Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session G3: Surface Tension II |
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Chair: Shelley Anna, Carnegie Mellon University Room: 303 |
Monday, November 21, 2011 8:00AM - 8:13AM |
G3.00001: Using bulk convection to approach kinetic-limited surfactant dynamics Lynn Walker, Nicolas Alvarez, Douglas Vogus, Shelley Anna Many processes involving transport of surfactants to interfaces are in the regime where kinetics, diffusion and convection are comparable. Using the principle that the timescale for diffusion depends on curvature, we previously developed a microtensiometer to accurately measure surfactant dynamics at spherical microscale liquid-fluid interfaces. In the present study, we introduce a low Reynolds number flow in the bulk solution to further increase the rate of diffusion. Dynamic surface tension is measured as a function of Peclet number and the results are put into the context of a simplified convection-diffusion model. Although a transition from diffusion to kinetic- limited transport is not observed experimentally for the surfactants considered, lower bounds on the adsorption and desorption rate constants are determined. These lower bound values are much larger than previously reported rate constants. This experimental tool and analysis allows the governing mechanisms of surfactant transport at liquid-fluid interfaces to be quantified. The addition of flow near an interface is effective in decreasing the length scale for diffusion, and shifting the relevant timescales. The results show that the detailed nature of the flow field does not need to be controlled as long as the local Reynolds number is low. [Preview Abstract] |
Monday, November 21, 2011 8:13AM - 8:26AM |
G3.00002: Spinodal decomposition in particle-laden Landau-Levich flow Justin Kao, A.E. Hosoi We investigate Landau-Levich coating of a solid wall by a suspension of large spherical particles. Capillary forces lead to self-assembly of a monolayer of particle aggregates, and experiments show two regimes of pattern formation, corresponding to sporadic and continuous deposition. We obtain coating fraction as a function of bulk particle volume fraction and wall speed, and propose a continuum spinodal decomposition (Cahn-Hilliard) model for this pattern formation process. Solutions of the corresponding model equations are presented and compared to experimental results. [Preview Abstract] |
Monday, November 21, 2011 8:26AM - 8:39AM |
G3.00003: Nanoparticle monolayers under stress: mechanically forced desorption from a fluid-fluid interface Valeria Garbin, John C. Crocker, Kathleen J. Stebe Nanoparticle-laden interfaces are studied for applications to materials with tunable electronic and optical properties, as emulsion stabilizers, and in catalysis. The mechanical response of nanoparticle monolayers under applied stress is of emerging interest since it impacts the success of these applications. Here we focus on the response of nanoparticle-laden interfaces to compression. A monolayer of nanoparticles is allowed to spontaneously form by adsorption from an aqueous suspension onto a pendant drop of oil. The effective surface pressure $\Pi$ of the composite interface is monitored by pendant drop tensiometry. As the drop is compressed, the nanoparticles are mechanically forced out of the interface into the aqueous phase. A new optical method is developed to measure the nanoparticle area density \emph{in situ}. We show that desorption occurs at a coverage that corresponds to close packing of the ligand-capped particles, suggesting that ligand-induced repulsion plays a crucial role in the desorption process. [Preview Abstract] |
Monday, November 21, 2011 8:39AM - 8:52AM |
G3.00004: The effect of lipid monolayers on Faraday waves Stephen Strickland, Lake Bookman, Michael Shearer, Karen Daniels Surface tension is known to affect the critical driving acceleration for Faraday waves and their spatial wavenumber at onset. We perform experiments in the subharmonic regime, on water whose free surface is contaminated with up to one monolayer of fluorescent NBD-PC lipid. A circular container of water is vibrated vertically at single frequencies ranging from 15 Hz to 70 Hz, and we measure the acceleration and wavenumber at the onset of Faraday waves. We observe that the critical acceleration is larger than predicted by recent models, if the effect of the contaminant is assumed to simply lower the surface tension. Critical wavenumbers are largely unaffected. We examine whether a non-uniform lipid distribution is responsible for these effects. [Preview Abstract] |
Monday, November 21, 2011 8:52AM - 9:05AM |
G3.00005: Surfactant-Driven Fracture Formation in Soft Gels Mark Schillaci, Joshua Bostwick, Karen Daniels The formation of fractures in gels well above the solid-liquid transition has been previously shown to initiate through a Poisson process, indicating that thermal fluctuations play a significant role. Here, we present experiments quantifying fracture formation in gels close to the solid-liquid transition. We utilize a spreading surfactant droplet to apply small forces to the surface of the gel. Fractures form along the contact-line and propagate outward in a star-burst pattern. By varying the droplet surface tension and gel modulus, we are able to tune the fracture formation and control the mean number of fractures formed. We interpret the number of fractures formed in the context of a linear elastic model for the uncompensated, Young-Dupr\'e (out-of-plane) force acting at the contact-line. However, we also observe that there is an inherent variability in both the number of fractures formed and the delay for fractures to form. In the regime where single fractures form, we observe a range of delay times consistent with a Poisson distribution. In the regime where multiple fractures form, we observe that all fractures appear simultaneously and the long delays are suppressed. [Preview Abstract] |
Monday, November 21, 2011 9:05AM - 9:18AM |
