Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session L08: Drops: Coalescence |
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Chair: Xin Yong, University at Buffalo Room: Ballroom H |
Monday, November 25, 2024 8:00AM - 8:13AM |
L08.00001: Double Crown Formation During the Drop Impact onto Thin Film Muhammad Faheem Afzaal, Abdulrahman B Aljedaani, Kenneth R Langley, Ziqiang Yang, Sigurdur T Thoroddsen The Edgerton crown is an iconic manifestation of drop impact splashing with a prominent cylindrical edge decorated with detaching droplets. Herein, we identify the formation of an intriguing double crown when a high-viscosity drop impacts a thin film of a lower-viscosity immiscible liquid. The first inner crown evolves in a regular manner, while the second outer crown forms near its base from the tip of the horizontally spreading drop when it approaches the outer free surface. The flow squeezed out between the drop and the solid surface generates counter-rotating vortex rings, which drive out the second crown. We mapped out the narrow parameter range in which this ephemeral structure emerges. Images were captured using a high-speed Phantom camera and long-distance microscope with adjustable magnification to visualize the impact dynamics. Direct numerical simulations were chosen to reproduce the impact using the Basilisk software, with multifluid setup comprising a drop, thin oil film, and surrounding gas, using dynamic adaptive grid refinement. This provided insight into the fine details, allowing us to identify the underlying dynamics of the double crown formation. |
Monday, November 25, 2024 8:13AM - 8:26AM |
L08.00002: High-speed impact of a drop on a liquid pool Hiranya Deka, Channaveera Shastry D M When a liquid drop impacts a liquid pool at a high velocity, a circular liquid sheet is ejected upward, forming a crater underneath the liquid pool. The liquid sheet may splash outward or bend inward. We study the impact of a liquid drop on a liquid pool, focusing on the dynamics of sealing and no-sealing of the liquid sheet. Both simulations and experiments are performed for a range of drop sizes and pool heights. The simulations show that the surface sealing depends on the size of the drop and impact velocity. The vortices generated by the air rushing into the cavity play a role in the sealing dynamics of the cavity |
Monday, November 25, 2024 8:26AM - 8:39AM |
L08.00003: Droplet impact on pools with base topography Thomas C Sykes, Radu Cimpeanu, Daniel M Harris, J. Rafael Castrejón-Pita, Alfonso A Castrejón-Pita Our recent work has shown that pool depth can significantly influence outcomes when droplets impact shallow pools, which is relevant to important processes such as raindrop impact, icing, and crop spraying. Here we consider the case where the pool base is not flat, but instead has a topography that breaks the axisymmetry of traditional normal droplet impact onto a static pool. In particular, we focus on configurations with linearly varying and abruptly changing pool depths, such as a submerged step. These ostensibly small modifications to the traditional setup can have a dramatic effect post-impact, generating inherently three-dimensional dynamics and influencing both the number and direction of ejected satellite droplets. Besides quantifying the resulting asymmetric behavior, we establish the limits within which base topography affects impact outcomes, in terms of both pool depth and impact position, for use in applications. |
Monday, November 25, 2024 8:39AM - 8:52AM |
L08.00004: Viscous dissipation during water droplet impact on an oil layer over a water pool Donghoon Lee, Ildoo Kim, Jinkee Lee Droplet impact phenomenon is commonly observed in our daily life such as when a raindrop falls into a puddle. Despite its ubiquity, impact of a droplet on another liquid is inherently complex, involving numerous forces. This complexity increases with the introduction of an additional liquid layer. In this study, we investigated the impact of a water droplet on a thin oil layer (~1 mm) deposited on a water pool. We released droplets of different diameters from a consistent height toward oil layers with seven variants of viscosities, ranging from 5 cSt to 500 cSt. As expected, our quantitative measurements revealed that as viscosity increases, the maximum cavity size decreases, indicating that the presence of an oil layer damps impact energy. We found that the damped energy is primarily used to push the oil outward from the impact point. To explain this observation, we developed an analytical model approximating viscous dissipation based on the volume displacement of oil due to impact, defined by the cavity growth rate. However, direct estimation of viscous dissipation was challenging for oil viscosities ν≤200 cSt because the oil layer was too thin to be measured. To address this, we introduced an oil layer volume conservation factor into the model, validating the equation indirectly as the calculated values of this factor matched the experimental range. |
Monday, November 25, 2024 8:52AM - 9:05AM |
