Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session G19: Surface Tension II |
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Chair: Karen Daniels, North Carolina State University Room: 28E |
Monday, November 19, 2012 8:00AM - 8:13AM |
G19.00001: Surfactant-driven fracture of gels: Initiation Joshua Bostwick, Mark Schillaci, Karen Daniels A droplet of surfactant spreading on a gel substrate can produce fractures on the gel surface, which originate at the contact-line and propagate outwards in a star-burst pattern. Experiments show that the number of arms is controlled by the ratio of surface tension contrast to the gel's shear modulus. To further understand the mechanism behind crack initiation, we model the gel as a linear elastic solid and compute the state of stress that develops within the substrate from the uncompensated contact-line forces. The elastic solution yields an effective metric to predict the number of fractures. We also show that the depth of the gel is critical parameter in the fracture process, as it can help mitigate large surface tractions. This observation is confirmed in experiments. [Preview Abstract] |
Monday, November 19, 2012 8:13AM - 8:26AM |
G19.00002: Surfactant-driven fracture of gels: Growth Karen Daniels, Mark Schillaci, Joshua Bostwick A droplet of surfactant spreading on a gel substrate can produce fractures on the gel surface, which originate at the contact-line and propagate outwards in a star-burst pattern. Fractures have previously been observed to initiate through a thermal process, with the number of fractures controlled by the ratio of surface tension differential to gel shear modulus. After the onset of fracture, experiments show the arm length grows with universal power law $L=t^{3/4}$ that does not scale with any material parameters (Daniels et al. 2007, PRL), including super-spreading surfactants (Spandangos et al. 2012, Langmuir). We develop a model for crack growth controlled by the transport of an inviscid fluid into the fracture tip. While treating the gel as a linear material correctly predicts power-law growth, we find that only by considering a Neo-Hookean (incompressible) material do we obtain agreement with the experiments. [Preview Abstract] |
Monday, November 19, 2012 8:26AM - 8:39AM |
G19.00003: Hole-Closing of a Surfactant Layer on a Thin Fluid Film Rachel Levy, Matthew Hin, M. Richard Sayanagi, Eric Autry, Jeffrey Wong, Karen Daniels The spreading of surfactants on a thin fluid layer has been most commonly studied in an outward-spreading geometry. We perform simulations and experiments on the inverse, the inward spreading of surfactant into a clean disk-shaped region, known as hole-closing. In both cases, we observe that the inward force from the surface tension gradient produces a transient distention, in which the underlying fluid is raised within the closing region. We observe that the height of the distension is controlled by a combination of fluid depth and the surface tension gradient between the two regions. We compare the evolution of the distension height over time to a coupled system of partial differential equations that have been used to model surfactant spreading for more than two decades. [Preview Abstract] |
Monday, November 19, 2012 8:39AM - 8:52AM |
G19.00004: The Adventures of the Diving-Bell Spider Raphaele Thevenin, Guillaume Dupeux, Keyvan Piroird, Christophe Clanet, David Quere The Argyroneta Aquatica is a unique spider that has every features of a usual terrestrial spider, but constantly lives under water. To however still be able to breath oxygen, it builds an underwater bell of air (hence its other name ``the diving-bell spider''): using its superhydrophobic abdomen, it pulls an air bubble at the surface by leaving the latter very rapidly. It then enters the bell formed under aquatic plants or under its under-water web, and leaves it more slowly so as to entrain the least air possible. We study these dynamics that take place at the air/water interfaces. We reduce the spider to two beads, one for the hydrophobic abdomen, one for the hydrophilic head, and measure and model the air entrainment according to the size and surface properties of the abdomen and to the velocity of motion. [Preview Abstract] |
Monday, November 19, 2012 8:52AM - 9:05AM |
G19.00005: Tension induced phase transitions in biomimetic fluid membranes Marc Shapiro, Petia Vlahovska Membranes in eukaryotic cells are mixtures of hundreds of lipid species. The lipid diversity enables membranes to phase separate and form domains, called rafts, which play a critical role in cell functions such as signaling and trafficking. The phase transitions underlying raft formation have been extensively studied as a function of temperature and composition. However, the third dimension of the phase diagram, i.e., the tension (2D pressure), is still unexplored because membrane tension is difficult to control and quantify. To overcome this challenge, we develop two approaches, capillary micromechanics and electrodeformation, in which the tension is regulated by the area dilation accompanying deformation of a vesicle (a closed membrane). The first technique consists of forcing an initially quasi-spherical vesicle through a tapered glass microcapillary, while the second method utilizes uniform electric fields to deform the vesicle into an ellipsoid. Domains are visualized using a fluorescent dye, which preferentially partitions in one of the phases. The experimental results suggest that the miscibility temperature (at which domains form in an initially homogeneous membrane) increases with applied tension. Domain motions and coarsening are also investigated. [Preview Abstract] |
