Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session A04: Focus Session: The Physics of Microscale Fluid Structure Interactions: Fully Coupled Flow and Deformation Mechanics I |
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Chair: Ivan C. Christov, Purdue University Room: Georgia World Congress Center B206 |
Sunday, November 18, 2018 8:00AM - 8:13AM |
A04.00001: Shallow Deformable Microfluidics: A passive Stokes flow rectifier for Newtonian fluids Aryan Mehboudi, Junghoon Yeom Directional dependence of hydrodynamic resistance in asymmetrically-shaped microchannels results in fluid flow rectification. Rigid nozzle/diffuser microchannels are widely used to rectify a fluid flow at sufficiently high Reynolds numbers (Re >>1), wherein the underlying mechanism is directional dependence of the hydrodynamic resistance due to the non-linear nature of the Navier-Stokes equations. Under the Stokes flow regime (Re << 1), however, the inertial terms forming the non-linear behavior of the governing equations diminish, causing the Newtonian fluid flow through a rigid nozzle/diffuser microchannel to exhibit a direction-independent hydrodynamic resistance disallowing the fluid flow rectification. In this work, we present an alternative approach to introducing nonlinear effects to the equations of motion for Stokes flow of Newtonian fluids by using the framework of deformable microchannels. The proposed nonlinearity stems from the coupled fluid-solid mechanics of the flow behavior in an asymmetrically-shaped microchannel with deformable ceiling. The flow rectification ratio of ∼ 1.4 has been theoretically and experimentally demonstrated for common Newtonian fluids like water and alcohol. |
Sunday, November 18, 2018 8:13AM - 8:26AM |
A04.00002: Arbitrary flow control in microfluidic systems using networks of bio-inspired soft valves Keunhwan Park, Magnus Paludan, Emil Østergaard, Stefan Akazawa, Kaare Hartvig Jensen For decades, semiconductor fabrication methods and mechanically actuated pumps and valves have been used to facilitate flow-control in microfluidic systems. However, these systems require energy input to regulate flow, and the cost increases accordingly. In this work, we propose a novel flow control system where a network of soft bio-inspired passive valves enables nearly arbitrary flow control. The valve action is facilitated by soft elements inside a microfluidic channel, which are deformed by the applied pressure. This leads to a highly nonlinear relation between applied pressure and flow rate. By building networks of these soft valves, we demonstrate a constant-current system - where flow is independent of applied pressure – as well as devices with nearly arbitrary pressure-drop / flow rate relations. Applications to flow control in chemical reactors, cell culturing systems, and drug delivery are discussed. |
Sunday, November 18, 2018 8:26AM - 8:39AM |
A04.00003: Elastic deformation instability in soft microfluidic configurations induced by non-uniform electro-osmotic flow Evgeniy Boyko, Ran Eshel, Amir D. Gat, Moran Bercovici We study theoretically and experimentally the deformation instability of an elastic sheet separated from a rigid surface by a thin liquid film subjected to non-uniform electro-osmotic flow. We first provide insight into the physical behavior of the system by considering a simplified model, inspired by electrostatic MEMS actuators, in which the elastic sheet is modeled as a rigid plate connected to a linear spring. Our theoretical analysis, supported by experimental observations, reveals an instability that is controlled by a non-dimensional parameter representing the ratio of electro-osmotic to elastic forces, and also indicates the existence of hysteresis for the onset of instability. We consider both a constant voltage and constant current actuation modes, and expand our analysis for the case of an elastic sheet that is free to deform under bending and tension conditions, providing approximate asymptotic solutions validated by numerical simulations. |
Sunday, November 18, 2018 8:39AM - 8:52AM |
A04.00004: Motion of asymmetric objects through soft lubricated tubes Bhargav Rallabandi, Jens Eggers, Mary Caswell Stoddard, Howard A. Stone The motion of tightly fitting objects through soft narrow tubes is a scenario that commonly arises in physiological processes. One example is that of avian egg laying, where it has been observed across species that eggs move through the oviduct pointy-end first, even though they are often laid blunt-end first. We investigate the mechanistic implications of this observation by considering the motion of fore-aft asymmetric intruders moving through lubricated elastic tubes. Using asymptotic theory for small speeds, we find that the thickness of the lubricating fluid layer scales inversely with the square root of the slope of the intruder surface near its nose in the direction of motion. Consequently, the force required to drive motion grows with the square root of this slope, while also depending on the translation velocity, the elastic properties of the tube and the viscosity of the lubricant. Our findings show that asymmetric objects are more efficiently moved pointy-end-first through lubricated soft tubes, suggesting a mechanistic rationalization for the observed orientation of eggs moving in avian oviducts. |
Sunday, November 18, 2018 8:52AM - 9:05AM |
