Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session A13: Drops: Splashing |
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Chair: Arne Pearlstein, University of Illinois Urbana-Champaign Room: Georgia World Congress Center B218 |
Sunday, November 18, 2018 8:00AM - 8:13AM |
A13.00001: Ejecta, corolla and splashes from drop impacts on viscous fluids Christophe Josserand, Florence Marcotte, Guy-Jean Michon, Thomas Seon We investigate both experimentally and numerically the impact of ethanol drops on deep pools of aqueous glycerol solutions with variable pool viscosity and air pressure. With this approach we are able to address |
Sunday, November 18, 2018 8:13AM - 8:26AM |
A13.00002: Multiple long-time solutions and helical wakes with drop spinning for nitrobenzene drops falling in water Louis L. Steytler, Arne J. Pearlstein We report three-dimensional numerical simulations of freely falling nitrobenzene drops in water, for a range of drop size (2 - 4 mm diameter), covering several flow regimes, from steady, axisymmetric, laminar flows with a wake without flow separation, to transitional flows with highly complex flows that are unsteady, and three-dimensional. Beyond an initial transient, a 4-mm diameter drop falls essentially vertically and "spins" about a vertical axis approximately coinciding with the drop center-of-mass. The spinning motion is accompanied by a helical-like wake. For a 3.8-mm drop, different prescribed initial transients give rise to two long-time solutions: an axisymmetric drop with time-periodic vortex shedding and shape oscillations, and a spinning drop with a helical-like wake, similar to the 4-mm drop. The wake structure is discussed and possible mechanisms for the spinning motion are discussed. |
Sunday, November 18, 2018 8:26AM - 8:39AM |
A13.00003: Spreading and splashing of droplets impacting superhydrophobic substrates Guillaume Riboux, Enrique S. Quintero, Jose M Gordillo A drop of radius R impacting a superhydrophobic substrate at a velocity V keeps its integrity and spreads over the solid for V<Vc or splashes, disintegrating into tiny droplets violently ejected radially outwards for V>Vc, with Vc the critical velocity for splashing. In contrast with the case of drop impact onto a hydrophilic substrate, Riboux & Gordillo [Phys. Rev. Lett. 113, 024507 (2014)], our experiments reveal that the critical condition for the splashing of water droplets impacting a hydrophobic substrate at normal atmospheric conditions is characterized by a value of the critical Weber number Wec=ρVc2R/σ ≈70, far smaller than the one corresponding to the partially wetting case. Here we present a self consistent model, in very good agreement with experiments, capable to predict Wec as well as the full dynamics of the drop expansion and disintegration for We>Wec. In particular, our model is able to accurately predict the time evolution of the rim bordering the expanding lamella, Rt(T) as well as the radii Rd(T)«R and velocities Vd(T)»V of the small and fast droplets ejected when We>Wec. |
Sunday, November 18, 2018 8:39AM - 8:52AM |
A13.00004: Rim thickness evolution in unsteady sheet fragmentation L. Bourouiba, Y. Wang, R. Dandekar, N. Bustos, S. Poulain Upon impact on a finite solid surface, a drop first expands into a sheet in the air, surrounded by a rim that destabilizes into ligaments that, in turn, shed secondary drops. The rim thickness is a key part of this system, set by the balance between incoming fluid from the expanding sheet and the fluid feeding the ligaments. This thickness is inherently unsteady, constantly changing with time. Linear stability analysis can explain the growth of an initial perturbation on a rim’s steady base-state, as long as the perturbation remains small. However, once the perturbation is large, the predictive power of such approach breaks down and the rim thickness is not known. We combine theory with advanced image analysis to study the unsteady rim destabilization and thickness evolution. We show that, at all times, the rim thickness is governed by a local instantaneous Bond number equal to unity, defined with the instantaneous, local, unsteady rim acceleration. We found that this criterion is robust and universal for a family of unsteady inviscid fluid sheet fragmentation phenomena, from impacts of drops on various surface geometries to impacts on films. We further discuss under which viscous and viscoelastic conditions the criterion continues to govern the unsteady rim thickness. |
Sunday, November 18, 2018 8:52AM - 9:05AM |
A13.00005: Ligament dynamics along unsteady rims Y. Wang, L. Bourouiba Upon impact on a finite solid surface, a drop first expands into a sheet in the air, surrounded by a rim that destabilizes into ligaments that, in turn, shed secondary droplets. As the sheet expands, the thickness of the rim surrounding it continuously evolves, as do the ligaments that shed fluid from it. The time evolution of the number of ligaments and the distribution of the distances between them is not captured by classical stability analysis. We combine advanced image processing algorithms and theory to elucidate the evolution of the population of ligaments along such unsteady rims. |
Sunday, November 18, 2018 9:05AM - 9:18AM |
A13.00006: Splashing of a water drop containing a few particles Weiwei Zhao, Shiji Lin, Longquan Chen, Sigurdur T. Thoroddsen, Marie-Jean Thoraval We study the effects of adding one or a few solid particles in a drop impacting onto a solid surface. The solid particles are inserted at the bottom of the drop before it impacts onto a solid surface at different impact velocities. The particles first hit the solid surface and rebounds vertically, first through the drop and then eventually emerging from its apex. For the case of a single particle, we have systematically varied the particle size, density and impact velocity, to characterize the velocity threshold above which the particle separates from the drop. We have then extended the study to adding up to four particles simultaneously in the impacting drop. We propose some scaling arguments to model the separation of the particle and the fascinating splashing morphology observed. |
Sunday, November 18, 2018 9:18AM - 9:31AM |
A13.00007: Stick or Splash? Thixotropy decides Samya Sen, Randy H Ewoldt We use high-speed imaging to study droplet impacts of thixotropic non-Newtonian fluids on pre-coated horizontal surfaces to explore the effect of thixotropic aging and rejuvenation on impact behavior, an aspect missing from the literature on this subject. Our results show that thixotropic aging has a significant effect on the stick-or-splash dynamics of yield-stress fluids: aged samples are more likely to stick. We test existing dimensionless groups with fluids of different microstructure (Carbopol and Laponite) and various degrees of thixotropic aging properties. Characterizing flow properties of aged samples is challenging since any deformation inherently rejuvenates and breaks down the microstructure and rheological properties. Steady state properties are therefore undefined for aged samples, and we propose alternative methods for characterizing aged rheological properties and using these in dimensionless groups that rationalize the stick-or-splash regime maps. |
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