L08.00005: Rolling Splash Lyes Kahouadji, Mosayeb Shams, Debashis Panda, Abdullah M Abdal, Seungwon Shin, Jalel Chergui, Damir Juric, Omar K Matar A new configuration of drop splashing on a thin liquid film is investigated numericallyconsidering the rotational effect of the drop before its impact. In addition to the well-known outcomes in classic drop splash configurations(impact, liquid crown, liquid rim, etc.), unexpected phenomena can also occur, such as curved liquid bridgeformation, traveling crispswave, and colliding jetsinducing unsteady fluid chain. This numerical study requires a fully three-dimensional direct numerical simulation without any symmetry assumptions. We employ a high-fidelity hybrid front-tracking/level-set method over a wide range of non-dimensional numbers, including Reynolds, Weber, and rotation ratio numbers. |
Monday, November 25, 2024 9:05AM - 9:18AM |
L08.00006: Splash on a liquid pool: coupled cavity-sheet unsteady dynamics Naijian N Shen, R. Dandekar, B. Naar, L. Bourouiba Splash from impacts of drops on liquid pools are ubiquitous and generate secondary droplets important for a range of applications in healthcare, agriculture, and industry. Despite having been investigated for more than a century, the physics of splash continues to comprise central unresolved questions. Combining experiments and theory, we study the sequence of topological changes from drop impact on a deep, inviscid liquid pool, with a focus on the regime of crown splash with developing air cavity below the interface and crown sheet above it. We here focus on developing coupled evolution equations for the cavity-crown system, leveraging asymptotic theory for the cavity and conservation laws for the crown. Using the key coupling of sheet and cavity, we derive similarity solutions for the sheet velocity and thickness profiles, and asymptotic prediction of the crown height evolution. Unlike the cavity whose expansion is opposed by gravitational effects, the axial crown rise is mostly opposed by surface tension effects. We show that our analytical results are in good agreement with the experimental measurements. Our cavity-crown coupling enables us to obtain explicit estimates of the crown splash spatio-temporal unsteady dynamics paving the the way to decipher ultimate splash fragmentation and the properties of the secondary droplets so produced. |
Monday, November 25, 2024 9:18AM - 9:31AM |
L08.00007: Bouncing to coalescence transition for drop impact onto moving liquid pools Daniel M Harris, Radu Cimpeanu, Oliver Sand, Eli Silver, Luke F Alventosa, Arman Mohammadi, Thomas C Sykes, Alfonso A Castrejón-Pita When a droplet impacts a bath of the same fluid, it can rebound completely provided that the mediating air layer remains intact during the process. However, above a critical impact velocity, the air layer is forced to drain and coalescence is initiated. While this problem has been studied extensively for the axisymmetric scenario of normal impacts on a still bath, little is known about the more general scenario of oblique impacts or impacts onto a moving liquid layer. In this work, we experimentally demonstrate that the critical normal velocity required to transition from the bouncing to coalescence regime is reduced for impact onto a moving liquid bath. Experimental measurements are compared to results obtained via direct numerical simulation that resolve the gas layer dynamics, and ultimately allow us to identify the physical mechanism responsible for the reduction in threshold, as well as extend our understanding to the case of oblique impacts. |
Monday, November 25, 2024 9:31AM - 9:44AM |
L08.00008: Effect of drop oscillations on the drop splash Raghavendra NAIDU S, Kamal Poddar, Sanjay Kumar The dynamics of a liquid drop impact on a liquid surface is studied experimentally on deep water by using high speed imaging and particle image velocimetry techniques. This study has provided deeper understanding of the effect of drop shapes on the fully developed splashing. Based on the experimental observations, it can be concluded that the liquid drop after impact with the target liquid, the kinetic energy possessed by the impinging drop divides between the drop and the target. Prolate drop, after the impact on the free surface of water, distributes kinetic energy more in the vertical direction than in the horizontal direction. Oblate drop, after the impact on the free water surface, distributes kinetic energy more in the horizontal direction than in the vertical direction, and spherical drop distributes it almost symmetrically. This uneven distribution of transferred kinetic energy from the drop to the target liquid is possibly the reason for variations in the various features observed in crown, cavity, and jets for different drop shapes at the time of impact. Such as crown shape, cavity shape and size, and maximum jet height. Prolate drop at the time of impact gives rise to unevenly expanded unsymmetrical thick crown, oblate drop at the time of impact gives rise to unevenly expanded unsymmetrical thin crown and spherical drop at the time of impact gives rise to symmetrically expanded crown. The maximum cavity depth for the prolate drop impact is greater than the maximum cavity depth for the oblate drop impact, and the maximum depth of cavity for the spherical drop impact is almost equal to that for prolate drop impact, and the radius of cavity for the spherical drop impact is greater than the cavity radius for both prolate and oblate drop impacts. Prolate drop at the time of impact gives rise to longer jets than the jets formed due to oblate and spherical drop at the time of impact. Oblate drop at the time of impact gives rise to the smallest jets. It is also found that the velocity field around the cavity is majorly influenced by the drop shapes at the time of impact. |
