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 G37: Drops: Impacts including Irregular Surfaces |
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Chair: Jeremy Marston, Texas Tech University Room: Portland Ballroom 252 |
Monday, November 21, 2016 8:00AM - 8:13AM |
G37.00001: Impact dynamics of liquid marbles Jeremy Marston, Tinku Supakar The impact of particle coated droplets (a.k.a. liquid marbles or armored droplets) onto solid substrates is assessed experimentally with high-speed video. The impact is characterized by the maximum spread diameter, which conforms to scaling laws in terms of the impact Weber number, meaning that the marbles behave similar to water droplets during this stage. However, the motion of the particles across the surface allows us to observe both clustering and divergence of the particle shell and, in particular, we observe the formation of arrested shapes (i.e. jammed interfaces) after impact onto hydrophobic surfaces, from an initially spherical shape. In this case, we postulate that the speed of retraction and rate of change of surface coverage is a key ingredient leading to arrested shapes. [Preview Abstract] |
Monday, November 21, 2016 8:13AM - 8:26AM |
G37.00002: Impact cratering on granular beds: from the impact of raindrops to the strike of hailstones Leonardo Gordillo, Junping Wang, Fred Japardi, Warren Teddy, Xiang Cheng The craters generated by the impact of a spherical object onto a granular bed strongly depend on the material properties of impactors. As an example, impact cratering by liquid drops and by solid spheres exhibit qualitatively different power-law scalings for the size of resulting impact craters. While the basic energy conservation and dimensional analysis provide simple guiding rules, the detailed dynamics governing the relation between these power-law scalings are still far from clear. To analyze the transition between liquid-drop and solid-sphere impact cratering, we investigate impact cratering by liquid drops in a wide range of impact energies, viscosities, surface tensions and drop sizes. Using high-speed photography and laser profilometry to survey more than 8000 laboratory-controlled impact cratering events, we fully delineate the solid-to-liquid transition and unveil a rich set of regimes with different scaling laws and crater morphologies. Our research provides a unified framework for understanding the scaling relations in granular impact cratering—a phenomenon ubiquitous in nature ranging from daily-life raindrop and hailstone impacts on sandy surfaces to catastrophic asteroids strikes on planetary bodies. [Preview Abstract] |
Monday, November 21, 2016 8:26AM - 8:39AM |
G37.00003: ABSTRACT WITHDRAWN |
Monday, November 21, 2016 8:39AM - 8:52AM |
G37.00004: Failure Mechanisms of Air Entrainment in Drop Impact on Lubricated Surfaces Min Pack, Han Hu, Dong-Ook Kim, Zhong Zheng, Howard Stone, Ying Sun Lubricated surfaces have recently been introduced and studied due to their potential benefit in various applications. Combining the techniques of total internal reflection microscopy and reflection interference microscopy, we examine the dynamics of an underlying air film upon drop impact on a lubricated substrate. In contrast to drop impact on solid surfaces where asperities cause random breakup of the entraining air film, we report two air film failure mechanisms on lubricated surfaces. In particular, using thin liquid films of high viscosity, we show that air film rupture shifts from a randomly driven to a controlled event. At low Weber numbers (We) the droplet bounces. At intermediate We, the air film fails at the center as the drop top surface crashes downward owing to impact-induced capillary waves; the resulting liquid-liquid contact time is found to be independent of We. In contrast, at high We, the air film failure occurs much earlier in time at the first inflection point of the air film shape away from the drop center, where the liquid-liquid van der Waals interactions become important. The predictable failure modes of the air film upon drop impact sheds light on droplet deposition in applications such as lubricant-infused self-cleaning surfaces. [Preview Abstract] |
Monday, November 21, 2016 8:52AM - 9:05AM |
G37.00005: Asymmetry of Drop Impacts on Patterned Hydrophobic Microstructures Geoff Willmott, Simon Robson, Matheu Broom When a water drop falls on to a flat solid surface, asymmetries in the geometry of the spreading drop can be specifically determined by patterned surface microstructures. For hydrophobic (or superhydrophobic) micropillar arrays, the most important asymmetric mechanisms appear to be the surface energy of contact lines, and pathways for gas escaping from penetrated microstructure [1]. In this presentation, static wetting and drop impact experiments will be discussed in relation to drop asymmetries. In addition to micropillar arrays, natural superhydrophobic surfaces (leaves) have been studied [2], and may suggest possibilities for controlling drop impacts in applications. Some of the clearest large scale drop asymmetries on leaves, which are similar to those associated with low drop impact contact times on synthetic surfaces [3], appear to be caused by features which generate high contact angle hysteresis, and are therefore indicative of poor superhydrophocity. [1] S. Robson and G. R. Willmott, Soft Matter 12, 4853 (2016). [2] A. Fritsch, G. R. Willmott and M. Taylor, J. R. Soc. N. Z. 43, 198 (2013). [3] J. C. Bird, R. Dhiman, H. –M. Kwon and K. K. Varanasi, Nature 503, 385 (2013). [Preview Abstract] |
Monday, November 21, 2016 9:05AM - 9:18AM |
G37.00006: Droplet impact on a needle Ben Lovett, Andrew Merritt, Tadd Truscott A droplet impinging a hydrophobic surface (low {\it We}) spreads to a maximum diameter before retracting to the center of impact and sometimes lifting off. If the impact surface is augmented by a small ridge, the droplet will often split at this surface feature upon spreading resulting in a shorter time to lift off [Bird et. al., Reducing the contact time of a bouncing drop. {\it Nature}, {\bf 503}, 2013]. We investigate how a singular feature (needle point) generates a similar reduction in droplet contact time. Droplets of diameter ($<$ 2 mm) were controlled to impact varying needles at various impact velocities ({\it We} range: 16 - 256). While it was initially supposed that splitting the droplet into more pieces would further decrease contact time, this was not observed. Rather, the spreading event at the center of the droplet has a greater effect on contact time. [Preview Abstract] |
