preview Abstract] |
Tuesday, November 22, 2011 4:49PM - 5:02PM |
S5.00009: Dimensional crossover in the viscous dynamics of coalescence of liquid drops confined in between two plates Maria Yokota, Ko Okumura Recently, it has been established that the dynamics of coalescence of liquid drops, driven by capillary force, is balanced by viscous force at shorter times (or in viscous drops) and by inertial force at longer times (or in less-viscous drops) [1, 2]. This has been confirmed also in two-dimensional coalescence [3]. Here, we study coalescence of a liquid droplet to a bath of the same liquid in a confined geometry of a Hele-Shaw cell [4]. We followed the dynamics of the neck which bridges the drop and the liquid bath. We find, in a single coalescence event, a crossover from a three dimensional viscous dynamics to another quasi two-dimensional viscous dynamics. This crossover is established by demonstrating clear data collapse, thanks to simple dimensional arguments. We discuss further the third scaling regime between the two viscous regimes, together with an unusual charge effect on the coalescence which makes the dynamics self-similar. \newline [1] Eggers J, Lister JR, Stone HA (1999) J. Fluid. Mech. 401:293-310. \newline [2] Aarts DGAL, Lekkerkerker HNW, Guo H, Wegdam GH, Bonn D (2005) Phys. Rev. Lett. 95:164503. \newline [3] Burton JC, Taborek P (2007) Phys. Rev. Lett. 98:224502. \newline [4] Yokota M, Okumura K (2011) Proc. Nat. Acad. Sci. 108:6395. [Preview Abstract] |
Tuesday, November 22, 2011 5:02PM - 5:15PM |
S5.00010: The Effect of Surfactants on the Breakup of an Axisymmetric Laminar Jet Justin Walker, Richard Calabrese The breakup of a laminar axisymmetric jet is a well-studied fluid dynamics phenomenon, first studied by Savart (1833) and Rayleigh (1879). Many papers have been published over the years describing the theory of jet breakup, such as the paper by Tomotika (1935). More recently, many studies have been performed using various computational simulations to better understand the mechanics of jet breakup, notable among these are Homma et al. (2006). Despite the extensive literature on the topic, the impact of surface active agents on jet breakup has received limited attention, whether due to the system's inherent complexity or a poor understanding of the mechanics of the action of surface active agents themselves. In this study, the drop size distribution and jet breakup length resulting from the breakup of liquid jet systems were studied experimentally. Jets were formed by forcing a fluid through a narrow capillary using pneumatic pressure. Experiments involving oil-water jets with aqueous surfactants were performed. Several distinct regimes were identified based on hydrodynamic and physicochemical conditions. Jet length was found to increase with surfactant concentration, while droplet diameter was found to decrease (dependent on jet regime). A Semiempirical model to predict the breakup length of Jets in the presence of surfactants is also proposed. [Preview Abstract] |
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