Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session A35: Drops: Impact and Splashing |
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Chair: Arnout Boelens, University of Chicago Room: Ballroom B |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A35.00001: Computational comparison of high and low viscosity micro-scale droplets splashing on a dry surface Arnout Boelens, Andrzej Latka, Juan de Pablo Depending on viscosity, a droplet splashing on a dry surface can splash immediately upon impact, a so called prompt splash, or after initially spreading on the surface, a late splash. One of the open questions in splashing is whether the mechanism behind both kinds of splashing is the same or not. Simulation results are presented comparing splashing of low viscosity ethanol with high viscosity silicone oil in air. The droplets are several hundred microns large. The simulations are 2D, and are performed using a Volume Of Fluid approach with a Finite Volume technique. The contact line is described using the Generalized Navier Boundary Condition. Both the gas phase and the liquid phase are assumed to be incompressible. The results of the simulations show good agreement with experiments. Observations that are reproduced include the effect of reduced ambient pressure suppressing splashing, and the details of liquid sheet formation and breakup. While the liquid sheet ejected in an early splash breaks up at its far edge, the liquid sheet ejected in a late splash breaks up close to the droplet. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A35.00002: The role of substrate wetting and drop shape in splashing dynamics Andrzej Latka, Sidney Nagel The impact of a liquid drop on a solid surface yields a beautiful splash via an intricate interaction of the liquid, the substrate and, most surprisingly, the surrounding air. Varying the liquid's viscosity or surface tension and the substrate's roughness or elasticity results in strikingly different splash morphologies. While one might also have expected the wetting properties to affect splashing, we show here that changing the substrate from fully wetting to non-wetting does not significantly alter the splashing behavior. We also investigate how the drop's evolving shape influences the dynamics. When the drop first contacts the surface, there is a region of high negative curvature; the bulk of the descending liquid must feed the sheet rapidly spreading over the substrate by flowing around this concavity. After the drop has spread sufficiently, this concavity disappears and the flows within the drop change their shape. We find that the effect of air on splashing is significantly stronger in the initial regime. This dependence sheds light on low-velocity drop impacts. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A35.00003: Time-resolved interference imaging of the air disc under an impacting drop E. Q. Li, S. T. Thoroddsen Water drop impacting on dry, solid surface, is rapidly decelerated by an air cushion. This thin air layer is formed by lubrication pressure in the gas, which is strong enough to stop the inertia of the drop liquid and deform its bottom tip. The contact of the drop with the solid therefore occurs along a ring, entrapping a central bubble. For very large impact velocities the lubrication pressure becomes large enough to compress the gas. We use the Kirana ultra-high-speed video camera and 50 ns pulsed laser-diodes for interferometric imaging, at time-resolution of 200 ns. We capture the evolution of the air-layer thickness profile over the entire bubble entrapment process. The maximum diameter of the air disc is in perfect agreement with earlier theoretical models, if one uses the bottom radius of curvature of the drop. The air-layer thickness is also in agreement with available theoretical models, if one assumes adiabatic compression of the gas. For the largest impact velocities the air is compressed by more than a factor of 10. Immediately after first contact, the air disc expands rapidly in the vertical. The outer edge of the air-disk forms a kink in the free surface. This kink can move radially outwards just before contact, at speed as large as 50 times the impact velocity. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A35.00004: Splash of a liquid drop on a dry solid surface Shruti Mishra, Shreyas Mandre, Chris Rycroft, Michael Brenner We study the early-time fluid mechanical phenomena of the splash of a liquid drop on a solid surface, focusing on the dynamics before contact through the intervening air layer. Previous theoretical work (e.g. Mani, Mandre and Brenner [Journal of Fluid Mech., (2010), vol. 647, pp. 163–185) on this problem neglected viscous effects in the liquid. However, a set of recent experiments show definitively that even at early times viscous effects in the liquid are important, and in particular have the ability to dramatically change the shape of the interface before contact. We describe a set of computations aimed to reproduce these experimental features. The simulations couple lubrication flow in the gas layer with nonsteady Stokes flow in the liquid, and surface tension at the liquid-air interface. