Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session F02: Drops: Impact, Bouncing, Wetting and Spreading II |
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Chair: Kevin Murphy, Virginia Tech Room: North 120 CD |
Sunday, November 21, 2021 5:25PM - 5:38PM |
F02.00001: Arrested dynamics of droplets on ice Venkata Yashasvi Lolla, Farzad Ahmadi, Hyunggon Park, Andrew P Fugaro, Saurabh Nath, Jonathan B Boreyko Dynamics of droplets impacting a chilled icy substrate or fiber were studied using high-speed imaging. Two spreading regimes were observed for droplets impacting planar ice: (i) at low Weber numbers, the spreading diameter exhibited a capillary-inertial 1/2 power law before being arrested; (ii) at high Weber numbers, the 1/2 power law deviated to an unexpected inertio-viscous 1/6 power law. The behavior of contact line arrest was also bimodal: (i) at low Weber numbers, the capillary-inertial spreading was halted as soon as the droplet’s bulk temperature cooled down to 0 °C, (ii) at high Weber numbers, the speed of a lateral non-equilibrium freeze front had to match the spreading velocity to arrest the inertial spreading. For droplets impacting a chilled fiber, droplet capture was enabled by a temperature-induced viscous dissipation effect, with wicking also playing a role for frosted fibers. |
Sunday, November 21, 2021 5:38PM - 5:51PM |
F02.00002: Blood backspatter drops moving toward the shooter can land behind the victim due to interaction with propellant gases Gen Li, Nathaniel Sliefert, James B Michael, Alexander L Yarin The theoretical/numerical results of the present work reveal a significant interaction of the oncoming vortex ring of propellant muzzle gases with backward blood spatter, especially in cases of the short-range shooting, it is shown that there is even possibility that a blood drop from the backspatter will fully turn around by a powerful turbulent vortex ring and land behind a victim. Such a theoretically predicted outcome is confirmed by experimental data of fully reversed drop trajectories observed in the experiments. A parametric study conducted in this work reveals the totality of the outcomes of the vortex ring interaction with the backward blood spatter and the resulting deflections and landing locations of blood drops. Furthermore, a secondary vortex ring is considered here too to account for a continuous effect of the propellant gas. The trajectories of multiple blood drops whose initial conditions are determined by the Rayleigh-Taylor instability are predicted up to their landing locations on the floor accounting for the gravity force and the drag force they experience from the surrounding air and the vortex ring/rings propagating from the rifle in the opposite direction. The pair of parameters which characterize the self-similar turbulent vortex rings, namely α associated with the semi-empirical turbulence model used and the vortex impulse P0, can be established using the experimental data for each type of weapon. |
Sunday, November 21, 2021 5:51PM - 6:04PM |
F02.00003: Multi-Scale Textured Surface Designs with Passive Frost-Resistant Capabilities Christian Machado, Haiyue Huang, Jiaxing Huang, Kyoo-Chul K Park The need for anti-frosting surfaces is continued and ever-present. These surfaces aim to intrinsically resist frost formation. Long-term frost resistance is incredibly valuable for regions that regularly experience freezing conditions, especially for above-ground power cables, aircraft airfoils, and satellite dishes, among numerous others. Here, we introduce new guidelines for multi-scale surface designs that passively promote increased frost-free regions over long periods of time. By combining geometries and material properties over multiple length scales, we can effectively manipulate the diffusion flux field of incoming water vapor to create two distinct regions: (1) a sacrificial region with the majority of frost growth and (2) a large non-frosted region that resists frost growth. While millimeter-sized features alone can partially have this effect, it can be enhanced by ~50% and in some cases up to a three-fold increase in the frost-free areal portion by the addition of Graphene Oxide (GO) dough. Utilizing the properties of GO doughs, namely permeability and anisotropic heat transfer, in conjunction with anisotropic mass transfer on macrotextures, we can successfully control the size of the frost-free region, thereby mitigating frost-related material/performance degradation. |
Sunday, November 21, 2021 6:04PM - 6:17PM |
