Bulletin of the American Physical Society
75th Annual Meeting of the Division of Fluid Dynamics
Volume 67, Number 19
Sunday–Tuesday, November 20–22, 2022; Indiana Convention Center, Indianapolis, Indiana.
Session U12: Drops: Impact, Bouncing, Wetting and Spreading IV |
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Chair: Andrew Bayly, Univ of Leeds Room: 139 |
Tuesday, November 22, 2022 8:00AM - 8:13AM |
U12.00001: Impact Dynamics of Natural Snowflakes on Engineered Surfaces Farshad Barghi Golezani, Abdel Hakim Abou Yassine, Hossein Sojoudi Blockage of air intake systems in vehicles due to snow accumulation in winter reduces engine power. To understand the risk better and mitigate it, computational fluid dynamics (CFD) models need to be developed for predicting the snow tracking property and the snow ingress amount into the air intake system. Such models require understanding of snow impact dynamics with various surfaces and determining so-called coefficient of restitution (COR). Here, a particle image velocimetry (PIV) method is used to obtain COR values of natural snowflakes impacting various engineered surfaces. Impact of surface properties (i.e., roughness and energy) and properties of snow (wetness, liquid water content, flake size) on the COR values are investigated. The results show that vertical COR substantially reduced as the impact angle increased. The horizontal COR dropped as impact velocity increased, but it was not significantly affected by impact angle. The findings of this study enable better prediction of snow accumulation and design of systems to minimize it, when needed. |
Tuesday, November 22, 2022 8:13AM - 8:26AM |
U12.00002: Role of compressible air film entrapped during droplet impact on a solid surface Joita Chakraborty, Saptarshi Basu, Ashoke De The impact of liquid droplets on solid surfaces occurs widely in the natural world and in various industrial applications such as falling raindrops, inkjet printing, spray coating and spray cooling. Before impact, as the droplet approaches the solid surface, a thin air layer is formed between them. The air in this thin region gets compressed as it cannot escape completely when the droplet comes closer to the surface. The high pressure of the compressed air layer deforms the bottom portion of the drop into a dimple shape, and a thin peripheral air disk surrounds this central dimple. The first liquid-solid contact occurs due to the collapse of this peripheral air disk. In this work, we explore the mechanism and the exact location of collapse using high-fidelity numerical simulations. We also monitor the evolution of the complete entrapment region using direct numerical simulation and quantitatively validate the air layer dynamics with existing experimental data. |
Tuesday, November 22, 2022 8:26AM - 8:39AM |
U12.00003: Role of surface active molecules and nanoparticles on the spreading dynamics of drops on solid surfaces parisa bazazi, Hossein Hejazi The characteristics of an aqueous drop spreading on solid surfaces are the keys to many deposition processes including coating, printing, and enhanced oil production. We study the wetting dynamics of aqueous drops, loaded with either silica nanoparticles or sodium dodecyl sulfate surfactants, on a hydrophilic glass surface that is submerged in high viscosity oil. The balance between capillary and viscous forces determines the early-time spreading dynamics in clean systems, characterized by a droplet radius that grows linearly with the time which finally evolves to the late-time Tanner regime. Along recovering the viscous and Tanner regimes, silica increases the early time spreading rate and the final wetted area. The drop affinity to glass and the osmotic forces driven by the silica concentration gradient dictate the wetting characteristics of silica drops. Unlike clean water and silica drops, surfactants impose an early time retardation regime. The non-uniform distribution of surfactants at the interface generates Marangoni stresses before the drop-solid contact, consequently suppressing the film drainage. The addition of both surfactants and nanoparticles also imposes an early time retardation regime where its duration is an increasing function of interfacial viscoelasticity. |
Tuesday, November 22, 2022 8:39AM - 8:52AM |
U12.00004: Droplet impact on oscillating hydrophobic substrates: jetting and atomization Aditya Potnis, Abhishek Saha Droplet impact on various surfaces has been studied extensively in literature, owing to its wide range of applications. However, impact on oscillating hydrophobic substrates has not been thoroughly investigated. This type of impact is critical in both naturally occurring processes, such as rain droplet impact on fluttering leaves or wings of insects, and in industrial applications, for manipulating droplet deposition. Our prior work on droplet impact on an oscillating hydrophobic substrate reaffirmed that substrate movement significantly affects the spreading characteristics. In this work, we will show that an oscillating substrate can also modify the recoiling dynamics of the droplet and thus affect its long-time behavior, particularly the induced jetting and subsequent atomization of the droplet. We will explore such atomization of a recoiling droplet for a range of frequency and amplitude of oscillations. Aided by experiments, we will identify the two regimes of atomization characterized by distinct timescales. Furthermore, we will endeavor to delineate these regimes and put forward a scaling analysis to predict the dynamics of post-impact atomization. |
Tuesday, November 22, 2022 8:52AM - 9:05AM |