Monday, November 21, 2016 9:18AM - 9:31AM |
G37.00007: Grating droplets with a mesh Dan Soto, Antoine Le Helloco, Cristophe Clanet, David Quere, Kripa Varanasi A drop thrown against a mesh can pass through its holes if impacting with enough inertia. As a result, although part of the droplet may remain on one side of the sieve, the rest will end up grated through the other side. This inexpensive method to break up millimetric droplets into micrometric ones may be of particular interest in a wide variety of applications: enhancing evaporation of droplets launched from the top of an evaporative cooling tower or preventing drift of pesticides sprayed above crops by increasing their initial size and atomizing them at the very last moment with a mesh. In order to understand how much liquid will be grated we propose in this presentation to start first by studying a simpler situation: a drop impacting a plate pierced with a single off centered hole. The study of the role of natural parameters such as the radius drop and speed or the hole position, size and thickness allows us to discuss then the more general situation of a plate pierced with multiple holes: the mesh. [Preview Abstract] |
Monday, November 21, 2016 9:31AM - 9:44AM |
G37.00008: Drop impact onto semi-infinite solid surface Huanchen CHEN, Alidad Amirfazli The drop impact onto solid surfaces has been studied intensively due to its importance in different applications, e.g. spray coating, inkjet printing and agricultural sprays. The previous studies on this topic were typically focused either on the drop impact onto an infinite solid surface (i.e. a solid surface that is large, and the impact happens far away from the surface edges), or onto a finite solid surface (e.g. drop impact onto a target smaller than the droplet). However, in practice, it is also possible for the impact onto a large surface but close to its edge (named as semi-infinite surface). In this first study of its kind, the process of drop impact onto a semi-infinite surface (both hydrophobic and hydrophilic) was investigated experimentally. During the impact process, part of the liquid lamella can spread out of the surface (free lamella). Depending on the distance between the impact point and surface edge, the free lamella can recede, or partially recede back to the surface, or completely break apart at the surface edge. The behavior of free lamella can also affect the morphology of the part of liquid lamella which remains in contact with the solid surface, especially in the receding phase (e.g. occurrence of drop rebound). Various morphologies observed for lamella breakage at the surface edge will also be discussed for surfaces of different wettabilities. [Preview Abstract] |
Monday, November 21, 2016 9:44AM - 9:57AM |
G37.00009: Droplet impact onto a solid sphere: effect of wettability and impact velocity Alidad Amirfazli, S.A. Banitabaei Collision of a droplet onto a still spherical particle was experimentally investigated. Effect of droplet impact velocity and wettability of the particle surface on collision outcomes were studied (0.05\textless V\textunderscore 0\textless 5.0 and $\theta =$70\textdegree ,90\textdegree ,118\textdegree ). Compared to the literature, the range of Weber number variations was significantly extended (0.1\textless We\textless 1146), and while focus of the previous works was only on impacts in which particle is larger than the droplet (D\textunderscore r\textless 1), the drop to particle diameter ratio in this work was larger than one. Therefore, formation of a thin liquid film, i.e. lamella, was observed due to impact of a relatively high velocity droplet onto a hydrophobic particle. It was shown that for hydrophobic targets with $\theta $\textgreater ?110?\textasciicircum \textdegree , change in particle wettability does not affect the lamella geometry. Temporal variations of various geometrical parameters of collision outcomes including lamella length and lamella base diameter were investigated during the impact. A comprehensive map of all the available works in drop impact on a spherical target was also provided. [Preview Abstract] |
Monday, November 21, 2016 9:57AM - 10:10AM |
G37.00010: Spreading of Impacting Droplets on Wettability-Patterned Surfaces Mohamed Elsharkawy, Antonio Russo, Pietro Asinari, Constantine Megaridis Droplet collision on solid surfaces is a long-studied field that has focused mostly on droplets striking uniform-wettability surfaces. As of now, very few studies exist that analyzed droplet impact on non-uniform (spatially) wettability surfaces. More importantly, no model exists for predicting droplet impact behavior on spatially non-uniform surfaces. Using photolithograhically-produced surfaces, we study droplet impact on axially-symmetric, non-uniform wettability surfaces. We expand upon previously presented models for uniform-wettability surfaces, and predict the maximum spreading diameter of droplets impacting on symmetric patterns on varying wettability surfaces. The present model is expanded to account for $n$ annular regions of different wettabilities, and calculate the corresponding maximum spreading diameter. In addition, within the model we explore the concept of a wettability contrast barrier that must be overcome by the impacting droplets in order to continue their spreading phase. We show under which conditions a droplet can successfully overcome this barrier, and under which conditions it cannot. The model put forth makes strong use of the previously-reported droplet impact model of Passandideh-Fard et al. It draws upon geometric assumptions, such as cylindrical shape for the expanding liquid and spherical cap for the impacting droplet. The work is fundamental in nature, but offers valuable insight that helps understand droplet impact dynamics on non-uniform wettability surfaces. [Preview Abstract] |
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