Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session G11: Drops: Elastic Surfaces and FibersDrops FSI
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Chair: Konrad Rykaczewski, Arizona State University Room: 504 |
Monday, November 20, 2017 10:35AM - 10:48AM |
G11.00001: Contact angles on stretched solids Liz Mensink, Jacco Snoeijer The surface energy of solid interfaces plays a central role in wetting, as they dictate the liquid contact angle. Yet, it has been challenging to measure the solid surface energies independently, without making use of Young’s law. Here we present Molecular Dynamics (MD) simulations by which we measure the surface properties for all interfaces, including the solids. We observe change in contact angles upon stretching the solid substrates, showing that the surface energy is actually strain dependent. This is clear evidence of the so-called Shuttleworth effect, making it necessary to distinguish surface energy from surface tension. We discuss how this effect gives rise to a new class of elasto-capillary phenomena. [Preview Abstract] |
Monday, November 20, 2017 10:48AM - 11:01AM |
G11.00002: Elasto-capillary interactions of drops and particles Jacco Snoeijer, Anupam Pandey, Stefan Karpitschka, Charlotte Nawijn, Lorenzo Botto, Bruno Andreotti The interaction of solid particles floating on a liquid interface is popularly known as the Cheerios effect. Here we present similar interactions for particles and droplets on elastic surfaces, mediated by elastic deformation. We start with the Inverted Cheerios effect, by considering liquid drops on a solid gel. Remarkably, the interaction can be tuned from attractive to repulsive, as shown experimentally and theoretically. We then turn to more general cases of particles on elastic layers, for which new interaction laws are derived. An overview is given on the various regimes, including the crossover from purely elastic to purely capillary interfaces. [Preview Abstract] |
Monday, November 20, 2017 11:01AM - 11:14AM |
G11.00003: Deformation of an Elastic Substrate Due to a Resting Sessile Droplet Aaron Bardall, Karen Daniels, Michael Shearer On a sufficiently soft substrate, a resting fluid droplet will cause significant deformation of the substrate. This deformation is driven by a combination of capillary forces at the contact line and the fluid pressure at the solid surface. These forces are balanced at the surface by the solid traction stress induced by the substrate deformation. Young's Law, which predicts the equilibrium contact angle of the droplet, also indicates an a priori radial force balance for rigid substrates, but not necessarily for soft substrates which deform under loading. It remains an open question whether the contact line transmits a non-zero force tangent to the substrate surface in addition to the conventional normal force. This talk will present a model for the static deformation of the substrate that includes a non-zero tangential contact line force as well as general interfacial energy conditions governing the angle of a two-dimensional droplet. We discuss extensions of this model to non-symmetric droplets and their effect on the static configuration of the droplet/substrate system. [Preview Abstract] |
Monday, November 20, 2017 11:14AM - 11:27AM |
G11.00004: Peeling without precursors John Lister, Dominic Skinner, Tim Large The peeling by fluid injection of an elastic sheet away from a substrate is often regularised by invoking a thin prewetting film or a low-viscosity phase in the tip. Here we analyse fluid-driven peeling without such precursors, and consider an elastic sheet either bonded to, or simply laid on, an elastic substrate. To resolve the `elastic contact-line problem' that arises from viscous flow and beam theory, we determine the near-tip behaviour from lubrication theory coupled to the full equations of elasticity and fracture. The result is a law for the tip propagation speed in terms of the remote loading and the toughness of the sheet-substrate bonding (which might be zero). There are distinct modes of failure, according to whether there is slip ahead of the fluid front. The propagation-speed law gives rise to new similarity solutions for the spread of a fluid-filled blister in different regimes. [Preview Abstract] |
Monday, November 20, 2017 11:27AM - 11:40AM |
G11.00005: The Influence of Geometry on The Gel-Liquid Contact Line Shih-Yuan Chen, Karen Daniels A sufficiently soft solid, such as gel, can be deformed by surface tension when in contact with a liquid, an effect known as elastocapillarity. It remains an open question whether the force at the gel-liquid contact line depends on the geometry, chemical composition, and/or the Poisson ratio of the gel. In our experiments, we focus on three distinct geometries: (1) a droplet on a soft substrate, (2) a gel thread immersed in a liquid bath, and (3) liquid with in a single pore of the gel. We quantify how the internal strain is influenced by the choice of geometry and chemical composition. [Preview Abstract] |