Monday, November 19, 2012 9:05AM - 9:18AM |
G19.00006: Wicking flow in optimized capillary channels Bruno Figliuzzi, Cullen Buie Many technological applications rely on the phenomenon of wicking flow induced by capillarity. However, despite a continuous interest on the subject, the influence of the geometry of the capillary channel on the dynamics of wicking is still poorly understood. In the case of a cylinder, the well-established Washburn law indicates that, at short time, the height of the rising liquid increases as the square root of time. However, Reyssat et al. demonstrated that shape variations affect the dynamics of the capillary rise at longer times. In numerous applications, being able to favor wicking in a capillary channel is a key issue. Starting from the Washburn-Lucas equation, we've developed a model describing the capillary rise of a liquid in a tube of varying circular cross-section. In this model, the dynamics of wicking is described by an ordinary differential equation whose second term depends on the shape of the capillary channel. Using optimal control theory, we have designed optimal shapes which favor wicking flow. Numerical simulations were conducted which show that the height of the rising liquid is up to 40 percent higher with the optimized shapes than with a cylinder tube of optimal radius. [Preview Abstract] |
Monday, November 19, 2012 9:18AM - 9:31AM |
G19.00007: Capillary rise within superhydrophilic channel Jungchul Kim, Myoung-Woon Moon, L. Mahadevan, Ho-Young Kim While the capillary rise within smooth channels is a classical topic in hydrodynamics, the dynamics of liquid rise through superhydrophilic, microscopically rough channels have rarely been studied so far. Here we experimentally show that within superhydrophilic channels, a bulk flow rises against gravity in a similar fashion to one in smooth channels in the initial stages. However, as the bulk approaches Jurin's height, a thin film that wicks into the rough surface emerges, a novel feature characteristic to superhydrophilic capillary rise. We construct a scaling law to explain the wicking rate of the thin film, which depends on the bulk height as well as the surface roughness and liquid properties. This study is potentially useful in understanding transport of sap through porous xylems of plants. [Preview Abstract] |
Monday, November 19, 2012 9:31AM - 9:44AM |
G19.00008: ABSTRACT WITHDRAWN |
Monday, November 19, 2012 9:44AM - 9:57AM |
G19.00009: Capillary interactions between spherical Janus particles at liquid-liquid interfaces Hossein Rezvantalab, Shahab Shojaei-Zadeh We study the non-equilibrium behavior of Janus particles at a flat liquid-liquid interface. If the Janus boundary is completely sharp and smooth, no interface deformation occurs due to uniform wetting around the particles. However, if the neighboring particles possess different orientations or are pinned at specified angles, they interact due to the induced deformation at the fluid-fluid interface. The tendency to minimize high energy surface areas of the Janus particle distorts the contact line from a circular shape and results in attracting forces between the particles. We examine the energetic interactions among spherical Janus particles as a function of their separation distance, orientation angle, and their wettabilities. It is found that the extent of interface deformation strongly depends on the difference between the wettabilities of the two hemispherical sides. We show that the interface distortions at the near sides of the two spheres join, under appropriate conditions, to form an interfacial structure resembling a capillary bridge. Our calculation provides a detailed insight into the interface deformation and inter-particle forces that arise between randomly oriented Janus spheres before reaching their equilibrium orientation. [Preview Abstract] |
Monday, November 19, 2012 9:57AM - 10:10AM |
G19.00010: The Short-range Capillary Force on Floating Objects Andong He, Khoi Nguyen, Shreyas Mandre We develop a general method to study the capillary force between objects of arbitrary shape which float close to each other on an interface, a regime in which multipole expansion is not useful. The force is represented as a power series in the small distance between the objects, of which the leading-order is finite. For objects with size $a$ much larger than the capillary length $l_c$, the force scales as $\sqrt{a/l_c}$ and the prefactor depends on the mean radius of curvature $R$ at the closest points. After contact the objects roll and/or slide with respect to each other to locally maximize $R$. For smaller objects ($a\ll l_c)$, the force scales as $(a/l_c)^{-1}\log(a/l_c)^{-2}$, and the prefactor depends only weakly on the shape and relative orientation of the objects. [Preview Abstract] |
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