Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session L27: Surface Tension Effects: Interfacial Phenomena I |
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Chair: Joshua Bostwick, Clemson University Room: Georgia World Congress Center B315 |
Monday, November 19, 2018 4:05PM - 4:18PM |
L27.00001: Marangoni-driven spreading of miscible liquids in the binary drop geometry Claas Willem Visser, Robin Koldeweij, Bram van Capelleveen, Detlef Lohse When two liquids with different surface tensions come into contact, the liquid with lower surface tension spreads over the other. This Marangoni-driven spreading has been studied for various geometries and surfactants, but the dynamics of the binary geometry (drop-drop) has hardly been quantitatively investigated despite its relevance for drop encapsulation applications. Here we use laser-induced fluorescence (LIF) to temporally resolve the distance $L(t)$ over which a low-surface-tension drop spreads over a miscible high-surface-tension drop. $L(t)$ is measured as a function of the surface tension difference between the liquids and the viscosity, revealing power-law behavior $L(t)\sim t^{\alpha}$ with a spreading exponent $\alpha \approx 0.75$. This value is consistent with viscosity-limited spreading over a deep bath. A single power law of rescaled distance as a function of rescaled time reasonably captures our results as well as different geometries, miscibility, and surface tension modifiers (solvents and surfactants). This result enables engineering the spreading dynamics of various liquid-liquid systems. However, we also observe deviations between this scaling law and literature experiments, which give rise to open questions that deserve attention in future work. |
Monday, November 19, 2018 4:18PM - 4:31PM |
L27.00002: Marangoni Enhanced Contact Line Advancement in Surfactant-Driven Superspreading: Going Beyond the deGennes-Tanner Paradigm Hsien-Hung Wei Since Rafai et al. (Langmuir 18, 10486, 2002) reported that a drop of a trisiloxane surfactant solution can spread in radius growing linearly with time, the origin of this superspreading phenomenon is still not fully understood. Here we crack this longstanding puzzle beyond the common deGennes-Tanner paradigm. We show that the curious linear spreading law can be resolved by a distinctive contact line structure with a tiny surfactant leak to the substrate, capable of driving the contact line at a constant speed with a locally concentrated Marangoni force. The criterion for superspreading is also derived, explaining why superspreading occurs in a range of surfactant concentration and why it is merely limited to a certain class of surfactants. We further show that the late time spreading can be governed by either the 1/6 or 1/2 power law, depending on the ability of interfacial surfactant to transfer onto the substrate. All these results can account for a variety of findings seen in experiments. Analogy to thermocapillary spreading is also made, reverberating the ubiquitous role of Marangoni effect in dynamic wetting driven by non-uniform surface tensions. |
Monday, November 19, 2018 4:31PM - 4:44PM |
L27.00003: Evaporation can stabilize a bubble at a free surface without surfactants Mark Menesses, Matthieu Roche, Laurent Royon, James C Bird When a bubble arrives at a free surface, we typically expect the film of the bubble cap to thin over some period of time until it ruptures. Traditionally, the drainage of this film has been considered inevitable, and even the addition of surfactants cannot prevent its eventual rupture. Here we present air bubbles at the free surface of liquids which appear to defy traditional drainage rules and can avoid rupture, persisting for hours until dissolution. Using pure, volatile liquids free of any surfactants, we highlight and model a phenomenon in which liquid surrounding the bubble is continuously drawn into the bubble cap, effectively overpowering the drainage effects. We compare our model to data obtained from white light interferometry and thermographic images. |
Monday, November 19, 2018 4:44PM - 4:57PM |