F02.00004: Life on stormy seas: water striders are impervious to raindrop impacts Daren A Watson, Andrew Dickerson Water striders are abundant in areas with high humidity and rainfall. Raindrops can weigh 42 times the adult water strider and some pelagic species spend their entire lives at sea, never contacting ground. In this experimental study, we use high-speed videography to film drop impacts on water striders and dynamically-scaled mimics for Froude number Fr=850. Drops force the insect subsurface upon direct contact. As the ensuing crater collapses, the water strider is shot into the air by a Worthington jet. We show the water strider's locomotive responses, low density, resistance to wetting when briefly submerged, and ability to regain super-surface rest state, render it impervious to impacting water drops. When pulled subsurface during secondary crater formation, water striders face the possibilities of being ejected above surface, or submerged below surface. Water striders trapped within the gas-filled crater at the point of crater retraction are expelled from the liquid bath with the ensuing Worthington jet. Water striders with center of mass below the air-water interface of the secondary crater wall at the point of crater retraction remain submerged due to the rapid collapse of the crater causing separation from the insects. Submersion makes the water strider poised on penetrating the air-water interface from below, which appears impossible without the aid of a plastron, or proper locomotive techniques. Submerged water striders employ a series of power strokes in locomoting the aquatic environment. Drops impacting the liquid bath several body lengths away from water striders elicit escape jumps as striders maneuver surface perturbations. Our results show water striders are robust to adverse super-surface conditions and augur well for the development of biomimetic robots. |
Sunday, November 21, 2021 6:17PM - 6:30PM |
F02.00005: The water entry of cups: how burping alters cavities and splashes JESSE L BELDEN, Nathan B Speirs, Aren M Hellum, AJ Paolero, Tadd T Truscott When a rigid body with a convex nose impacts water it wets progressively, and the splash separation is a function of geometry and surface properties. However, when an inverted thin-walled cup impacts the water, the splash and cavity evolve quite differently. Here we show experimentally that air initially trapped inside the cup "burps" out as the body descends, creating a toroidal cavity emanating from the cup bottom. The timescale associated with this air leakage depends on the impact velocity and cup depth, and is rationalized by an analytical model. Depending on these experimental conditions, the leaked gas may reconnect to the free-surface, allowing for the cavity to be inflated. Under other conditions, the toroidal cavity seals on the body and prevents inflation through the free-surface, yielding a small air donut that descends with the body. These different scenarios directly couple to the splash behavior above the surface, influencing the motion of the splash and ultimately the seal time on the body. In this talk, we classify these different regimes, which are revealed through high speed imaging. |
Sunday, November 21, 2021 6:30PM - 6:43PM |
F02.00006: Spreading dynamics of droplets impacted on an oscillating substrate Aditya Potnis, Abhishek Saha Droplet impact on oscillating substrates is ubiquitous in many natural and industrial processes. For example, impact, spreading, and subsequent atomization of rain droplets on beating insect or bird wings and on fluttering leaves govern the spread of pathogens and cellular matters from these surfaces. In industrial applications, oscillating substrates are often used to control atomization, heat transfer, and solidification mechanisms in printing, spraying, or coating processes. Recognizing the importance of the dynamics that arise from the coupling between droplet-transport and substrate motion, in this work, we present an experimental investigation of the spreading of a droplet impacting on a sinusoidally oscillating hydrophobic substrate. The work particularly focuses on the maximum spread of droplets as a function of various parameters of the substrate oscillation. We will show that the maximum spread for impacts on vibrating substrates differs from the impact on static substrates. Specifically, we will elucidate how the frequency, amplitude of vibration, and the phase at the impact significantly affect the outcome. Subsequently, the work will present a scaling analysis to connect the spreading dynamics with an average velocity of the substrate. |
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