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 C22: Drops: Interaction with Elastic Surfaces, Particles, and Fibers |
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Chair: Farzad Ahmadi, Virginia Tech Room: 604 |
Sunday, November 24, 2019 8:00AM - 8:13AM |
C22.00001: Elastocapillary flow driven blood-plasma separation. Alwar Samy Ramasamy, Ashis Kumar Sen We report a lab-on-a-membrane device which can separate plasma from whole blood due to a combined effect of capillary flow and sedimentation of blood cells. The initial length of the polydimethylsiloxane (PDMS) membrane is made hydrophilic by exposing to oxygen plasma and it is stuck to the PDMS substrate forming a 90\textdegree wedge. As the dispensed whole blood comes in contact with the membrane, force due to surface tension deflects the membrane and the consequent evolution of blood meniscus results in a Laplace pressure drop that drives the flow. The high velocity gained in the hydrophilic region is sufficient to drive the flow in to the hydrophobic region. As the flow advances, sedimentation of blood cells occurs along the length resulting in cell free layer of plasma. The geometry of the membrane and the hydrophilic length are adjusted such that the time scale required for sedimentation of blood cells is smaller than the flow time scale. As a proof of concept, a detection strip is embedded with the device to detect the level of glucose present in blood plasma. [Preview Abstract] |
Sunday, November 24, 2019 8:13AM - 8:26AM |
C22.00002: The droplet on a sugar fiber Stephane Dorbolo, Floriane Weyer, Nicolas Vandewalle, Alexandre Delory The shape and the motion of a droplet on a fiber are determined by the volume of the droplet, the radius of the fiber and the contact angle (static, advancing and receding) of the liquid on the material of the fiber. We consider a particular case in which the radius of the fiber is modified by the very presence of the droplet. The chosen system consists of a fiber made of glucose on which a water droplet is released. When the fiber is vertical, the droplet slides down the fiber before stopping at the extremity of the fiber. At this point, the droplet dissolves the fiber until the droplet moves upwards. This surprising motion of the droplet is analyzed regarding the dissolution dynamics. [Preview Abstract] |
Sunday, November 24, 2019 8:26AM - 8:39AM |
C22.00003: Redwood-inspired fog harps Weiwei Shi, Thomas Van Der Sloot, Brandon Hart, Brook Kennedy, Jonathan Boreyko In California, coastal redwoods (\textit{Sequoia sempervirens}) obtain 34{\%} of their annual hydrologic input from fog drip, as fog droplets are able to effectively slide along the parallel needle arrays to fall onto the soil. Inspired by the redwoods, we recently developed anti-clogging ``fog harps" comprised of an array of vertically oriented wires that harvested 2-3X more water compared to traditional meshes during experiments with scale-model in lab. Here, we conduct outdoor field tests for a full-scale fog harp placed side-by-side with a mesh harvester. The harp harvested anywhere from 5X to 70X more water compared to the mesh, depending upon the weather conditions. This enhancement is attributed to the harp's minimal contact angle hysteresis along the drainage pathway, which completely prevents clogging and even allows for water collection in subpar fog conditions. On the harp, droplets tended to slide along a single wire at a critical volume of only about 0.1 mm$^{\mathrm{3}}$. In contrast, most of the fog collected by the mesh remained pinned, resulting in its gradual evaporation. We expect that the fog harp's unprecedented collection efficiency will expand the regions where fog harvesting is a viable means of water harvesting. [Preview Abstract] |
Sunday, November 24, 2019 8:39AM - 8:52AM |
C22.00004: Impact and imbibition of blood drops with textiles: Where does this stain come from? Daniel Attinger, Richard Faflak Forensic investigators are sometimes asked if a given stain on fabric could have originated from a blood source at a specific relative location. A wide range of values of the maximum distance that a blood drop can travel have been reported, from less than one meter to more than 10 meters. The formation of bloodstains on fabrics can involve fast atomization mechanisms, flights of deforming droplets, and capillary interactions of a complex fluid with a multiscale substrate. Here we solve the above forensic question with fluid dynamics and inverse data search. Fluid dynamic simulations involve Newton's equation of motion, models for aerodynamic drag forces on deformable particles, and an in-house capillary model of blood wicking in fabrics. Key parameters are the drop size, launch velocity and launch angle. It is assumed that the ambient air is quiescent, and that stains exhibit similar areas on the front and back of the fabric. The latter assumption is verified for stains larger than the fabric thickness. Simulation results are then mined for the stain size, a parameter directly measurable on a crime scene. Experiments of blood spattered on fabric are in agreement with the simulation results. Findings are presented in a simple chart which discriminates whether a stain of specific size, on a specific fabric, can originate from a blood source at a specific relative location, or not. [Preview Abstract] |
Sunday, November 24, 2019 8:52AM - 9:05AM |
C22.00005: A model of droplet durotaxis driven by the elastocapillary response of a soft viscoelastic substrate Saiful Tamim, Joshua Bostwick Dynamic wetting of soft solids has many industrial and biological applications which require a rigorous understanding of the underlying fluid transport mechanism. One such phenomena, known as durotaxis, involves the spontaneous motion of liquid drops on a soft substrate with a thickness gradient. A passive driving force is generated by the elastocapillary deformation of the compliant substrate due to the interaction with the drop. We model the interactions of a two-dimensional drop with a neutrally-wetting viscoelastic substrate with thickness gradient. The substrate response is characterized by a sharp triangular wetting ridge at the contact line whose geometry changes the macroscopic contact angle of the liquid drop from its equilibrium value. The gradient in substrate thickness causes the contact angle to be less on the thicker side, which generates a driving force that moves the drop in the direction of increasing thickness. The drop velocity is determined by the viscoelastic relaxation of the substrate (viscoelastic braking), and computed from a self-consistent model that relates the soft wetting force to the viscoelastic dissipation. We find our results to be in close agreement to the velocities observed in experiment by Style et al. 2013, \textit{PNAS}. [Preview Abstract] |