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A35.00005: Drop impact on flowing liquid films: asymmetric splashing Renad Ismail, Zhizhao Che, Lauren Rotkovitz, Idris Adebayo, Omar Matar The splashing of droplets on flowing liquid films is studied experimentally using high-speed photography. The flowing liquid films are generated on an inclined substrate. The flow rate of the liquid film, the inclination angle, and the droplet speed are controlled and their effects on the splashing process studied. Due to the flow in the liquid film and the oblique impact direction, the splashing process is asymmetric. The propagation of the asymmetric crown and the generation of secondary droplets on the rim of the crown are analysed through image processing. The results show that the flow in the liquid films significantly affects the propagation of the liquid crown and the generation of secondary droplets. [Preview Abstract] |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A35.00006: Droplet Impact onto an Immiscible, Floating Oil Layer: Splash Behavior and Droplet Sizes David Murphy, Cheng Li, Vincent d'Albignac, David Morra, Joseph Katz The high speed impact of a raindrop on a fluid surface at We$_{\mathrm{d}}=\rho $u$^{\mathrm{2}}$d/$\sigma $\textgreater 2000 affects environmental processes like marine aerosol production. High speed imaging shows that a floating immiscible oil layer, such as a crude oil slick, modifies the splash behavior. Tests performed for a wide range of layer thicknesses (h), viscosities, and surface and interfacial tensions facilitate behavioral categorization in terms of We$_{\mathrm{h}}=\rho _{\mathrm{h}}$u$^{\mathrm{2}}$h/$\sigma_{\mathrm{h}}$ and ReFr$_{\mathrm{h}}=\rho_{\mathrm{d}}$u$^{\mathrm{3}}$d/$\mu _{\mathrm{h}}$gh, where h and d subscripts refer to layer and droplet properties, respectively. Included are multi-layer/level crowns, and due to the high Oh$=\mu $/($\rho \sigma $d)$^{\mathrm{1/2}}$ of oil, formation of an intact ejecta sheet within 50 $\mu $s after impact, which subsequently ruptures to form aerosolized oil droplets. High speed holographic microscopy provides the size and spatial distributions of airborne droplets, which are bimodal with peaks at 50 and 225 $\mu $m. Small droplets (50 $\mu $m) are ejected primarily at shallow angles and remain at low elevation by microligament breakup within the first 50 $\mu $s of impact. Larger droplets (225 $\mu $m) are ejected at a steeper angle and produced later by breakup of larger ligaments protruding vertically from the splash crown. Small droplet frequency at high elevation increases when crude oil is introduced, mostly as satellite droplets resulting from the large ligament breakup. [Preview Abstract] |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A35.00007: Crown-splash by a cylinder impact Jalil Hasanyan, Sean Gart, Sunghwan Jung The impact of a droplet onto a liquid bath creates a crown splash of the thin liquid sheet. Similarly, we can observe a crown-splash phenomenon when a rigid hydrophilic cylinder impacts a liquid bath. After the cylinder impacts the air/liquid interface, the liquid sheet splashes upwards and creates a crown-forming instability. However, unlike the drop-generated splash, the solid-generated splash does not expand radially, but stays on the side of the cylinder. In this present study, we examined the vertical splash depending on the cylinder size, impact speed, and liquid properties. Also, the instability of the leading edge of the splash is characterized and compared with capillary instability theories. [Preview Abstract] |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A35.00008: Numerical simulation of drop impact on a thin film: the origin of the droplets in the splashing regime Zhihua Xie, Zhizhao Che, Renad Ismail, Chris Pain, Omar Matar Drop impact on a liquid layer is a feature of numerous multiphase flow problems, and has been the subject of numerous theoretical, experimental and numerical investigations. In the splashing regime, however, little attention has been focused on the origin of the droplets that are formed during the splashing process. The objective of this study is to investigate this issue numerically in order to improve our understanding of the mechanisms underlying splashing as a function of the relevant system parameters. In contrast to the conventional two-phase flow approach, commonly used to simulate splashing, here, a three-dimensional, three-phase flow model, with adaptive, unstructured meshing, is employed to study the liquid (droplet) – gas (surrounding air) – liquid (thin film) system. In the cases to be presented, both liquid phases have the same fluid property, although, clearly, our method can be used in the more general case of two different liquids. Numerical results of droplet impact on a thin film are analysed to determine whether the origin of the droplets following impact corresponds to the mother drop, or the thin film, or both. [Preview Abstract] |
Sunday, November 22, 2015 9:44AM - 9:57AM |
A35.00009: Numerical simulation of droplet impact on interfaces Lyes Kahouadji, Zhizhao Che, Omar Matar, Seungwon Shin, Jalel Chergui, Damir Juric Simulations of three-dimensional droplet impact on interfaces are carried out using BLUE, a massively-parallel code based on a hybrid Front-Tracking/Level-Set algorithm for Lagrangian tracking of arbitrarily deformable phase interfaces. High resolution numerical results show fine details and features of droplet ejection, crown formation and rim instability observed under similar experimental conditions. [Preview Abstract] |
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