Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session E53: Fluid Mechanics for Soft Matter I: Flows |
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Sponsoring Units: GSOFT GSNP DFD Chair: Eric Davis, Clemson Univ Room: LACC 513 |
Tuesday, March 6, 2018 8:00AM - 8:12AM |
E53.00001: Programmable aerodynamic drag control in morphable cylinders Pedro Reis, Mark Guttag, Dong Yan We study active aerodynamic drag control on morphable cylinders, whose topography can be modified pneumatically. Our design was initially inspired by the Saguaro cactus, which possesses an array of axial grooves that are thought to help reduce drag. In our analog experiments, we fabricate samples covered by an outer elastomeric film that is supported by a rigid inner skeleton. This cylindrical skeleton comprises either (i) a series of axial grooves or (ii) a hexagonal arrangement of holes. Decreasing the sample’s inner pressure deforms the outer film to produce (i) axial grooves or (ii) a pattern of dimples, whose depth can be varied on demand. First, we combine mechanical experiments and finite element simulations to characterize the depth of the topographic features vs. pneumatic loading. Secondly, we perform wind tunnel tests to measure the aerodynamic drag coefficient (as a function of Reynolds number) of the samples, for different surface depths. Finally, we leverage the morphable nature of our samples to customize their drag profile, in a fully ‘programmable’ manner. |
Tuesday, March 6, 2018 8:12AM - 8:24AM |
E53.00002: Wetting, disorder, and pattern formation Amir Pahlavan, Luis Cueto-Felgueroso, Gareth McKinley, Ruben Juanes Displacement of a more viscous fluid by a less viscous one in a homogeneous medium leads to an interfacial instability. This description, however, neglects the inherent heterogeneity observed in most real systems, as well as the non-hydrodynamic interactions between the fluids with the solid surfaces. Here, we revisit the Saffman-Taylor instability using a roughened Hele-Shaw cell as an analogue disordered porous medium and show that the interplay between disorder and wettability of the medium can fundamentally alter the pattern formation dynamics. Our observations show the critical role of the contact-line dynamics in the macroscopic pattern formation, and point to new directions for controlling flows with applications ranging from microfluidics to CO2 sequestration. |
Tuesday, March 6, 2018 8:24AM - 8:36AM |
E53.00003: Oscillatory Fingering in an Elasto-Rigid Hele-Shaw Channel Callum Cuttle, Anne Juel, Draga Pihler-Puzovic We study the propagation of an air finger in a Hele-Shaw channel where the top boundary is an elastic membrane. The injection of air at a constant flow rate works against elastic, viscous and capillary forces to form an approximately steadily propagating blistering finger (Juel et al. 2018), which depends on the capillary number Ca -- the ratio of viscous to surface tension forces -- and the level of initial collapse of the channel cross-section. A stable Saffman–Taylor finger is formed when the membrane is initially flush with the side walls, and in the rigid channel this mode is linearly stable for all values of Ca. By contrast, increasing the initial deflection of the membrane leads to the broadening of the finger and the formation of small-scale fingering at its tip, which is advected around the finger as it advances. This form of propagation is oscillatory in the reference frame of the moving finger. We present evidence of a simple geometric mechanism for the switching between two modes of oscillatory fingering. One resembles the tip-splitting instabilities observed to occur subcritically in rigid channels above a critical value of Ca and the other is linked to stubby finger formation in tapered channels, such as in the famous tape-peeling study of McEwan & Taylor (1961). |
Tuesday, March 6, 2018 8:36AM - 8:48AM |
E53.00004: Integer lattice gas with Monte Carlo collision operator recovers the entropic lattice Boltzmann method with MRT collision operator and exact fluctuations Thomas Blommel, Alexander Wagner We present a new kind of lattice gas that closely resembles modern lattice Boltzmann methods. This new kind of lattice gas, that we call a Monte Carlo Lattice Gas, has interesting properties that shed light on the origin of the multi-relaxation time (MRT) collision operator and it derives the equilibrium distribution for entropic lattice Boltzmann. Furthermore these lattice gas methods have Galilean invariant fluctuations given by a Poisson statistics, giving further insight into the properties that we should expect for fluctuating lattice Boltzmann methods. We report on our progress in making our collision operator more computationally efficient by factorizing the collisions into one dimensional components. We also examine the physical justification for over-relaxation through a scaling analysis of our lattice gas. |
Tuesday, March 6, 2018 8:48AM - 9:00AM |
