Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session C18: Focus Session: Competing Roles of Surfactants in Free Surface Flows with Hydrodynamic Singularities |
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Chair: Pritish Kamat Room: 400 |
Sunday, November 24, 2019 8:00AM - 8:13AM |
C18.00001: The Surprising Influence of Marangoni Stress on Near-Singular Dynamics in Breaking Surfactant-covered Liquid Threads Pritish Kamat, Osman Basaran The singularity arising during thread breakup is a precursor to drop formation and is accompanied by flows directed away from the breakup location that grow stronger as pinch-off nears. When surfactants are present at interfaces, these outward flows convect surfactants away from the singularity such that the dynamics proceeds in local absence of surfactants. Despite this, it has been observed that even small amounts of surfactants can have a substantive effect on breakup. Experiments show reduced rates of thinning, altered satellite sizes, and formation of fractal structures---microthread cascades---whereas simulations show that thinning filaments can recover and escape pinch-off in the presence of surfactants. Here, we show that evacuation of surfactants from the breakup singularity indirectly leads to the generation of large concentration (surface tension) gradients in the vicinity of but not at the breakup location. The resultant gradients generate Marangoni stresses that are large enough to compete with other forces, and are key to how surfactants influence near-singular dynamics. Here, we use simulations to shed light on two distinct contexts where the competition between Marangoni stresses and other forces perceptibly influences the fate of the breakup singularity. [Preview Abstract] |
Sunday, November 24, 2019 8:13AM - 8:26AM |
C18.00002: Influence of the surface viscosities on the pinching of a pendant droplet loaded with SDS Miguel A. Herrada, Alberto Ponce-Torres, Manuel Rubio, Jose M. Montanero The interfacial viscosities of relatively viscous insoluble surfactants are known to affect the dynamics of a liquid thread close to the free surface pinching [1]. Here, we analyze both numerically and experimentally the breakup of a pendant water drop loaded with SDS. We focus on the influence of the monolayer of SDS on the free surface minimum radius $R_{\textin{min}}$. The results show remarkable agreement between the experiments and numerical simulations for the pure DIW case. When a surfactant is introduced, it creates a monolayer that alters the pinchoff dynamics. If only solute-capillarity and Marangoni convection are considered in the numerical simulations, there is a measurable deviation with respect to the experimental results for $R_{\textin{min}}(\tau) <~ 5$ $\mu$m. We considered the surface shear and dilatational viscosities in the simulation to reproduce the entire range of experimental data. The value of the shear viscosity is consistent with the upper bound reported in the literature [2]. [1] A. Ponce-Torres, J. M. Montanero, M. A. Herrada, E. J. Vega, and J. M. Vega, Phys. Rev. Lett. 118,024501 (2017). [2] Z. A. Zell, A. Nowbahar, V. Mansard, L. G. Leal, S. S. Deshmukh, J. M. Mecca, C. J. Tucker, and T. M. Squires, Proc. Natl. Acad. Sci. 111,3677 (2014) [Preview Abstract] |
Sunday, November 24, 2019 8:26AM - 8:39AM |
C18.00003: The role of surface viscous stresses in the liquid thread breakup Hansol Wee, Pritish Kamat, Osman Basaran Many industrial processes involving emulsions, foams, and inkjet printing exploit the ability of surfactants to adsorb onto and lower the surface tension of water-air and water-oil interfaces. In addition to lowering surface tension, surfactants may induce Marangoni stresses and cause surface rheological effects. Therefore, the dynamics of free surface flows can be significantly altered by their presence. Although much attention has been paid to date to the influence of Marangoni stresses and solutocapillarity, the effect of surface viscous stresses has been inadequately studied given the difficulty in measuring surface viscosities due to the presence of surfactants. Using a simple model that they vary linearly with concentration, we examine their effect on thread breakup by 1D simulations using the slender-jet approximation. The results obtained with the 1D algorithm are confirmed by direct comparison against predictions made with a 3D but axisymmetric free surface solver. [Preview Abstract] |
Sunday, November 24, 2019 8:39AM - 8:52AM |
