Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session S05: Drops: Coalescence (5:45pm - 6:30pm CST)Interactive On Demand
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S05.00001: Substrate Wettability Affects Mixing During Droplet Coalescence Thomas C. Sykes, David Harbottle, Zinedine Khatir, Harvey M. Thompson, Mark C. T. Wilson Internal jets can vastly improve mixing efficiency during droplet coalescence. For free droplets, jet formation depends on geometry, initial configuration and fluid properties. When at least one of the coalescing droplets is on a substrate, we systematically demonstrate that the three-phase contact line and substrate wettability are crucial too. In particular, using both high-speed imaging and quantitatively validated numerical simulations incorporating the Kistler dynamic contact angle model (with hysteresis) we investigate internal jet formation during the coalescence of an initially static free droplet and a sessile droplet of the same fluid. We identify and elucidate a mechanism of jet formation arising for surprisingly low sessile to free droplet volume ratios, showing that the presence of a substrate can improve mixing efficiency. Moreover, this mechanism is found to depend on substrate wettability with the importance of advancing contact angle subordinated to that of receding contact angle. Droplet geometry and fluid properties are also considered to thoroughly explain the dynamics observed. [Preview Abstract] |
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S05.00002: The role of surface tension gradients in coalescence of two unequal-size drops Swati Singh, Arun K Saha The coalescence of two drops in a gaseous surrounding has received special attention due to its importance in both natural and industrial process, such as raindrop/cloud formation, spray atomization and microfluidic devices. In the present work, the coalescence dynamics of two drops made of miscible but distinct fluids, placed one over another in a vertical position are studied. The five non-dimensional parameters are used to describe the flow mechanism: Ohnesorge number, Bond number, Schmidt number, drop diameter ratio and lower to upper drop surface tension ratio. A two-dimensional axisymmetric simulation using Coupled Level set and Volume of fluid method (CLSVOF) has been performed to unveil the underlying physics under varying drop diameter ratio (1.0-3.0), Ohnesorge number (0.003-0.02) and surface tension ratio (0.3-1.5). Result shows three distinct regimes: the appearance, disappearance and re-appearance of partial coalescence with decreasing surface tension ratio. With increasing Ohnesorge number, the effect of Marangoni flow is dampened by the high viscous forces resulting in complete coalescence of drop into the bulk liquid. Moreover, a reduction in the parent size ratio is also noted to result in total coalescence. [Preview Abstract] |
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S05.00003: Molecular Events Kick-off Droplet Coalescence Sreehari Perumanath, Matthew K. Borg, Mykyta V. Chubynsky, James E. Sprittles, Jason M. Reese Ranging from the formation of thunderstorms to the operation of office printers, droplet-based systems are ubiquitous in our everyday life. However, thus far we have only had a partial understanding of the mechanisms by which two or more droplets coalesce to form a larger droplet. The classical mechanism underlying coalescence of two droplets is that surface tension drives the process right from the beginning. Using computationally expensive molecular simulations, here we show that it is in fact thermal capillary waves on the droplets' surface that initiate single or multiple contacts between nanodroplets, and coalescence thereafter commences in a `thermal regime'. Here, the bridge expands linearly in time due to collective molecular jumps near the bridge fronts. In this non-classical regime, surface tension instead acts to suppress the bridge growth rather than enhance it. Transition to the classical hydrodynamic regime only occurs once the bridge radius exceeds a size-dependent thermal length scale, which needs to be considered in hydrodynamic analyses of droplet coalescence. [Preview Abstract] |
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S05.00004: How sedimenting droplets grow and stir the fluid Akshay Bhatnagar, Bernhard Mehlig, Dhrubaditya Mitra, Prasad Perlekar We study a suspension of droplets in the presence of gravitational settling by using Cahn-Hilliard-Navier-Stokes model. In the early stage of time evolution, droplets diffuse and merge into each other. During this stage, the typical size of a droplet $L$ grows as $t^{1/3}$. As the droplet-size becomes larger they sediment and grow faster: $L$ grows as $t^{2/3}$. The settling droplets stir the flow and produce pseudo-turbulence. The kinetic energy of the flow grows as $t^{5/3}$. [Preview Abstract] |
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S05.00005: Relaxation of coalescing drops with insoluble surfactants at high Capillary number: the surprising encapsulation of a tiny drop inside the mother drop Carolina Vannozzi A new phenomenon is reported, discovered while extending previous studies on the relaxation of two viscous drops, previously undergoing a head-on collision in an extensional flow for low Re[1], to the case with insoluble surfactants at the interface. As the Capillary number increases above 0.1, the drops are highly deformed, to the point that each drop forms two lobes separated by a thinning neck. Surfactants are convected outside the neck area and, given enough time, the drops will break up into four smaller drops. However, for certain neck thicknesses and surfactant concentrations and diffusivities, if the flow is stopped, the drops will relax back towards a spherical shape, but the Marangoni stresses, generated by the initial surfactant distribution, will also create a flow that will push the matrix fluid towards the inside of the drops. Eventually two facing spherical drops will be formed, each having inside another surfactant rich tiny drop, resulted from a break up mechanism resembling tip streaming or tip dripping [2]. [1]Vannozzi, C. Relaxation and coalescence of two equal sized viscous drops in a quiescent matrix J. Fluid Mech. 2012.[2]Egleton C.D., Tsai T.-M., Stebe K.J., Tip streaming from a drop in the presence of surfactants , Phys. Rev. Lett. 2001. [Preview Abstract] |
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