A04.00005: Wettability-independent droplet transport by Bendotaxis Alexander Bradley, Dominic J Vella, Finn Box, Ian J Hewitt When a drop is confined in a thin channel with deformable walls, a combination of bending and capillarity causes a pressure gradient that, in turn, results in the spontaneous movement of the liquid. Surprisingly, the direction of this motion, which we refer to as bendotaxis, is always the same, regardless of the wettability of the channel; bendotaxis may therefore be a useful means of transporting droplets on small scales, with various technological applications. This talk will present details of macroscopic experiments and a simple mathematical model used to study this motion, focussing in particular on the time scale associated with the motion, and we discuss the implications of these results. |
Sunday, November 18, 2018 9:05AM - 9:18AM |
A04.00006: To Leak or not to Leak through Holey Sheets Matteo Pezzulla, Lorenzo Siconolfi, Francois Gallaire, Pedro M 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, elastic object with a surrounding viscous fluid has received much less attention. Here, we use precision desktop experiments and finite element simulations to focus on the prototypical problem of a perforated elastic plate moving through a viscous fluid, at low Reynolds numbers. We seek to provide a predictive framework for the large 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. Moreover, we perform ad-hoc numerical simulations to capture the details of the fluid-structure interactions, highlighting the effect of permeability on the drag experienced by the structure. We hope that our findings may lead to a better understanding of the interactions between porous slender structures and viscous flows, across biological and technological applications. |
Sunday, November 18, 2018 9:18AM - 9:31AM |
A04.00007: Mode-switching of fingering instabilities in an elastic Hele-Shaw channel Callum Cuttle, Draga Pihler-Puzovic, Anne Juel We study the propagation of a curved front in a Hele-Shaw channel where the top boundary is an elastic membrane, and the channel is initially collapsed and filled with liquid – a benchtop model of pulmonary airway reopening. The injection of air at a constant flow rate works against elastic, viscous and capillary forces to form an approximately steadily propagating blistering finger, which depends on the level of initial collapse of the channel cross-section and the capillary number Ca – the ratio of viscous to surface tension forces. We find that the initial deflection of the membrane destabilises the finger into an oscillatory propagation mode, where small-scale fingering at the finger-tip is advected towards the rear of the finger as it advances. We present evidence of a switching transition between two oscillatory modes which are connected to the steady pushing and peeling regimes underlying airway reopening. One resembles the tip-splitting instabilities observed to occur sub-critically in rigid channels above a critical value of Ca, while the other is linked to stubby finger formation in tapered channels, e.g. tape peeling and the printer’s instability. |
Sunday, November 18, 2018 9:31AM - 9:44AM |
A04.00008: Stability and deformation of a wet elastic substrate Chris Boamah Mensah, Gregory Chini, Oliver Jensen Motivated by an application to pulmonary alveolar micro-mechanics, the stability and deformation of an elastic septum lined with liquid films is investigated. The normal force balance on the septum, which is modeled as an inertia-less, kinematically nonlinear Euler-Bernoulli beam, yields an equation for the septal curvature that is coupled to time dependent lubrication equations for the adjacent liquid films. Linear stability analysis of the (compressed) flat configuration reveals that the presence of the films simultaneously stabilizes the buckling instability of the dry septum while introducing a new mode of flexural instability. Numerical simulations complemented by asymptotic analysis of the coupled system are used to explore the nonlinear development of the instabilities. |
Sunday, November 18, 2018 9:44AM - 9:57AM |
A04.00009: Morphology models for cracking drying droplets Arandeep Uppal, Matthew Hennessy, Richard V Craster, Omar K Matar During the drying of complex fluids, such as polymer solutions or colloidal dispersions, a liquid phase is transformed into a solid. Compared to the case of a pure drop of volatile liquid, the presence of a particulate phase changes the evaporative process and leads to hydrodynamical and mechanical instabilities, sometimes resulting in cracking. The onset of cracking during thin-film deposition, photolithography, and colloidal assembly can ultimately lead to failure of the fabricated product. In contrast, cracking plays an advantageous role in a number of applications ranging from medical diagnostics to high-resolution nano-patterning. Cracking during the evaporative process is an area of ongoing study. We propose a hierarchy of models to capture a range of possible fracture behaviours. In our models, we account for a variety of phenomena, including compaction, evaporation, and substrate interactions. We show that the interplay between these physical mechanisms lead to a variety of crack patterns, which can be seen experimentally. |
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