E53.00005: Buoyant Bretherton: gravity driven rise of a bubble in a capillary Wassim Dhaouadi, Francois Gallaire, John Kolinski The pressure driven motion of a bubble in a capillary has long been used to measure fluid flow rates in myriad industrial processes. Advances in microfluidics and small-scale flow applications highlight the importance of the limit when the capillary’s radius is significantly smaller than the capillary length for the fluid of interest, increasing the relative importance of viscous resistance as the bubble moves through the capillary. For buoyancy driven bubble motion, when the bubble is subjected to a constant force, the bubble’s motion is critically dependent on the dynamics of the surrounding liquid film. The absence of quantitative measurements of the liquid film thickness led to contradictory predictions and experimental data for this problem. Here, we present quantitative measurements of the liquid film surrounding the bubble for a liquid with a strong interfacial attraction to the capillary wall. Within the bounds of our measurement, the film thickness reaches a constant value, and the bubble always rises, even for capillary diameters significantly less than the critical value predicted by the theory. |
Tuesday, March 6, 2018 9:00AM - 9:12AM |
E53.00006: Flow of Deformable Droplets: Discontinuous Shear Thinning and Velocity Oscillations. Martina Foglino, Davide Marenduzzo We study the rheology of a suspension of soft deformable droplets subjected to a pressure-driven flow. Through computer simulations, we measure the apparent viscosity as a function of droplet concentration and pressure gradient, and provide evidence of a discontinuous shear thinning behavior, which occurs at a concentration-dependent value of the forcing. We further show that this response is associated with a nonequilibrium transition between a “hard” (or less deformable) phase, which is nearly jammed and flows very slowly, and a “soft” (or more deformable) phase, which flows much more easily. The soft phase is characterized by flow-induced time dependent shape deformations and internal currents, which are virtually absent in the hard phase. Close to the transition, we find sustained oscillations in both the droplet and fluid velocities.We also analyse the reversibility properties of our system, performing simulations where our suspension is subjected to an oscillatory shear or where a selected droplet sees a periodic increase of its volume. Preliminary results suggest that hydrodynamics interactions can lead to irreversible behavior. |
Tuesday, March 6, 2018 9:12AM - 9:24AM |
E53.00007: Microstructure and Dynamics of Self-Assembled Ordered Drop Arrays in Strongly Confined Creeping Flows Sagnik Singha, Mauricio Zurita-Gotor, Michael Loewenberg, Jerzy Blawzdziewicz Flow-driven strongly confined systems of deformable drops in parallel wall channels can spontaneously form ordered structures, such as flow-aligned chains and lattice formations. Using a Hele–Shaw multipolar approximation, we explore the microstructural evolution in such systems. We focus a on confined Couette flow, in which, to the leading order, the microstructural dynamics is driven by the Hele–Shaw quadrupolar hydrodynamic interactions between drops. Systems at different drop area fractions are described in terms of the longitudinal and transverse pair correlation functions to characterize spontaneous formation of periodic structures, and bond-orientation order parameters are introduced to quantify the evolution of structural defects. The effect of drop polydispersity on the defect annealing process is evaluated. We also investigate the microstructure of drop arrays in a pressure-driven flow, in which both dipolar and quadrupolar interparticle hydrodynamic interactions are active. |
Tuesday, March 6, 2018 9:24AM - 9:36AM |
E53.00008: Universal emergent dynamics of micro-spinners above a wall Enkeleida Lushi, Florencio Balboa Usubiaga, Michael Shelley We study the collective behavior in ensembles of micro-spheres driven to rotate above a wall. This many particle system is Hamiltonian and the ensemble rotates about its center of mass. Using large-scale computations that resolve the interactions with the wall, hydrodynamics and particle collisions, we study the emergence of collective rotation, edge currents and universal behaviors that depend on the density and height. |
Tuesday, March 6, 2018 9:36AM - 9:48AM |
E53.00009: What makes fabric waterproof? Simon Torbeyns, Denis Terwagne When laying a liquid droplet on a piece of fabric, the droplet can bounce off the fabric, sit on it or totally impregnate it. In this paper, we investigate the transition in between the sitting situation and the impregnating situation. As it has been shown in a number of recent works, totally wetting liquid droplets interacting with a network of fibers lead to different impregnation morphologies. Here, we use partially wetting liquid droplets that lead to other morphologies where droplets sit on the fabric without impregnating the network of fibers. We show that a transition between "sitting on" and "passing through" occurs that depends on the number of fibers and their geometry, the liquid that we use and its wettability on the surface of the fibers. Based on our observation and careful measurements, we developed a model that predicts the transition and the final morphology of the droplets on the fibers array. This work opens the path to study the impregnation of liquids in more complex fibers geometries and should find application in the textile and wearable industries. |