U12.00005: Candle Soot is a Low-cost Alternative for Water Repelling Surfaces? Bhaskarjyoti Sarma, Amaresh Dalal, Dipankar Narayan Basu The ubiquitous phenomenon of superhydrophobic surfaces repelling water droplets is exploited in numerous industrial processes which include self-cleaning and thermal management. However, the longer fabrication time and higher cost of the robust and stable superhydrophobic surfaces have remained their Achilles heel. Interestingly, candle soot-based superhydrophobic surfaces can be an economical alternative to tackle the earlier issue, even when produced using the simplest methods like manual flame deposition. We have evaluated the non-wetting behavior of such carbon-based fractal surfaces using a wide range of droplet impact phenomena, which remain scarcely reported to date. The critical limit of the key events namely, rebounding, contact time, impalement transition and splashing dynamics in different impact conditions have been plotted in a working regime map. The most remarkable observation is the absence of the partial rebound regime during the impact of water droplets, unlike several existing superhydrophobic surfaces. Similarly, the contact time of impacting water droplets even shows a similar scale to that of Leidenfrost droplets for a wide range of conditions. There is a need for further investigation of the non-wetting behavior of carbon soot-based surfaces in the future. |
Tuesday, November 22, 2022 9:05AM - 9:18AM |
U12.00006: The role of drop shape in impact and splash Jack Lo, Ye Li, Qingzhe Liu, Yuan Liu, Lei Xu The impact and splash of liquid drops on solid substrates are ubiquitous in many important fields. However, previous studies have mainly focused on spherical drops while the non-spherical situations, such as raindrops, charged drops, oscillating drops, and drops affected by electromagnetic field, remain largely unexplored. Using ferrofluid, we realize various drop shapes and illustrate the fundamental role of shape in impact and splash. Experiments show that different drop shapes produce large variations in spreading dynamics, splash onset, and splash amount. However, underlying all these variations we discover universal mechanisms across various drop shapes: the impact dynamics is governed by the superellipse model, the splash onset is triggered by the Kelvin-Helmholtz instability, and the amount of splash is determined by the energy dissipation before liquid taking off. Our study generalizes the drop impact research beyond the spherical geometry, and reveals the potential of using drop shape to control impact and splash. |
Tuesday, November 22, 2022 9:18AM - 9:31AM |
U12.00007: Why are drop-impact-crater jets so singular? Sigurdur T Thoroddsen, Yuan S Tian, ZIQIANG YANG Fast microjets are known to emerge during the rebounding of drop-impact craters, or after a bubble bursts at a pool surface. The thinnest jets are the fastest and generate the smallest secondary droplets of importance to aerosol formation and the aroma of champagne. These jets are associated with the small dimple which forms at the bottom of the crater and rebounds without pinching off a bubble. The radial collapse of this dimple has been shown to be purely inertial but is highly sensitive on initial and boundary conditions [1,2]. We perform high-resolution volume-of-fluid numerical simulations using Gerris to reveal a novel focusing mechanism, which reveals the mechanism behind the fastest jets. This involves the entrainment of a conical air-sheet. This jetting configuration explains the extreme sensitivity to the precise impact conditions which have been observed in the detailed experiments of the phenomenon. It also shows why liquid viscosity does not strongly affect the jetting velocity. |
Tuesday, November 22, 2022 9:31AM - 9:44AM |
U12.00008: Splash-free Urinals Inspired by Nautilus Shells and Dogs Kaveeshan A Thurairajah, Mabel Song, JD Zhu, Ethan Barlow, Randy Hurd, Zhao Pan The practice of marking territory with urine is repulsive to most people; ironically, about half of the world's population (males) inadvertently marks bathroom floors, and themselves, through unintentional urinal splash back. This problem has persisted since the invention of the urinal over a century ago. We found that when a liquid jet or droplet train impacts a rigid surface below a certain critical impinging angle, almost no splatter is generated. Thus, a surface designed to always intersect the urine stream equal to or smaller than the critical angle prevents splash back. Inspired by nautilus shells and dog urination, we designed urinal surface geometries that effectively eliminate splatter by satisfying the splash-suppressing intersect criteria. Our numerical and experimental validations show that our urinal designs are superior to the typical use of a popular urinal available on the market as well as scenarios where urine steams are highly unstable (e.g. urinals in ships and airplanes undergo perturbation). Our new urinal designs will keep bathrooms cleaner and reduce the labor, water, and chemicals required for periodic cleaning to promote more sustainable bathroom maintenance. |
Tuesday, November 22, 2022 9:44AM - 9:57AM Not Participating |