C18.00004: How surfactants influence the drop size in sprays Daniel Bonn Spraying is a widely used method to produce a liquid sheet that break up into droplets of a certain size distribution. When spraying simple liquids, it is known which experimental parameters determine the droplet size distribution. For many applications however, surfactants are added, producing a hitherto unknown effect on the droplet size distribution. Using two generic types of spraying nozzles, we sprayed solutions of different types of aqueous surfactants and measured the droplet size distribution of the sprays. We find that the breakup of surfactant solutions is similar to that of pure water but results in droplets that are on average smaller. The resulting droplet size distribution can be well described using the predictions for simple liquids provided that we replace the parameter of the equilibrium surface tension with the dynamic surface tension of the surfactant solution at a surface age of 20 ms, which is the characteristic time for destabilization and breakup of a liquid sheet. By rescaling them with the mean droplet size, the droplet size distributions of water and sprays with different concentrations of surfactants all collapse onto a single curve and can be well described using the compound Gamma function found previously for pure liquids. [Preview Abstract] |
Sunday, November 24, 2019 8:52AM - 9:05AM |
C18.00005: Restoring universality to the pinch-off of a bubble Amir Pahlavan, Howard Stone, Gareth McKinley, Ruben Juanes We observe the formation of bubbles and drops on a daily basis, from dripping faucets to raindrops entraining bubbles on the surface of a lake. The pinch-off of a bubble is an example of the formation of a singularity, exhibiting a characteristic separation of length and time scales. Because of this scale separation, one expects universal dynamics that collapse into self-similar behavior determined by the relative importance of viscous, inertial, and capillary forces. Here, we report on the intriguing observation that confinement makes the pinch-off of a bubble a universal process, as opposed to the unconfined case, where pinch-off is sensitive to the details of the experimental setting. We show that the pinch-off dynamics of a bubble confined in a capillary tube undergo a sequence of two distinct self-similar regimes, even though the entire evolution is controlled by a balance between viscous and capillary forces. We demonstrate that the early-time self-similar regime restores universality to bubble pinch-off by erasing the system's memory of the initial conditions. Our observations have implications for immiscible flow phenomena from microfluidics to geophysical flows, where confinement, together with fluid-solid physicochemical interactions, play a key role. [Preview Abstract] |
Sunday, November 24, 2019 9:05AM - 9:18AM |
C18.00006: Flow structure of Marangoni contracted sessile drops Stefan Karpitschka, Olinka Ramirez, Michiel A. Hack, Wojciech Kwiecinski, E. Stefan Kooij, Tim J. Segers, Jacco H. Snoeijer A droplet of two miscible liquids should spread over a high-energy surface until complete wetting. However, if one component is more volatile and has a higher surface tension, a quasi-stationary non-vanishing apparent contact angle can be observed. This is caused by the enrichment of the residual component near the contact line and the associated surface tension gradient. A hydrodynamic-evaporative model, using a long-wave approximation for the droplet coupled to diffusion limited evaporation predicts a balance between Marangoni and capillary flows and a power law between the apparent contact angle and the ambient humidity [Karpitschka et al., Langmuir (2017)]. This explanation differs from a recent model, where the low surface tension of a precursor around the droplet is held responsible [Benusiglio et al., Soft Matter (2018)]. A discrimination between possible mechanisms requires experimental resolution of the flow in the drop. We present uPIV measurements and relate them to the apparent shape of the drop, for aqueous solutions of various short chain carbon diols. Depending on the surface activity of the diol, its concentration, and the ambient humidity, we observe different regimes, indicating that multiple mechanisms lead to the observed angles. [Preview Abstract] |
Sunday, November 24, 2019 9:18AM - 9:31AM |
