Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session H16: Drops:Splash and Shatter |
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Chair: Sigurdur Thoroddsen, King Abdullah University of Science and Technology Room: D133/134 |
Monday, November 21, 2016 10:40AM - 10:53AM |
H16.00001: Experiments on the breakup of drop-impact crowns by Marangoni holes Abdulrahman Aljedaani, C. L. Wang, A. Jetly, E. Q. Li, S. T. Thoroddsen High-speed video experiments investigate the crown break up due to Marangoni instability when a highly viscous drop impacts on a thin layer of lower-viscosity liquid, which also has lower surface tension than the drop liquid. The presence of this low-viscosity film modifies the boundary conditions, giving effective slip to the drop, which forms a regular bowl-shaped crown, which rises vertically away from the solid and subsequently breaks up through the formation of a multitude of holes. Previous experiments [1] have proposed that the breakup of the crown results from a spray of fine droplets ejected from the thin film. These droplets can hit the inner side of the crown forming spots with lower surface tension, which drive the hole formation. We test the validity of this assumption by doing close-up imaging to identify individual spray droplets, to show how they hit the crown and influence the hole formation. For all the impact experiments, the release height was kept constant at H$=$5.4 m, leading to an impact velocity of U$=$9.5 m/s on the thin liquid film. [1] Thoroddsen, S. T., Etoh, T. G. {\&} Takehara, K., Crown-breakup by Marangoni instability. \textit{J. Fluid Mech.}, \textbf{557}, pp. 63-72 (2006). [Preview Abstract] |
Monday, November 21, 2016 10:53AM - 11:06AM |
H16.00002: Binary drop interaction on surfaces: onset and bounding ligaments of Crescent-Moon fragmentation Lydia Bourouiba, Yongji Wang Drop impacts on surfaces can splash and create secondary droplets. These have important implications for industrial, environmental, and health processes such as air contamination by secondary pathogen-bearing droplets shaping disease transmission. Most studies of splash on surfaces have focused on the impact of one drop on a dry surface. Nevertheless, the outcome of impacts by spray or rain are shaped by the presence of adjacent sessile drops on the surface. Recently, in the context of rain and spray-induced disease transmission in crops, one particular binary drop interaction, the crescent-moon splash, was identified as a frequent and efficient source of secondary droplets (Gilet and Bourouiba ICB 2014 and JRSI 2015). The crescent-moon results from the interaction of an impacting drop with a sessile drop in the neighborhood of the impact point. Here, we report and rationalize the existence of a critical transition of impact parameters that enables the crescent-moon fragmentation to emerge. We also report and rationalize the peculiar, yet universal emergence of two bounding ligaments that are important in shaping the crescent-moon sheet. [Preview Abstract] |
Monday, November 21, 2016 11:06AM - 11:19AM |
H16.00003: Numerical simulation of droplet splashing over varying thin liquid film. Amaresh Dalal, Jai Manik, Ganesh Natarajan Droplet impact on wet surfaces is observed in various industrial processes and natural phenomenon. Behavior of droplet impact over thin liquid film is a complex phenomenon involving strong interface deformations. In the past, various studies have been performed to investigate the dynamic behavior of droplets using different geometries and physical conditions. But all the studies were primarily with constant film thickness. The present work is focused on the deformation of single and multiple droplets falling over thin liquid film with variable film thickness. The varying thicknesses of the film may be achieved by considering a sinusoidal varying bottom wall of two different amplitudes. It has been observed that the velocity with which the crown is spreading actually get decreased with the increase in the amplitude of the sinusoidally varying film. Similar behavior has been observed irrespective of the location of drop fall i.e. either falling over crest or over the trough. Also it has been noted that, in the case when droplet is falling over crest, the thickness of the lower portion of the crown rim also gets increased with the increase in amplitude of the film. [Preview Abstract] |
Monday, November 21, 2016 11:19AM - 11:32AM |
H16.00004: Liquid dynamics and surface wettability in splashing Andrzej Latka, Arnout Boelens, Juan de Pablo, Sidney Nagel The impact of a drop results in a beautiful splash that depends on the properties of the liquid, the substrate, and the surrounding air. The interactions of the three phases, particularly those of the ambient gas, have proven difficult to understand. Here, we focus on two aspects of splashing that make it exceptionally challenging: the surprising role of substrate wetting and the complicated hydrodynamics of drop impact. Splashing theories, by analogy to forced wetting, have predicted a strong dependence on wetting properties. By using high-speed interference imaging and simulations, we find that the dynamics of the air-liquid interface at the contact line are independent of substrate wetting properties. We also investigate the effect of the drop’s evolving shape. When the drop first contacts the surface, it initially exhibits a region of high negative curvature that later disappears after the drop has spread sufficiently. We find that the effect of air on splashing is significantly stronger in the initial regime and demonstrate that this difference leads to a high and low impact velocity regime. [Preview Abstract] |
Monday, November 21, 2016 11:32AM - 11:45AM |
