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 H33: Drops: Bouncing, Impact, and Dynamic Surface Interactions II |
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Chair: Sigurdur Thoroddsen, KAUST Room: Ballroom A |
Monday, November 23, 2015 10:35AM - 10:48AM |
H33.00001: The first contact of a droplet impacting a dry solid surface S.T. Thoroddsen, E.Q. Li, I.U. Vakarelski The first contact of a drop hitting a dry solid surface, does not occur at a point but along a ring, owing to viscous lubrication pressure in the intervening air layer. This always leads to the entrapment of a small bubble under the center of the drop. The nature of the actual first contact is affected by the roughness of the solid. We use ultra-high-speed imaging, with 200 ns time resolution, to observe the structure of this first contact between the liquid and a smooth solid surface. For a water drop impacting onto regular micro-scope glass slide we observe a ring of micro-bubbles as observed by Thoroddsen et al.\footnote{Thoroddsen et al., The air-bubble entrapped under a drop impacting on a solid surface. {\it J. Fluid Mech.},{\bf 545}, 203-212.} which conveniently marks the original diameter of the air-disc. This ring of bubbles arises owing to multiple initial contacts just before the formation of the fully wetted outer section. These contacts are spaced by a few microns and quickly grow in size until they meet each other, entrapping the bubbles. We thereby conclude that the localized contacts are due to nanometric roughness of the glass surface and the presence of the micro-bubbles can therefore distinguish between glass with 10 nm roughness from perfectly smooth glass. [Preview Abstract] |
Monday, November 23, 2015 10:48AM - 11:01AM |
H33.00002: Bouncing and bursting in a wedge Etienne Reyssat, Caroline Cohen, David Quere Placed into an inhomogeneous confined medium, non-wetting drops tend to be expelled from the tightest regions, where their contact with the walls would be maximized. They preferentially explore more open areas which are favorable from the point of view of capillary energy. Following this principle, one may thus use the geometry of confined environments to control fluid droplets in various ways : displacing, filtering, fragmenting... In this communication, we present experimental results on the dynamics of Leidenfrost drops launched into a wedge formed by two quasi-horizontal glass plates. Influenced by the gradient of confinement, these non-wetting liquid pucks approach the apex of the wedge to a minimal distance where they bounce back. At higher impact velocity, we observe that drops tend to penetrate deeper into the wedge but often burst into a large number of small fragments. We also discuss ways to control the deviation of droplets from their initial trajectory. We propose scaling law analyses to explain the characteristics of the observed bouncing and bursting phenomena. [Preview Abstract] |
Monday, November 23, 2015 11:01AM - 11:14AM |
H33.00003: Walking droplets interacting with planar boundaries Giuseppe Pucci, Pedro Sáenz, Adam Damiano, Daniel Harris, Pierre-Thomas Brun, John Bush A decade ago, Yves Couder and co-workers discovered that droplets may self-propel along the surface of a vibrating fluid bath. We here present the results of an experimental investigation of the interaction of such walking droplets with boundaries. Particular attention will be given to characterizing reflection from plane boundaries and diffraction through an aperture. [Preview Abstract] |
Monday, November 23, 2015 11:14AM - 11:27AM |
H33.00004: Viscous drops bounce faster: prompt tumbling-rebound from a sublimating slope Carlo Antonini, Stefan Jung, Andreas Wetzel, Emmanuel Heer, Philippe Schoch, Ali Mazloomi M., Shyam S. Chikatamarla, Ilya Karlin, Marco Marengo, Dimos Poulikakos We discovered a new drop rebound regime, characteristic of highly viscous liquids impacting onto tilted sublimating surfaces. By focusing on non-axisymmetric impact conditions at increasing viscosity, we demonstrate that low viscous drops show a ``slide, spread, recoil and rebound'' behavior, whereas viscous drops exhibit a ``prompt tumbling-rebound'' behavior. As such, viscous glycerol drops surprisingly rebound faster than three orders of magnitude less viscous water drops. This is made possible by a small conversion of translational to rotational kinetic energy, at non-axisymmetric impact conditions, as also confirmed by additional Lattice Boltzmann simulations: a rapid transition of the internal angular velocity prior to rebound to a constant value, as in a tumbling solid body, promotes a rapid rebound of more viscous drops, which are capable to rebound without recoiling. By studying drop impact dynamics, we explore the drop behavior in contactless and frictionless conditions, and identify the Ohnesorge number as the primary parameter to predict the transition between different impact regimes on tilted sublimating slopes, with tumbling observed for Ohnesorge numbers higher than unity. [Preview Abstract] |
Monday, November 23, 2015 11:27AM - 11:40AM |
H33.00005: Reducing the residence time of a bouncing drop with spoked macrotexture Colin Patterson, James Bird Liquid drops can bounce when they impact non-wetting surfaces. Recently, studies have demonstrated that the time that the bouncing drop resides at the surface can be adjusted with the presence of ridged macrotextures. When non-parallel macrotextures are present, they intersect to create spoked junctions. At sufficient velocity, a drop impacting a junction might be expected to breakup into smaller droplets; yet it is unclear how many droplets would be produced and the time for these droplets to clear the surface. Here, we show that the number droplets and overall residence time depends on both the number of spokes and the Weber number. We experimentally demonstrate that the center-assisted recoil framework extends beyond superhydrophobic surfaces to those above the Leidenfrost temperature. Finally, we present a physical model that rationalizes our results. [Preview Abstract] |