L27.00004: Evaporation Driven Oscillatory Marangoni-Capillary Flow in Thin Films of a Binary Mixture Xingyi Shi, Gerald G. Fuller, Eric S G Shaqfeh Thin film systems are ubiquitous: from soap bubbles to industrial lubricants. It is well known that inhomogeneities in thin film composition can generate a surface tension gradient leading to soluto-Marangoni flows. One simple system that exhibits such behavior is a binary liquid thin film over a bubble surface. If the liquid film contains miscible components with different surface tension and evaporation rate, then when the evaporative species leaves the film, the surface tension of the mixture changes and liquid is drawn toward the high surface tension region, thereby increasing the film curvature. We demonstrate that under certain conditions the capillary pressure may ultimately overwhelm the Marangoni effects such that the film thickness and composition are again homogenized. Evaporation then recreates a composition gradient and the above interplay between Marangoni and capillary stresses repeats, resulting in an oscillatory flow. Such an oscillatory flow was observed both in numerical simulations and in dynamic fluid-film interferometer experiments with binary mixtures of low molecular weight silicone oil. This talk will focus on elucidating the interplay among evaporation, soluto-Marangoni flow, and capillary flow and their effects on oscillation amplitude and frequency. |
Monday, November 19, 2018 4:57PM - 5:10PM |
L27.00005: Evaporation-driven solutocapillary flow of thin films over curved substrates Mariana Rodriguez Hakim, Joseph M. Barakat, Eric S. G. Shaqfeh, Gerald G. Fuller Evaporation-driven solutocapillary flows arise when the evaporation of a volatile species induces concentration nonuniformities that give rise to spatial gradients in surface tension and subsequent Marangoni flows. These flows are prevalent in foams and emulsions, biological systems, and coating processes. We study evaporation-driven solutocapillarity in the context of ultrathin liquid films resting atop solid, spherical substrates in contact with a fluid reservoir. Experiments are conducted with low molecular weight silicone oil mixtures composed of a volatile solvent and trace amounts of a nonvolatile solute. A theoretical model based on the thin-film approximation is developed and numerically solved to give the film thickness, solute concentration, and pressure profiles. Our results reveal that both Marangoni stresses and stabilizing van der Waals interactions between the substrate and free surface can induce flow reversals and film regeneration. Furthermore, increasing the solvent’s rate of evaporation enhances the rate of film regeneration. This talk focuses on these effects and their resulting film profiles. |
Monday, November 19, 2018 5:10PM - 5:23PM |
L27.00006: Axisymmetric Membranes under External Force Thomas Powers, Leroy Jia, Steven Pei, Robert A Pelcovits Motivated by recent experimental work on colloidal membranes, we calculate the shape and force vs. extension for an axisymmetric membrane of fixed area connecting two rigid rings. For sufficiently small area, the catenoid always solves the Euler-Lagrange equation for this problem, but the area constraint means that the shape is a catenoid only for a single value of the extension. At large extensions, structures can form where the two rings are connected by a thin cylindrical tether. We describe the conditions under which there is a maximum extension as in the soap film problem, and when there is no maximum limit to the extension. |
Monday, November 19, 2018 5:23PM - 5:36PM |
L27.00007: Dynamics below an ethanol film spreading on a water layer. Anurag Pant, Baburaj A Puthenveettil We study the flows in the bulk of a water layer, which develop as a consequence of an ethanol film spreading on its surface. Ethanol-water surface tension difference gives rise to surface tension driven spreading of a lighter, miscible droplet of ethanol on the surface of water layer in form of a film. The film spreads radially outwards with vortices underneath the film front. PIV technique was used to acquire the velocity field below the surface. The velocity field shows two counter rotating vortices moving outwards and away from the point of drop deposition. We obtained the vortex core positions for various time instances (t) to find the vortex translation velocity (Uv) which scales as t-0.75. The scaling shows similarity to the film spreading velocity uf~ t-0.75 proposed by Dandekar et al.(2017) . From the velocity field, we observe the development of an upward flow in the bulk during the course of drop spreading . The upward flow is directed toward the region in between the vortices at any instant. The upward flow velocity (uy) is found by averaging the vertical components of velocity over space in between the vortex cores. This velocity increases once the drop touches the surface, maintains a constant value and decrease to zero as the drop spreading comes to an end. |