U12.00009: Ferrofluid Droplets Falling and Impacting in a Non-Uniform Magnetic Field Geoffrey R Willmott, Matheu Broom, Steve Wells, Amelia Cordwell, Alex Chapple, Stephen Chung Ferrofluids, i.e. fluids containing magnetic colloids, are most famous for forming spiky Rosensweig instabilities when placed in a magnetic field. The dynamics of their deformation within a magnetic field are less well studied. Here, high-speed photography experiments have been carried out in which drops of a ferrofluid fall vertically through the non-uniform magnetic field generated by a simple disc magnet, before impacting on a glass slide placed above the magnet. As the drop falls, it becomes elongated in the direction of motion. Following image analysis, the forces acting on the drops and their consequent deformation and trajectory may be modelled. The modelling has suggested that the surface component of the magnetic force should not be neglected. Following impact, instabilities can form as early as the initial spreading phase, apparently seeded by crown-rim instabilities. The dynamics of spreading affect the eventual distribution of instabilities. Unlike static drops, the largest peaks are nucleated at the edge of the drops. These experiments used a hydrocarbon and magnetite ferrofluid with magnetic susceptibility χ = 2.64, and a magnet with an on-axis B-field up to 22 mT. The droplets were typically 2 mm in diameter, and were released from heights up to 200 mm. |
Tuesday, November 22, 2022 9:57AM - 10:10AM |
U12.00010: Solidification dynamics of impinging droplets in distinct regimes due to intertwined influences of Weber and Stefan numbers: a unifying theoretical and experimental framework Peiwen Yan, Pirouz Kavehpour In contact with subcooled substrates, droplets spread and solidify simultaneously, an interplay of fluid dynamics, heat transfer, and phase transition with its broad occurrence in nature and industry. Existing hypotheses attribute contact line pinning (CLP) mechanism to critical contact angle, volume, or temperate is reached. The extent of substrate subcooling determined by Stefan numbers, however, significantly alters physics of CLP and triggers various solidifying modes, rendering a possibility to unify seemingly contradicting hypotheses in their respective regimes. This work focuses on impacting droplet solidification at intermediate Weber numbers and its morphology at the onset of splash under intertwined effects of We and Ste. It aims to reveal the ultimate underlying physics behind CLP for impacting and spreading droplets with distinct solidification modes. Impacting hexadecane droplets on subcooled glass surfaces are experimentally studied with ranges of We from 1.0 to 800 and Ste from 0.01 to 3.0. The asymptotic spreading threshold is non-monotonic, where fingering and satellite droplets formation are “optimally” suppressed with certain Ste for moderately high We, suggesting fundamental influences of solidification modes on spreading dynamics. |
Tuesday, November 22, 2022 10:10AM - 10:23AM |
U12.00011: Singular jets from compound drop impact Marie-Jean THORAVAL, Zeyang MOU, Zheng ZHENG, Zhen JIAN, Carlo ANTONINI, Christophe JOSSERAND The impact of a water drop onto a solid surface can either lead to the deposition of the drop liquid or its rebound, depending on the wetting properties of the target surface. If the water drop is covered by an oil layer to form a compound drop, the water core can rebound from the target surface independently of the wetting properties. Under such conditions, the rebound dynamics can produce very thin and fast jets, similar to the singular jets observed for single phase drops impacting onto a superhydrophobic surface. We combine high-speed imaging experiments with high-resolution numerical simulations to study the formation of these jets. We demonstrate that the maximum jet velocity can reach two different peaks when varying the impact velocity. Furthermore, we show that the collapse dynamics of the cavity at the two peaks follows different scaling laws, due to two different mechanisms responsible for the singular flow at the two peaks. We explain how the complex dynamics of the two fluid interfaces are responsible for these differences, with the first jet due to the convergence of capillary waves, while the second jet is produced by the accumulation of oil above the recoiling water core. |
Tuesday, November 22, 2022 10:23AM - 10:36AM Author not Attending |
U12.00012: Experimental Analysis of Droplet Impact Behavior on Aluminum Plate with Variation in Droplet Height and Surface Temperature Ahsan Naveed, Olivier COUTIER-DELGOSHA Aircraft icing is one of the most challenging problem that aerospace and aviation industry is facing. Over the years, research is going on to study the impact and freezing process of water droplets on cold surfaces and to subsequently devise counter measures to avoid icing. In the current work, the behavior of water droplet impacting on aluminum plate with surface roughness of 0.1 μm. The effect of droplet height variation and change in surface temperature is observed. The experiments are done at room temperature and it is found that with the increase in height, the droplet impact velocity increases, has more energy to overcome the viscous effects, and spreads more on the surface in relatively less time as compared to small heights. The droplet behavior is further studied through non-dimensional parameters. Higher Reynolds number with increase in height indicates, viscous effects are not prominent and Bond number depicts that gravity does not affect surface tension. Weber number analysis indicates that the kinetic energy is more prominent at higher heights. Then the surface temperatur is varied and experiments are performed at 0°C, -5°C, -10°C and -15°C. The results indicate that contrary to the room temperature, the viscous effects are more prominent at lower temperature and the droplet spreads less on the surface and exerts more energy in overcoming the viscous effects. The retraction rate of the droplet, after reaching the maximum spread diameter, increases with height and decreases with lower surface temperature. |
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