Monday, November 20, 2017 11:40AM - 11:53AM |
G11.00006: Elasticity modulated Electrowetting of a sessile liquid droplet Sumit Kumar, Sri Ganesh Subramanian, Sunando DasGupta, Suman Chakraborty The sessile liquid droplets on the elastic and soft deformable surface produce strong deformation near the three-phase contact line (TPCL). The capillary and elastic forces play an important role during this deformation, and deteriorate the wetting behaviour of a sessile drop. The present work combines the effects of liquid viscosity and substrate elasticity on the dynamics of EWOD. The influence of decreasing film elasticity and viscosity on the electrowetting response of a sessile drop is experimentally investigated by delineating the changes in equilibrium apparent contact angles on substrates with varying Young's modulus of elasticity. The increase in viscosity of the liquid leads to greater electrowetting for non-deformable substrates whereas; the dynamics are not greatly affected in case of soft substrates. Although the viscosity appears to be an influential factor, the dynamics are more skewed towards the substrate rigidity. The vertical component of Young's force creates a wetting ridge at the three-phase contact line, the height of which is a direct function of the substrate rigidity. The produced ridges reduce the overall wettability of the droplet. [Preview Abstract] |
Monday, November 20, 2017 11:53AM - 12:06PM |
G11.00007: Dropwise condensation on soft hydrophobic coatings Konrad Rykaczewski, Akshay Phadnis Soft solids such as elastomers can increase droplet nucleation density due to substrate's deformation induced by combined effects of the condensate droplet's surface tension and Laplace pressure. Thus, since nucleation density strongly affects dropwise condensation (DWC) heat transfer, use of soft substrates could potentially enhance efficiency of this industrially important process. Here, we theoretically and experimentally investigate whether such benefits could indeed be attained. Specifically, we experimentally quantified the increase in nucleation density as well as droplet departure diameter as a function of substrate Young's modulus in the range of 75-500 kPa. Next, we combined analytical solutions of the elastomer deformation induced by droplets with finite element modeling of the heat transfer across them. By simulating the effect of Young's modulus of the coating on the heat transfer across the droplet size range relevant to DWC, we demonstrate that thermal resistance added by the condensate due to substrate depression is detrimental. By substituting heat transfer simulation of heat transfer in individual droplets and experimental data on nucleation density and droplet departure into DWC model, we demonstrate that softening of a hydrophobic coating would have detrimental effects on the overall heat transfer during this phase change process. [Preview Abstract] |
Monday, November 20, 2017 12:06PM - 12:19PM |
G11.00008: Equilibrium shapes of drops on membranes Ishan Sharma, Vineet Nair, Viswanathan Shankar Equilibrium shapes for axisymmetric sessile and pendant drops placed on / attached to geometrically nonlinear elastic membranes, in horizontal as well as inclined configurations, are obtained. The effective contact angle of the drop with the membrane, its contact radius, the maximum membrane displacement, and the volume of the drop is investigated for various values of Bond Number and membrane tension. [Preview Abstract] |
Monday, November 20, 2017 12:19PM - 12:32PM |
G11.00009: Refreshing Music: Fog Harvesting with Harps. Weiwei Shi, Mark Anderson, Brook Kennedy, Jonathan Boreyko Fog harvesting is a useful technique for obtaining fresh water in arid climates. The wire meshes currently utilized for fog harvesting suffer from dual constraints: coarse meshes cannot efficiently capture fog, while fine meshes suffer from clogging issues. Here, we design a new type of fog harvester comprised of an array of vertical wires, which we call ``fog harps.'' To investigate the water collection efficiency, three fog harps were designed with different diameters (254 $\mu $m, 508 $\mu $m and 1.30 mm) but the same pitch-to-diameter ratio of 2. For comparison, three different size meshes were purchased with equivalent dimensions. As expected for the mesh structures, the mid-sized wires performed the best, with a drop-off in performance for the fine or coarse meshes. In contrast, the fog harvesting rate continually increased with decreasing wire diameter for the fog harps, due to its low hysteresis that prevented droplet clogging. This resulted in a 3-fold enhancement in the fog harvesting rate for the harp form factor compared to the mesh. The lack of a performance ceiling for the harps suggest that even greater enhancements could be achieved by scaling down to yet smaller sizes. [Preview Abstract] |
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