Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session P21: Drops: Impacts General |
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Chair: Xiang Cheng, University of Minnesota, Twin Cities Room: 603 |
Monday, November 25, 2019 5:16PM - 5:29PM |
P21.00001: Pressure and Shear Stress Distribution of Drop Impacts. Ting-Pi Sun, Xiang Cheng Drop impacts are ubiquitous and relevant to many important natural and industrial processes. Although the kinematics of drop impact such as the morphology of impacting drops have been extensively studied experimentally due to the fast advance of high-speed photography techniques, the dynamic aspects of drop impacts remains largely unexplored. Here, we investigate the pressure and shear stress distributions of drop impacts via a newly-developed experimental tool, high-speed 3D stress microscopy. By combining laser-sheet illumination with high-speed photography, we track the fast movements of fluorescent particles embedded in elastic gels under the impact of liquid drops. The measurements enable us to obtain the strain of the elastic gels induced by the impact. The temporal evolution of impact pressures and shear stresses of liquid drops can then be extracted based on the strain-stress relation of continuum mechanics. Our study on the pressure distribution confirms the key prediction of the self-similar theory and numerical simulations, where the maximum impact pressure occurs near the contact line, rather than the center of impacting drops. In addition, we also quantify the fast temporal evolution of impact-induced shear stresses, information crucial for mitigating impact-induced damages on solid substrates. This research is support by NSF CAREER DMR-1452180. [Preview Abstract] |
Monday, November 25, 2019 5:29PM - 5:42PM |
P21.00002: Drop splashing on a thin particle layer Ehsan Esmaili, Seungho Kim, Sunghwan Jung In nature, high-speed raindrops impact and spread on particulate surfaces e.g. soil, pollens or spores on plant leaves. Although a drop impact on a surface is a traditional topic for industrial applications, drop-impact dynamics on a thin layer of particles in natural situations are less known about. Here we describe a single drop splashing on a thin layer of glass particles with different density of 1.1-4.35 (gcm$^{\mathrm{-3}})$, the size of 10-98 micrometer, and the surface wettability of 35\textordmasculine -105\textordmasculine . Both experiments and scaling law argument have been carried out to determine the phase diagram for drop splashing. This study suggested that splashing occurs as particles cover the rim and cause the lamella to slide and levitate from the surface. It was shown that the Weber number and the particle density are two major factors to determine the critical packing fraction for splashing. The splashing on a particulate bed can lead to a better understanding of soil loss and erosion dynamics due to the raindrops. [Preview Abstract] |
Monday, November 25, 2019 5:42PM - 5:55PM |
P21.00003: Numerical Study of Mechanism of Deep Erosion Pit in High-speed Liquid Droplet Impingement Yuka Iga, Hirotoshi Sasaki In the process of the liquid droplet impingement erosion from the deceleration period to the final steady period, certain number deep pits are caused, and the pits grows deeper and deeper with keeping the number and the diameter of the pit. Namely, even though the many droplets uniformly impinge on the material surface, the material surface is not evenly but selectively eroded with a certain number of the pit. The purpose of this study is to clarify the progress mechanism of deep erosion pit of the liquid droplet impingement erosion by numerical analysis. In the numerical results, the Rayleigh wave, which is a surface wave of a material, caused by the droplet impingement propagates on the inner surface of the pit and reaches the pit bottom, and it focuses. Then, the equivalent stress drastically increases in the pit bottom although the droplet does not impinge directly on there. According to increase of the offset between the pit and droplet centers, the focus becomes weak. When the droplet impinges the region of flat material surface, the impingement pressure attenuates inside the liquid film. Therefore, these numerical results explain why a certain number of deep pits are selectively formed on the material surface in the liquid droplet impingement erosion. [Preview Abstract] |
Monday, November 25, 2019 5:55PM - 6:08PM |
P21.00004: Droplet impact on a porous slice: visualization of simultaneous spreading and imbibition D. Jordan Bouchard, Sanjeev Chandra Experimentally studying droplet impact and imbibition on porous materials is difficult because most porous materials are opaque, and the time resolution of X-ray imaging techniques is not fast enough to capture the simultaneous spreading and imbibition of a drop. Currently, we must rely on computer simulations to predict how a drop impregnates a porous material during the spreading phase. To gather experimental measurements, we have fabricated thin, optically transparent porous slices using a single layer of glass beads, with average diameters of 500 and 100 microns, sintered between two panes of glass. Using a high-speed camera, we film a 2 mm deionized water drop impacting the porous slices at velocities of 0.06, 0.5, 1.2, and 1.9 m/s, so we can simultaneously capture droplet spread and imbibition into the porous slice. Droplet inertia can drive liquid flow and significantly shorten the penetration time of a drop. If the average pore size is large enough the drop can be entirely driven into the porous medium by the drop's kinetic energy. Smaller pores dissipate the kinetic energy of impact rapidly so that liquid inertia has a negligible contribution to the volume of the drop that penetrates the porous slice. We provide criteria based on our experimental measurements that can be used to predict when inertia driven flow into porous materials will contribute significantly to the total volume of liquid that penetrates the porous material. [Preview Abstract] |
Monday, November 25, 2019 6:08PM - 6:21PM |