Tuesday, March 6, 2018 9:48AM - 10:00AM |
E53.00010: Poro-elasto-capillarity of cellulose sponges Jonghyun Ha, Jungchul Kim, Yeonsu Jung, Ho-Young Kim It is a mundane experience that cellulose sponges swell while absorbing water. Fluid flows in deformable porous media, such as soils and hydrogels, are classically described based on the theories of Darcy and poroelasticity, where the expansion of the media arises due to increased pore pressure. However, the situation is qualitatively different in cellulosic porous materials like sponges because the pore expansion is driven by wetting of the surrounding cellulose walls rather than by increase of the internal pore pressure. The observation using the environmental scanning electron microscopy reveals the coalescence of microscale wall pores, which allows us to build a mathematical model for pore size evolution and the consequent wicking dynamics. The scaling law constructed through this work agrees well with the experimental results. Our study sheds light on the physics of water absorption in hygroscopically responsive porous materials, which have far more implications than everyday activities (e.g. cleaning, writing and painting) carried out with cellulosic materials (paper and sponge), including absorbent hygiene products, cooking, and soft robotics. |
Tuesday, March 6, 2018 10:00AM - 10:12AM |
E53.00011: Spatial fluctuations in flow through a deformable porous medium Nancy Lu, H. Jeremy Cho, Sujit Datta Fluid flow through a deformable porous medium, such as a filter cake or a biological tissue, is typically modeled by combining Darcy’s law with a solid stress balance. Unfortunately, this continuum picture neglects flow fluctuations inside the pore space. These fluctuations can impact solute dispersal in such processes as filter-cake washing and nutrient transport in tissues. Additionally, this method neglects variations in the stress within the solid matrix, which can further alter the variability in the fluid flow. We use direct visualization to disentangle this coupling between solid stresses and fluid flow in model deformable porous media. Specifically, we use confocal microscopy to characterize how the spatial fluctuations in the flow are altered by deformations of the solid matrix. Our results thus shed light on the pore-scale complexity that is hidden in continuum models. |
Tuesday, March 6, 2018 10:12AM - 10:24AM |
E53.00012: Deformation of perforated elastic sheets due to the hydrodynamic loading by a viscous fluid Matteo Pezzulla, Elizabeth Strong, Hussain Karimi, Pedro Reis From spider webs and insect wings, to wire fences and parachutes, Nature and technology present vast examples of porous and perforated flexible structures that deform due to fluid flow. Whereas fluid flow through porous media has been studied extensively, the fluid-structure interactions of a perforated, slender elastic object undergoing large deformations due to the hydrodynamic loading of a surrounding viscous fluid has received much less attention. Here, we use precision desktop experiments to focus on the prototypical problem of a perforated elastic plate moving through a viscous fluid, at low to moderate Reynolds number: 0.1 ≤ Re ≤ 10. We seek to provide a predictive framework for the deformation of perforated plates due to hydrodynamic loading so as to rationalize our experimental findings. For this purpose, we use a reduced theoretical model based on Kirchhoff-Euler beam theory coupled with a description of the fluid loading, at low Reynolds numbers. We quantify the effect of the interplay between elasticity, permeability, and fluid loading on the deformed shape of the structure. We hope that our findings may lead to a better understanding of fluid-structure interactions between porous slender structures and viscous flows, across biological and technological applications. |
Tuesday, March 6, 2018 10:24AM - 10:36AM |
E53.00013: Coarsening Kinetics of Ternary Polymer Solutions with Mobility and Viscosity Contrasts Jan Ulric Garcia, Douglas Tree, Kris Delaney, Glenn Fredrickson Nonsolvent induced phase separation (NIPS) is a technique to make polymer membranes. In NIPS, a polymer solution film is immersed in a nonsolvent bath, inducing phase separation and eventual solidification of the film into a membrane. Though NIPS has been in use for decades, the formation of important membrane features, such as asymmetric pore size distribution, still remains a mystery. To understand how these features are formed, we built a phase-field simulation software capable of solving coupled diffusion and momentum equations that describe the NIPS system. Our stable pseudo-spectral methods allow us to simulate systems at the micron length-scale for millisecond time-scales. In this talk, we report how coarsening—a major factor in shaping bulk membrane morphology—behaves when mobility and viscosity contrasts are present among the NIPS components. We consider both diffusion-only coarsening and coarsening with hydrodynamics. We also look at the effect of dimensionality, comparing coarsening in 2D and 3D. |
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