G3.00006: Rod-like microparticles at interfaces: near-field capillary interactions and implications for mechanics of particle-laden interfaces Lorenzo Botto, Lu Yao, Marcello Cavallaro, Kathleen J. Stebe Rod-like particles assemble at fluid interfaces owing to anisotropic capillary interactions. Experiments on cylindrical and ellipsoidal particles reveal that preferred orientation, interaction strength, and properties of resulting assemblies depend strongly on particle shape. While cylinders assemble end-to-end forming rigid linear chains, ellipsoids form flexible structures with particles side-by-side. Simulations of pairs of particles at contact reveal that chains of cylinders are rigid. Experiments bear this out: a chain of microcylinders rotated via a magnetic field remains rigid even under torques in excess of $10^5kT$. Above a yield stress, chains snap and dissipate stored capillary energy. Interactions between ellipsoids are comparatively weak, with no energy barrier as particle rotate about each other, consistent with the highly flexible chains formed by these particles. These interactions have profound implications for the mechanics of particle-laden interfaces including their viscoelastic properties. [Preview Abstract] |
Monday, November 21, 2011 9:18AM - 9:31AM |
G3.00007: Experimental and Theoretical Study of Chemically Driven Waves at an Oil-Water Interface Meir Basson, Rouslan Krechetnikov When an aqueous solution of TSAC surfactant is placed on top of a heavier organic solution of Iodine, a chemical reaction between the solutes causes traveling waves to form at the interface. These traveling waves have wavelengths on the order of centimeters and speeds on the order of centimeters per second. In this talk, we present an experimental study of these chemically driven interfacial waves that form and rotate around an annular container. Specifically, using stereo photography, we resolve the three dimensional structures and time scales of the waves and thus explore the effect of parameters such as container geometry (e.g. diameter, annular gap) and solute concentrations on the speed, shape, and lifetime of the formed interfacial waves. Also, a theoretical study describing the mechanism behind the traveling waves will be presented. In particular, we analyze the chemical reaction which causes the wave motion and propose a model for the chemical-to-mechanical conversion of energy at the liquid-liquid interface. [Preview Abstract] |
Monday, November 21, 2011 9:31AM - 9:44AM |
G3.00008: Thermocapillary Actuation of Binary Drops on Solid Surfaces Fangjie Liu, Yuejun Zhao, Chuan-Hua Chen Under thermocapillary actuation, aqueous drops typically remain stuck on hydrophobic solid surfaces subjected to a temperature gradient. We report the thermal actuation of water drops on a parylene-coated silicon substrate, where the actuation was enabled by encapsulating the water drop with a minute amount of long-chain alcohol. When placed near an initially stuck water drop, heptanol spread toward the water under thermocapillarity. The merged binary drop can take on a unique shape: the dome shaped water drop is capped by a thin layer of heptanol and the cap protrudes through a ``foot'' to a precursor film. For intermediate volumes of water drop with a fixed volume of heptanol, the speed of the binary drop was linearly proportional to its diameter and the imposed temperature gradient with an offset accounting for the hysteresis force. The thermocapillary actuation speed can be modeled by a lubrication theory adapted from the models of slender liquid ridges. The binary thermocapillary actuation also works for other combinations of secondary fluids and solid surfaces, making it an attractive scheme for transporting aqueous samples of biomedical and industrial relevance. [Preview Abstract] |
Monday, November 21, 2011 9:44AM - 9:57AM |
G3.00009: Shape oscillations of attached bubbles in pure liquids and in surfactant solutions Jiri Vejrazka, Lucie Vobecka, Nicolas Meunier, Maria Zednikova, Sandra Orvalho, Jaroslav Tihon The high-speed imaging is used for studying oscillations of air bubble, which are excited by a motion of a tip of a capillary, to which the bubble is attached. The frequency, damping and shape of decaying oscillations are determined for the lowest three eigenmodes. In pure water, the experimental results agree well with an inviscid analysis of Bostwick and Steen (\textit{Phys Fluids }\textbf{21}, 2009), if the bubble size is small compared to the detachment size. For larger bubbles, the effect of neck formation on the oscillations is documented experimentally. The addition of a surfactant to the system modifies both the oscillation frequency and damping. The frequency initially increases with the increasing surfactant concentration due to Gibbs elasticity, and starts to decrease only after exceeding some surfactant concentration. The damping strongly increases, passes through a maximum and then slightly decreases. The maximum damping is observed for the concentration, at which the oscillation frequency starts to decrease. Concluding, bubble oscillations are strongly sensitive to presence of a surfactant. [Preview Abstract] |
Monday, November 21, 2011 9:57AM - 10:10AM |
G3.00010: Solid Layer Formation at Oil-Water Interfaces Rielle de Ruiter, Willem Tjerkstra, Michel Duits, Frieder Mugele Metal stearates form at the interface between a decane solution of stearic acid (s.a.) and aqueous salt solutions of variable composition and pH. Studying the evolution of their mechanical, optical, and chemical properties as a function of time we find hardly any interfacial activity for pH$<$pKa of s.a. For pH $>$ pKa, s.a. deprotonates at the interface and forms metal stearates, eventually leading to the formation of macroscopic solid layers. Dynamic interfacial tension measurements reveal several stages of the process, including the subsequent formation of dilute and dense monolayers followed by three-dimensional growth. In the presence of divalent ions, the solid layers display a significant increase in the dilatational storage modulus. In the presence of multiple cation species (artificial seawater) the growth of the solid layers is particularly pronounced. The layers preferentially incorporate Ca2$\backslash$th as revealed by XPS and IR spectroscopy. Our results highlight in particular the importance of the synergistic effects of simultaneously present monovalent and divalent cation species on the interfacial adsorption. [Preview Abstract] |
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