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 ZC09: Drops: Drops with Additives |
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Chair: Ranjiangshang Ran, Emory University Room: Ballroom I |
Tuesday, November 26, 2024 12:50PM - 1:03PM |
ZC09.00001: Maximum spreading of impacting non-Newtonian drops Anahita Mobaseri, Satish Kumar, Xiang Cheng The maximum spreading diameter of impacting drops is a crucial parameter characterizing the outcomes of a drop-impact event. While the dependence of the maximum spreading diameter on various impact conditions has been extensively studied for Newtonian drops, a quantitative prediction for non-Newtonian drops remains elusive. The difficulty arises from the large spatiotemporal variations of shear rates during impact, which leads to highly non-uniform viscosity distributions within impacting drops. Here, by combining simulations, experiments and scaling analyses, we provide a universal description of the maximum spreading diameter of non-Newtonian drops on a superhydrophobic surface. Particularly, we identify the characteristic shear rate via a detailed analysis of the energy budget over a wide range of impact conditions. The finding allows us to map the maximum spreading of non-Newtonian drops to that of corresponding Newtonian drops. Our study addresses the long-standing challenge of predicting the maximum spreading of non-Newtonian drops and offers valuable guidelines for designing non-Newtonian liquids with desired impact dynamics. |
Tuesday, November 26, 2024 1:03PM - 1:16PM |
ZC09.00002: Early-time azimuthal undulation of liquid metal drop impacts on solid surface Peiwen Yan, Hossein P Kavehpour Splashing of an impact droplet on dry solid surfaces has attracted numerous studies. For Galinstan drops, an oxidized skin formed at drop surface "freezes" and visualizes early-time undulation and radial ligament formation at microseconds after impact, usually requiring ultra-high-speed cameras to capture. Via optical microscopy the earliest undulation onset is found to initiate at the cusp region between ejected lamella and bulk and the location of cusp formation Rc is observed at impact Weber numbers from 150 to 600. Cusp formation velocity is predicted based on scaling analyses involving lamella ejection and drop impact velocities; our model agrees reasonably well with previous theoretical scaling and direct numerical simulations. Rather than past works assuming effective deceleration takes place instantly after drop impacts, we correctly determine deceleration at Rc for cusp formation and justify the relevance of RT instability mechanisms in this early-time undulation. Phenomenologically similar to drop impacts on liquid surface, "triangular teeth" patterns are developed at Rc before the presence of ligament lines, potentially due to spanwise vortex shedding (Li et al, 2018). This work provides experimental evidence of cusp formation and models precisely the onset location of early-time undulation and its wavenumber at various drop impact velocities. |
Tuesday, November 26, 2024 1:16PM - 1:29PM |
ZC09.00003: Oscillations of slightly viscous liquid drops covered with a monolayer of insoluble surfacant Hansol Wee, Ajay Harishankar Kumar, Naresh Khushalchand Dhanwani, Benjamin D Fudge, Dominic Vella, J. Rafael Castrejón-Pita, Alfonso A Castrejón-Pita, Osman A Basaran A fundamental understanding of the oscillations of surfactant-laden drops in air is crucial for applications including measurement of interfacial properties and liquid atomization. In this study, the small-amplitude oscillations of a drop of an incompressible Newtonian liquid covered with a monolayer of an insoluble surfactant are analyzed. Here, the Navier-Stokes system governing liquid motion within the drop and the convection-diffusion equation governing surfactant transport along the liquid-gas interface are solved analytically using linear stability analysis and computationally using a finite element-based simulation technique. The presence of surfactant lowers surface tension and thereby reduces the oscillation frequency. On the other hand, its presence generates surface tension gradients and thus enhances damping rate. Special attention is devoted to the variation of the damping rate D with the surfactant strength parameter B. An approximate analytical solution in the limit of small viscosity (M) is examined to elucidate the key role played by a certain dimensionless number involving the ratio B/M. The role of vorticity in the dynamics is also explored, and insights gained from it are used to explain the variation of D as a function of the initial surfactant loading. |