H16.00005: Unified thickness profile of radially expanding sheets in the air Yongji Wang, Lydia Bourouiba The impact of a drop on a small solid surface or an edge results in a sheet expansion in the air. The sheet can then fragment into droplets. Understanding the dynamics of expansion and fragmentation in the air is important for a range of applications, including the transport of pathogen-bearing droplets created from contaminated leaves or surfaces. Here, we revisit the axisymmetric case of a radially expanding sheet formed from the impact of a drop on a small target of comparable size to that of the drop. We show that the temporal and spatial evolution of the sheet thickness profile is governed by a self-similar solution derived from first principles. The derived profile allows to collapse on a single curve the direct experimental measurements of sheet thickness profile for impacts on targets reported in the literature to date. A unified functional form governing the sheet thickness profile is proposed and reconciles the two conflicting theoretical profiles proposed in the literature thus-far. Finally, we show that the surface-to-drop size ratio plays an important role in affecting the thickness profile of the sheet in the air and rationalize the effects involved. Our findings allow to unify the thickness profile of unsteady expanding sheets in the air. [Preview Abstract] |
Monday, November 21, 2016 11:45AM - 11:58AM |
H16.00006: How a laser impact fragments a liquid drop Hanneke Gelderblom, Alexander L. Klein, Henri Lhuissier, Detlef Lohse, Emmanuel Villermaux The deposition of laser energy in a superficial layer of an unconfined liquid drop leads to propulsion, strong deformation of the drop into a thin sheet, and eventually fragmentation. Here we study the mechanisms leading to drop fragmentation by combining high-speed and stroboscopic imaging with analytical modelling. We investigate how ligaments and holes develop on the deforming drop, which eventually cause the drop to break up, and identify the dependence of this fragmentation process on the laser-pulse properties. We demonstrate that a Rayleigh-Taylor instability of the decelerating sheet rim leads to the formation of radial ligaments. The holes nucleating in the sheet result from a subtle interaction between the kinematic amplification of initial disturbances in the laser-beam profile and an intrinsic Rayleigh-Taylor instability caused by the rapid forward acceleration of the drop. [Preview Abstract] |
Monday, November 21, 2016 11:58AM - 12:11PM |
H16.00007: Shock wave-droplet interaction Hamed Habibi Khoshmehr, Rouslan Krechetnikov Disintegration of a liquid droplet under the action of a shock wave is experimentally investigated. The shock wave-pulse is electromagnetically generated by discharging a high voltage capacitor into a flat spiral coil, above which an isolated circular metal membrane is placed in a close proximity. The Lorentz force arising due to the eddy current induced in the membrane abruptly accelerates it away from the spiral coil thus generating a shock wave. The liquid droplet placed at the center of the membrane, where the maximum deflection occurs, is disintegrated in the process of interaction with the shock wave. The effects of droplet viscosity and surface tension on the droplet destruction are studied with high-speed photography. Water-glycerol solution at different concentrations is used for investigating the effect of viscosity and various concentrations of water-sugar and water-ethanol solution are used for studying the effect of surface tension. Here we report on how the metamorphoses, which a liquid drop undergoes in the process of interaction with a shock wave, are affected by varied viscosity and surface tension. [Preview Abstract] |
Monday, November 21, 2016 12:11PM - 12:24PM |
H16.00008: It’s harder to splash on soft solids Sam Howison, Christopher Howland, Arnaud Antkowiak, Rafael Castrejon-Pita, James Oliver, Robert Style, Alfonso Castrejon-Pita Droplets splash when they impact dry, flat substrates above a critical velocity that depends on parameters such as droplet size, viscosity and air pressure. By imaging ethanol drops impacting silicone gels of different stiffnesses we show that substrate stiffness also affects the splashing thresh- old. Splashing is reduced or even eliminated: droplets on the softest substrates need over 70% more kinetic energy to splash than they do on rigid substrates. We show that this is due to energy losses caused by deformations of soft substrates during the first few microseconds of impact. We find that solids with Young’s moduli < ∼ 100kPa reduce splashing, in agreement with simple scaling arguments. Thus materials like soft gels and elastomers can be used as simple coatings for effective splash prevention. Soft substrates also serve as a useful system for testing splash-formation theories and sheet-ejection mechanisms, as they allow the characteristics of ejection sheets to be controlled independently of the bulk impact dynamics of droplets. [Preview Abstract] |
Monday, November 21, 2016 12:24PM - 12:37PM |
H16.00009: How a drop splashes upon impact onto a moving surface. Hamed Almohammadi, Alidad Amirfazli Understanding whether drop spreads or splashes upon impact onto a moving surface is important due to its applications in printing, spraying, and icing. A systematic study was performed to understand how drop splashes upon impact onto moving hydrophilic and hydrophobic surfaces. High speed imaging from top and side views was used to capture the impact of drops ($D_{0}=$2.5 mm) of liquids with three different viscosities ($\mu =$1-4.1 mPa.s). Wide range of normal drop velocity ($V_{n}=$0.5-3.4 m/s) and surface velocity ($V_{s}=$0-17 m/s) were studied; such normal and tangential velocity ranges are not available in systems where a drop impacts at an angle relative to a surface. It was found that the splashing behavior of the drop upon impact onto a moving surface, unlike the understanding in the literature, is azimuthally different along the lamella contact line. Splashing probability decreases along the lamella contact line as velocity difference between the surface and the lamella decreases. A new model was developed to describe such azimuthally different behavior for splashing which is function of normal Reynolds and Weber numbers, $V_{s}$, and surface wettability. It is also found that the increase of the viscosity decreases the splashing threshold. [Preview Abstract] |
Monday, November 21, 2016 12:37PM - 12:50PM |
H16.00010: An octahedron model for oscillating, bouncing drops Francois Blanchette We present a model for oscillating and bouncing liquid drops. The model uses 6 point masses distributed as the vertices of an octahedron, connected by linear springs. We derive the physically relevant choice of parameters and use this model to study drops bouncing on solid surfaces, as well as drops bouncing on a nearly inviscid liquid surfaces. The surfaces may be stationary of subject to forced oscillations. [Preview Abstract] |
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