Monday, November 19, 2018 5:36PM - 5:49PM |
L27.00008: Dynamics of an Ideal Fluid in a Wedge Josh McCraney, Paul H Steen, Elizabeth Wesson, Joshua Bostwick It is common aboard spacecraft to passively contain/transport fuels, cryogens, waste, and other fluids through capillarity. Interior corners are commonly used to induce/retard flow. Residual accelerations (crew-docking, orbital maneuvers) can disturb the flow, ``sloshing'' the fluid, inducing unfavorable capillary surface instabilities. Here we consider a two-dimensional triangular wedge partially filled with an ideal fluid, with an interface held in equilibrium by surface tension. Small perturbations induce dynamical changes governed by the equations of motion, which are reduced to an operator equation. We report the natural frequencies and mode shapes for a pinned contact line. |
Monday, November 19, 2018 5:49PM - 6:02PM |
L27.00009: Frequency response of edge waves in soft viscoelastic materials Xingchen Shao, J.R. Saylor, Joshua Bostwick Herein we explore the frequency response of circular standing waves, ascertaining the effect of viscosity and elasticity on resonance. Edge waves were mechanically generated in a circular tank mounted on an electromechanical shaker. The wave field was measured using a camera and collimated light source. Water-glycerin mixtures and agarose gels were investigated for frequencies ranging from 4Hz to 22.9Hz and over a range of viscosity and shear modulus. The results showed that 1) the resonance modes for standing waves on water-glycerin mixtures shift to lower frequencies with increasing viscosity; 2) for fixed viscosity, the resonance modes on gels have a higher frequency than water-glycerin mixtures, i.e. elasticity increases the resonance frequency. A new theory for elastocapillary edge waves in a circular geometry is presented. The resonance frequencies predicted by the theory agree favorably with the experimental data. This work could potentially be used to develop a diagnostic method for measuring the complex modulus of soft materials. |
Monday, November 19, 2018 6:02PM - 6:15PM |
L27.00010: Thermocapillary dipole in a Hele Shaw configuration Valeri Frumkin, Moran Bercovici We present a theoretical and experimental study of thermocapillary flow in a Hele-Shaw cell containing a circular opening in the upper surface, and subjected to a linear temperature gradient. Under the assumption of shallowness (i.e. the depth of the reservoir is much smaller than its radius) we obtain a solution corresponding to a doublet flow (dipole) in the Hele-Shaw cell, providing good qualitative agreement with experimental results. We demonstrate how a superposition of dipoles can be applied for two-dimensional flow patterning, and how a confined dipole can act as a thermocapillary motor for driving liquids in microfluidic circuits. In addition, we show how the principles behind the thermocapillary dipole can be applied in order to drive thermocapillary surface swimmers on fluid-liquid interfaces |
Monday, November 19, 2018 6:15PM - 6:28PM |
L27.00011: Investigation of the Induced Flow in a Point to Plane Plasma Reactor Patrick Conlon, Douglas Bohl, Selma Mededovic Thagard, Mikhail Vasilev Plasma discharges have been shown to be useful in many technical and industrial settings. They have been used in applications including wound care, cancer treatment, and water purification. Plasma discharges have been shown to be effective in eliminating many different chemical contaminants (e.g. pharmaceuticals, PFOA) that are difficult to treat with other techniques. The exact physicochemical nature of plasma systems is not well understood. A corona discharge plasma reactor where the plasma is discharged in argon gas above the solution to be treated is studied. The chemical reactions are known to take place in a thin region along the free surface. The plasma discharge induces fluid motion in the liquid phase of reactor that refreshes the fluid at the free surface. The goal of the project is to determine the relationship between the degradation rate of various chemicals and the resulting flow fields. PIV was used to quantify the induced flow of the aqueous solution. The results show different fluid motions in the liquid phase are established based on the added chemical. The flow fields are dependent on the surface characteristics of the chemical and changes in surface tension during the plasma discharge. |
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