Monday, November 25, 2024 9:44AM - 9:57AM |
L08.00009: Deformation and Coalescence of Two Impacting Droplets in Microgravity Cabala R Newell, Natalie Violetta Frank, Karl Cardin, William Ruehle, Samira Shiri, Raúl Bayoán B Cal
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Monday, November 25, 2024 9:57AM - 10:10AM |
L08.00010: Complete, partial, and non-coalescence of sessile droplets on superhydrophobic surfaces Gopal Chandra Pal, Cheuk Wing Edmond Lam, Chander Shekhar Sharma Multiple droplets coalesce to form larger droplets during dropwise condensation on a water-repellant surface. Here, we report that such multi-droplet coalescence sequences do not necessarily result in the merging of all participating droplets. The non-coalescing droplets can bounce away from the coalescing droplets with significant tangential momentum. Existing studies have reported partial coalescence and bouncing of droplets in the presence of an additional external force field or due to the bulk motion of the droplets. However, the phenomenon reported here is observed without these factors, occurring on deposited or condensed droplets. It is challenging to discern the parameters influencing the outcome of these coalescence processes during condensation due to the small length and time scales involved. Hence, we perform controlled droplet coalescence experiments wherein two sessile droplets are made to coalesce in close vicinity of a third droplet. We observe three distinct phenomena across a wide range of droplet diameters: complete coalescence, coalescence followed by re-separation, and bouncing of the third droplet without coalescence. We discuss how the initial geometrical arrangement and the traveling capillary wave can affect the outcome. |
Monday, November 25, 2024 10:10AM - 10:23AM |
L08.00011: Magnetophoresis induced coalescence of equal-sized oil droplets in paramagnetic aqueous carrier phase Xueyong Lu, Kilian Ortmann, Kerstin Eckert, Zhe Lei The coalescence of liquid droplets in an immiscible liquid carrier phase is a crucial step in industrial solvent extraction processes. More spontaneous and efficient coalescence leads to improved demulsification and higher yield. Coalescence reduces surface energy and is thus thermodynamically favorable. However, various repulsive forces can hinder the process, resulting in a non-zero residence time for coalescence. As droplets come into close contact, film drainage occurs until it reaches a critical thickness where van der Waals forces dominate, forming a liquid bridge between droplets. In this study, we investigate the coalescence of two identical millimeter-sized oil droplets dispersed in a paramagnetic Mn(II) solution under a high magnetic field gradient. The induced Kelvin force causes droplet magnetophoresis, bringing the droplets into close contact. Using high-speed imaging and particle image velocimetry, we analyze the flow field in the carrier phase before and during coalescence and statistically evaluate the residence time of coalescence. To mimic the repulsive forces encountered during the demulsification process, we introduce controlled amounts of surfactants (DeTAB, SDS) into the aqueous carrier. This method allows us to tune the repulsive Gibbs-Marangoni and electrostatic forces by varying the ionic strengths and surface concentrations, providing some insights into this complicated system. |
Monday, November 25, 2024 10:23AM - 10:36AM |
L08.00012: Integrating Inertia and Surface Tension Effects in Droplet Coalescence Dynamics Ahmad Dousti, Vahid Azadeh Ranjbar, Samira Shiri Coalescence of droplets is a fascinating phenomenon observed both in nature and industry, influencing various processes and applications. In nature, the coalescence of sessile droplets can be seen in phenomena such as dew formation on leaves or spider webs. In industry, this phenomenon is applied in processes like spray coating and inkjet printing. When two droplets come into contact, a highly curved liquid bridge forms at the contact point, generating high Laplace pressure that propels the liquid into the bridge and facilitates their merging. Surface tension and viscosity significantly influence this process: higher surface tension increases Laplace pressure and accelerates coalescence, while higher viscosity prolongs the merging process by resisting deformation and flow. Previous studies show that the lubrication approximation can be applied to small, high-viscosity droplets on non-wetting surfaces. However, for larger droplets with lower viscosity, the lubrication equation falls short as inertial effects become significant and must be considered to accurately describe the coalescence dynamics. In this study, by incorporating inertia and surface tension gradients into theoretical models and comparing the results with experimental data, we aim to provide a more comprehensive description of droplet coalescence dynamics across different droplet sizes and liquid properties. This work advances our understanding of the coalescence process and paves the way for further research in this domain. |
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