Tuesday, November 26, 2024 1:29PM - 1:42PM |
ZC09.00004: Marangoni effects cause non-monotonic damping of droplet oscillations Benjamin D Fudge, Evangelina Antonopoulou, Martin Ball, Hansol Wee, Osman A Basaran, Alfonso A Castrejón-Pita, Dominic Vella Experiments have often shown that the oscillations of droplets coated with surfactant are more damped than those with pure droplets, though the frequency is largely unchanged. This is attributed to Marangoni effects: the non-uniform surfactant distribution causes gradients of surface tension and induced flows that oppose the oscillation. In this talk we will show that the decay rate is a non-monotonic function of surfactant strength: a peak in the decay rate is observed at relatively low surfactant strengths followed by a further decrease as the surfactant strength increases (though still more damped than the zero-surfactant case for sufficiently large Laplace numbers). We discuss the physical origin of this surprising phenomenon and the issues that this non-monotonicity causes when trying to experimentally determine surfactant strength from observations of oscillation. |
Tuesday, November 26, 2024 1:42PM - 1:55PM |
ZC09.00005: Studies on the Coalescence of Single and Mixed Surfactant Laden Droplets Kristo Kotsi, TENG DONG, Takeshi Kobayashi, Alexander Moriarty, Ian McRobbie, Alberto Striolo, Panagiota Angeli We investigated the coalescence behaviour of single and mixed surfactant laden droplets with an interface of a pure or surfactant laden deionised water and a 5 cSt silicone oil. The surfactants studied were the non-ionic tristyrylphenol ethoxylates (EOT) and the anionic sodium benzene sulfonate, with C10-C13 alkyl chain derivatives (NaDDBS) which have significantly different critical micelle concentration (CMC) values. Additionally, a mixture of these surfactants with a ratio of nEOT/nNaDDBS = 0.01 was examined. These surfactants are widely used in agrochemical applications, to stabilise emulsions that deliver active ingredients, such as herbicides, on weeds. |
Tuesday, November 26, 2024 1:55PM - 2:08PM |
ZC09.00006: Electrocoalescence Behavior of Two Drops Containing Hydrolyzed Polyacrylamide and Sodium Dodecyl Sulfate Rahul Painuly, Vikky Anand
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Tuesday, November 26, 2024 2:08PM - 2:21PM |
ZC09.00007: Direct numerical simulations for surfactant-laden interfacial flows with moving contact lines and above the critical micelle concentration Jalel Chergui, Debashis Panda, Lyes Kahouadji, Seungwon Shin, Damir Juric, Omar K. Matar High fidelity numerical simulations of complex fluids are the next frontier in advancing computational modelling for multiphase flows. A highly robust, parallelised, and accurate representation of surfactant-laden interfacial flows enable a wide scope of its applications in cleaning, mixing, agriculture, microfluidics, etc. In this work, we focus on extending our code BLUE [1] to inherit several new features to simulate surfactant-laden flows that are inevitable in the real world. We consider a comprehensive model that accounts for Marangoni stresses (arising from interfacial tension gradients), sorption kinetics (including adsorption/desorption associated with the deformable and liquid-solid interfaces), interfacial and bulk diffusion, and moving contact lines. This model also accounts for situations wherein the surfactant bulk concentration exceeds the critical micelle concentration above which micellar aggregates are expected to form. As an exemplar problem, we use surfactant-laden drop impact on a solid surface to showcase the rich physics of surfactant-laden transport phenomena. The surfactant species interact via adsorption at and desorption from the interfaces, and via micelle breakup to release monomers and re-formation via monomer aggregation; importantly, adsorption and desorption at the contact line is also taken into consideration. Such a highly coupled transport process involves a large number of dimensionless parameters (over 20). The Weber number in the problem is kept relatively low, and the contact angle sufficiently large to account for substrate hydrophobicity. We elucidate the rich mechanisms underlying surfactant-laden flows with micelles and contact lines through a parametric study within the above framework. |
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