Monday, November 23, 2015 11:40AM - 11:53AM |
H33.00006: Spontaneous droplet self-launching on superhydrophobic surfaces Thomas Schutzius, Stefan Jung, Tanmoy Maitra, Gustav Graeber, Dimos Poulikakos Spontaneous removal of droplets from surfaces is of significant importance in nature and many technologies, e.g., self-cleaning surfaces. Despite progress, the understanding of phenomena leading to such behavior, combined with surface design promoting their manifestation, remains a challenge. We show how water droplets in contact with superhydrophobic surfaces in a low-pressure environment can self-remove through sudden spontaneous launching and subsequent repeated bouncing behavior. We demonstrate that this bouncing results from the combined effect of droplet vaporization, vapor flow in the surface texture, and substrate adhesion leading to a forced, underdamped, mass-spring-damper system behavior. This work is a step toward understanding inherent physical phenomena of droplet-surface interactions manifesting themselves at conditions promoting vaporization, e.g. low-pressure environments, and shows how surface texture design aware of such phenomena alone, can prohibit water retention on surfaces. [Preview Abstract] |
Monday, November 23, 2015 11:53AM - 12:06PM |
H33.00007: Bubble entrapment under the impact of a viscous drops onto a solid surface K. Langley, E.Q. Li, S.T. Thoroddsen When a high-speed viscous drop impacts onto a solid surface, the lubrication pressure leads to the entrapment of an air-disc which contracts into an isolated central bubble. The maximum disc-diameter is marked by microbubbles, followed by a fully wetted outer region. However, the outer edge of this wetted region forms a lamellar jet traveling along the surface, which entraps a myriad of micro-bubbles.\footnote{Thoroddsen, S. T., Takehara, K. \& Etoh, T. G. 2010, {\it Phys. Fluids}, {\bf 22}, 051701.}$^,$\footnote{Palacios J., Hernandez J., Gomez P., Zanzi C., Lopez J. 2012, {\it Exp. Fluids}, {\bf 52}, 1449--1463.} The tip of the spreading lamella separates from the solid surface and levitates on a lubricating air layer. The local contacts between the levitated sheet and the solid surface form wetted patches and finally entrap bubbles when the wetted patches meet the advancing contact line, through topological change. Isolate bubbles are also entrained through the advancing contact line. These bubble entrapment mechanisms are investigated with high-speed video imaging and are found to be highly dependent on the drop viscosity and surface properties. We use high-speed interferometry to measure the thickness of the air-layer under the advancing lamella. [Preview Abstract] |
Monday, November 23, 2015 12:06PM - 12:19PM |
H33.00008: Water drop dynamics on a granular layer Coraline Llorens, Anne-Laure Biance, Christophe Ybert, Christophe Pirat Liquid drop impacts, either on a solid surface or a liquid bath, have been studied for a while and are still subject of intense research. Less is known concerning~ impacts on granular layers that are shown to exhibit an intermediate situation between solid and liquid. In this study, we focus on water drop impacts on granular matter made of micrometer-sized spherical glass beads. In particular, we investigate the overall dynamics arising from the interplay between liquid and grains throughout the impact. Depending on the relevant parameters (impact velocity, drop and grain sizes, as well as their wetting properties), various behaviors are evidenced. In particular, the behavior of the beads at the liquid-gas interface (ball-bearing vs imbibition) is shown to greatly affect the spreading dynamics of the drop, as well as satellite droplets formation, beads ejection, and the final crater morphology. [Preview Abstract] |
Monday, November 23, 2015 12:19PM - 12:32PM |
H33.00009: An Experimental Investigation on the Impingement of Water Droplets onto Superhydrophobic Surfaces Pertinent to Aircraft Icing Phenomena Haixing Li, Rye Waldman, Hui Hu Superhydrophobic surfaces have self-cleaning properties that make them promising candidates as anti-icing solutions for various engineering applications, including aircraft anti-/de-icing. However, under sufficient external pressure, the liquid water on the surface can transition to a wetted state, defeating the self-cleaning properties of superhydrpphobic surfaces. In the present study, an experimental investigation was conducted to quantify the transient behavior of water droplets impinging onto test surfaces with different hydrophobicity properties under different environmental icing conditions. The experiments were performed in the Icing Research Tunnel of Iowa State University (IRT-ISU) with a NACA0012 airfoil. In addition to using a high-speed imaging system to reveal transient behavior of water droplets impinging onto test surfaces with different hydrophobicity properties, an IR thermometry was also used to quantify the unsteady heat transfer and dynamic phase changing process within the water droplets after impingement onto the test plates with different frozen cold temperatures. The high-speed imaging results were correlated with the quantitatively temperature measurements to elucidate underlying physics in order to gain further insight into the underlying physics pertinent to aircraft icing phenomena. [Preview Abstract] |
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