Sunday, November 24, 2019 9:05AM - 9:18AM |
C22.00006: Stiffness-guided motion of a droplet on a solid substrate Panagiotis Theodorakis, Sergei Egorov, Andrey Milchev A range of technologies requires the directed motion of nanoscale droplets on solid substrates. A way of realising this effect is durotaxis, whereby a stiffness gradient of a substrate can induce directional motion without requiring an energy source. Here, we report on the results of extensive molecular dynamics investigations of droplets on a surface with varying stiffness. We find that durotaxis is enhanced by increasing the stiffness gradient and, also, by increased wettability of the substrate, in particular, when the droplet size decreases. We anticipate that our study will provide further insights into the mechanisms of nanoscale directional motion. [Preview Abstract] |
Sunday, November 24, 2019 9:18AM - 9:31AM |
C22.00007: Droplet motion on soft gels: comparing stiffness, strain, and surface energy gradient Shih-Yuan Chen, Aaron Bardall, Michael Shearer, Karen Daniels Motion of a droplet can be induced by depositing the droplet on a substrate with a gradient in substrate surface energy, surface stress, or stiffness. The motion is determined by the substrate’s surface stress, stiffness and strain state. The surface stress of a solid can be tuned via the surface energy or the surface strain of the solid, via the Shuttleworth effect. In this work, we describe our efforts to disentangle droplet motion induced by surface energy, surface strain, and stiffness. Using a 2-D model for a droplet on a soft substrate, we introduce a gradient in either solid surface energy or stiffness. The difference between the left and right contact angle sets a condition for gradient-induced droplet motion. We find that the droplet moves towards the lower solid surface energy or the lower stiffness. While the predicted threshold stiffness is too low to be relevant present for typically-used gel substrates, the threshold surface energy gradients fall within experimental ranges. Encouraged by our model, we cast a two-layer substrate: the bottom layer is UV-cured PAAM with a horizontal stiffness gradient, with a PVS layer on top. We then perform experiments by stretching the PAAM layer, inducing a strain gradient in the PVS which serves as an analog to the surface energy. [Preview Abstract] |
Sunday, November 24, 2019 9:31AM - 9:44AM |
C22.00008: Folding capsules with caged droplets Saverio Spagnolie, Xuanrong Guo, David Lynn We will describe surfactant-induced changes in the shapes of polymerizable oil droplets which are "caged" inside of deformable capsules. Using experiments, numerical simulations, and theory, we examine the roles of interfacial energies, capsule bending stiffness, and volume fraction. We find regions of parameter space where competing forces result in a sequence of flower-like shapes with 6-, 5-, 4-, and 3-fold symmetries, which are then polymerized and observed using high magnification scanning electron microscopy (SEM). Theory and simulation capture a wider range of possibilities, offering further support of this avenue for templating the synthesis of cross-linked polymer "designer" particles in the near future. [Preview Abstract] |
Sunday, November 24, 2019 9:44AM - 9:57AM |
C22.00009: Elastically driven relaxation of a fluid-filled blister on a porous substrate Danielle L. Chase, Ching-Yao Lai, Howard A. Stone The relaxation dynamics of a fluid-filled blister between an adhered elastic sheet and a porous substrate are controlled by the deformation of the elastic sheet and the viscous stresses in the blister and in the pores. We present experiments where fluid is injected at the interface between a porous substrate and an adhered elastic sheet. First, fluid invades the pores and subsequently, the elastic sheet is peeled and uplifted from the substrate resulting in a fluid-filled blister. Further injection causes the fluid front in the pores and the fracture front of the blister to propagate radially. After injection is stopped, the fracture front is static, and the fluid front continues to advance into the pores as the elastic stresses of the overlying sheet drive drainage of the blister. We develop a mathematical model for the relaxation dynamics, and after rescaling experimental data with characteristic scales for time and blister volume, we find that experiments with varying permeabilities of the porous substrate and moduli of the elastic sheets collapse onto a universal curve. The model has been applied to the relaxation of surface uplift following the drainage of a supraglacial lake and is relevant in other contexts involving fluid-driven fracture in porous media. [Preview Abstract] |
Sunday, November 24, 2019 9:57AM - 10:10AM |
C22.00010: Internal circulation and mixing in tight-squeezing deformable drops Jacob Gissinger, Alexander Zinchenko, Robert Davis Laminar flow has reduced mixing compared to turbulent flow, which is an inexpedient property for microfluidic applications such as droplet-based microreactors. A simple, two-step method is used to visualize and quantify the internal mixing of a drop in laminar Stokes flow, provided its interfacial velocity field. First, the internal Dirichlet problem is solved using the boundary-integral method. Second, solely the interface between two passive interior fluids is advected using an adaptive number of linked tracer particles. The reduction in dimension decreases the number of required tracer points, and also resolves arbitrarily-thin filaments, in contrast to backward cell-mapping methods. Interesting vector field topologies develop within 2D cross sections of drops suspended in far-field flow, after becoming trapped in various types of constrictions. For example, attracting fixed points are located on the plane of symmetry within drops trapped between cylindrical particles. For drops trapped in three-sphere constrictions, a repelling fixed point and an attracting periodic orbit are observed. The method is extended to 3D via an adaptive mesh scheme. The chaotic advection of 2D interfaces within drops of various shape is visualized, and the associated degree of mixing is quantified. [Preview Abstract] |
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