C18.00007: Marangoni effects enabling interfacial singularities and topological changes in fluid flows Rouslan Krechetnikov In this talk I will address two geometric effects of surface tension gradients: formation of interfacial singularities and change in the flow topology compared to the clean interface case. In the first part, I will focus on a necessary condition for the existence of geometric singularities -- divergence of curvature at fluid interfaces -- in the solutions of fluid dynamic equations. Besides establishing a relation to dynamic singularities -- unboundedness of the velocity field -- explicit asymptotic solutions of the Navier-Stokes equations are developed as well. Next, using as an example the phenomena of tip-streaming, with the help of asymptotic matching we resolve the associated Marangoni-driven singularities providing explicit asymptotic formulas for the scaling of the emitted droplets. In the second part, using the Landau-Levich problem of dip coating as a paradigm, we demonstrate how Marangoni stresses are capable of changing the flow topology by moving the stagnation point initially residing at the interface in absence of surfactants into the bulk once surfactants are added to the system. This finding not only explains thickening of the deposited film as induced by Marangoni effects, but also illustrates another geometric effect surface tension gradients may cause. [Preview Abstract] |
Sunday, November 24, 2019 9:31AM - 9:44AM |
C18.00008: Coalescence of surfactant-laden drops in liquid-liquid emulsions Vishrut Garg, Osman Basaran Determining the timescale over which liquid-liquid emulsions separate into their constituents is crucial for many processes, e.g. separating crude oil from brine. This timescale depends on the dynamics of collision and coalescence of liquid drops immersed in a second liquid which can be significantly altered by the presence of surfactants at the liquid-liquid interface. We simulate the approach, collision, and eventual coalescence of two drops immersed in an ambient liquid in the presence of insoluble surfactants where both liquids are incompressible Newtonian fluids. The governing equations are augmented to account for long range van der Waals interactions that become significant as the separation between the drops falls below a few hundred nanometers and solved using a Galerkin finite element based algorithm. In contrast to drops with clean interfaces which coalesce during their first approach, surfactant laden drops are seen to rebound on first approach before coalescing on their subsequent approach. This rebound results in increased drainage times. We examine the physics underlying drop rebound in the presence of surfactants. [Preview Abstract] |
Sunday, November 24, 2019 9:44AM - 9:57AM |
C18.00009: Coating and Crumpling of Particle–Coated Bubbles in Confined Geometries Shelley Anna, Charles Sharkey, Zixian Cui We examine bubble flow in a capillary filled with a suspension of surface-active particles. Silica nanoparticles are rendered surface active when mixed with cationic CTAB surfactant, which adsorbs to the silica surface in a glycerol-water mixture. Bubbles are dispensed via a co-flow nozzle at varying bubble lengths and capillary numbers. Fluid film thickness is measured along the length of the bubble. The measurements are compared with bubble flow through surfactant solutions and with predictions from a Bretherton-type model. The adsorbed particle-surfactant complexes form an evolving rigid layer at the trailing end of the bubble. Two critical bubble lengths are observed. Above the first critical length, the bubble contains two distinct film thicknesses. The thickened film at the trailing end arises from the rigid particle layer on the interface. Above the second critical length, the trailing bubble cap crumples and collapses. Crumpling occurs soon after the bubble is dispensed into the capillary, and the critical length varies with bubble velocity. These results allow us to infer rheological and mechanical properties for the interface that are associated with the crumpling phenomenon. [Preview Abstract] |
Sunday, November 24, 2019 9:57AM - 10:10AM |
C18.00010: Waves on an interface with surfactant Willem van de Water, YukMan Lau, Jerry Westerweel, Damir Juric, Jalel Chergui, Seungwon Shin The presence of surfactant on an interface between two immiscible fluids can dramatically change the interfacial tension. The question is whether this is still so when the interface is rippled through waves which redistribute surfactants. We study Faraday waves on an oil-water interface and use benchmark surfactants with increasing concentration that reaches into the realm of ultralow interfacial tension ($\sigma = {\cal O} (10^{-6}\; {\rm N/m}))$. In the experiments we measure the wavelength, wave height and threshold acceleration amplitude and compare them to a Floquet analysis $^1$. Surprisingly, the dispersion of capillary waves (frequency 20 Hz) points to a much stiffer interface at surfactant concentrations where it should be ultralow. We hypothesize the key role of surfactant dynamics. This is supported by numerical simulations$^2$ of Faraday waves in the presence of surfactant gradients.\\ 1. K. Kumar and L.S. Tuckerman, J. Fluid Mech. {\bf 279}, 49-68 (1994).\\ 2. S. Shina, J. Cherguib, D. Juricb, L.Kahouadjic, O. K. Matarc, and R. V. Crasterd, J. Comp. Phys., {\bf 359}, 409-435 (2018) [Preview Abstract] |
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