P21.00005: Shock wave-induced drop fragmentation upon raindrop impact on biological surfaces Seungho Kim, Brian Wu, Jason J. Dombroskie, Sunghwan Jung Rainfall on biological superhydrophobic surface (e.g. bird feathers, insect wings, plant leaves, etc) is ubiquitous in nature. Previous studies in the laboratory have focused on low-speed impacting drops (less than 1 m/s) only, which is far from the speed of real raindrops (more than 5 m/s). In this present work, we explore raindrop impact at high speeds, which exhibits unexpected drop dynamics: numerous shock-like waves are generated on a spreading drop in the presence of microscopic textures on biological surfaces. Then, the spreading drop with shock-like waves is fragmented soon after it approaches a maximal spreading extent, thereby reducing the residence/contact time more than twofold. Since it is known that the heat and momentum transfer of an impacting drop onto the substrate can be reduced by decreasing the contact time of impacting drop, our findings may help to understand how endothermic animals lower hypothermia risks, and how insects stabilize their flight position during rainfalls. Here, we visualize such salient high-speed drop dynamics using a high-speed camera, and validate the distinctive features through experiments and theoretical models. [Preview Abstract] |
Monday, November 25, 2019 6:21PM - 6:34PM |
P21.00006: Droplet Impact on dry solid surfaces: Traditional vs Bio-mimetic Saptarshi Basu, Durbar Roy, Khushboo Pandey, Rabibrata Mukherjee An experimental study of droplet impact has been conducted on four different substrates (2 traditional and 2 bio-mimetic) in the impact Weber number range of 6 to 130. The droplet shape dynamics have been visualized using high speed shadowgraphy (at 10 kHz). Glass and PDMS are the traditional substrates, whereas the other two surfaces are inspired by rose-petal and lotus-leave micro-structures. Various regimes are demarcated for all the substrates depending on the impact Weber number. The receding rebound, and breakup mechanisms of bio-mimetic surfaces are found to be strikingly different from that of the traditional substrates. Dimensional analysis, scaling arguments and energetics have been utilized to unearth the underlying dynamics of the impact. [Preview Abstract] |
Monday, November 25, 2019 6:34PM - 6:47PM |
P21.00007: Drop Impact on Liquid Film: Bouncing to Merging Transition for Two-Liquid System Abhishek Saha, Xian Wu Impact of a drop on liquid film is critical in several industrial applications, including inkjet printing and thermal sprays. Previous studies using single liquid for both drop and film showed that the impact could can result in two outcomes, namely bouncing and merging, and the transition between these two states is a function of impact Weber number and film thickness. It was also reported that for a range of Weber number, the impact outcome changes from merging to bouncing, back to merging and then to bouncing again, as we increase the film thickness. Since many of the advanced printing technologies such as 3D inkjet printers can print multiple materials, very often the drop and the impacted the liquid film are required to be of different liquids with varying properties. Recognizing its importance, in this talk, we will present a study on the dynamics of drop impact on liquid film using two liquids with similar surface tension, but different viscosities. The results with two-liquid systems show a shift in the transitional boundaries with respect to that of the single-liquid system. In addition to the two types of merging, early merging and late merging, reported for single-liquid systems, a third kind of merging was also observed for two-liquid systems. This third kind of merging was found to reduce the degree of non-monotonicity of the transitional boundaries between bouncing and merging states. [Preview Abstract] |
Monday, November 25, 2019 6:47PM - 7:00PM |
P21.00008: Dynamics of the ejecta sheet at extreme impact velocity Abdulrahman B. Aljedaani, Y.S. Tian, Tariq Alghamdi, S. T. Thoroddsen We have constructed a 26-meter-tall vacuum tube to study drop impacts at very high velocities, in a controlled manner. Under reduced ambient pressure we can attain impact velocities up to 23 m/s and Reynolds numbers as large as $10^5$. We focus on the ejecta sheets produced when the drop impacts on a thin layer of liquid. The ejecta dynamics are captured with two simultaneous high-speed video cameras, one focused on the ejecta details and the other on the overall crown evolution. Experiments are performed over a wide range of drop and liquid viscosity combinations. The ejecta breakup and splashing are greatly affected by the ambient air pressure, where air-drag and Bernoulli suction pressure can bend the sheets into intricate shapes. The tip of the sheet can bend up or down depending on the viscosity ratio between the two liquids. Under some impact conditions the sheet entraps a toroidal bubble as the crown rises away from the substrate. The sheets finally rupture to produce a spray with a myriad of micro-droplets. We construct a simplified model to describe the shape evolution. [Preview Abstract] |
Monday, November 25, 2019 7:00PM - 7:13PM |
P21.00009: Using Wagner theory to predict early-time jet properties in liquid-liquid impact problems Matthew Moore, Radu Cimpeanu Droplet impact problems have a wide range of applications throughout real-world phenomena and industrial processes, ranging from inkjet printing to aerosol formation. Due to the violent displacement of liquid free surfaces and the associated topological changes on short timescales and disparate lengthscales, impacts are notoriously difficult to model. With access to ever more sophisticated technology, we have a wealth of resources at hand to investigate impact phenomena in greater detail. However, since impact problems are highly nonlinear, it is desirable to use mathematical modelling to help predict certain properties, such as the location of the root of the ejecta, enabling us to focus numerical or experimental investigations on a subset of the full problem. In this analysis, we employ an inviscid, incompressible fluid model to perform a comprehensive analysis of droplet impacts for general droplet radii and impact speeds. We derive leading order predictions for the location, thickness and velocity of the root of the high-speed ejecta, as well its leading-order thickness and velocity. We compare the predictions to direct numerical simulations performed in the open-source software package Gerris. We discuss what the theory predicts well, where it struggles and applications. [Preview Abstract] |
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