Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session E7: Microfluids: Porous Media |
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Chair: Ho-Young Kim, Seoul National University Room: 329 |
Sunday, November 24, 2013 4:45PM - 4:58PM |
E7.00001: Regimes of gas transport through macroscopic areas of multi-layer graphene Michael Boutilier, Rohit Karnik, Chengzhen Sun, Nicolas Hadjiconstantinou Nanoporous graphene membranes have the potential to surpass the permeance and selectivity limits of current gas separation membranes. Recent experiments and simulations on individual graphene nanopores have demonstrated that molecule-size-selective nanopores can be created and used to separate components of a gas mixture. However, micrometer-scale tears and nanometer-scale intrinsic defects, inherently present in macroscopic areas of graphene, can severely limit the gas separation performance of graphene membranes of practical size. In this study, we measure the inherent permeance of macroscopic, multi-layer graphene membranes to various gases. A model for the transport of gases through these membranes is developed and shown to accurately explain the measured flow rates. The results quantify the separate contributions of tears and intrinsic defects to the inherent permeance of macroscopic areas of multi-layer graphene. The model is then extended to graphene membranes with engineered selective nanopores to optimize design parameters for defect-tolerant gas separation membranes. [Preview Abstract] |
Sunday, November 24, 2013 4:58PM - 5:11PM |
E7.00002: Molecule permeation and gas separation by nanoporous graphene membranes Nicolas Hadjiconstantinou, Chengzhen Sun, Michael Boutilier, Rohit Karnik Molecular simulations and experiments suggest that by introducing nanopores of appropriate size, nanoporous graphene membranes can exhibit permeability and selectivity exceeding those of existing state-of-the-art membranes by several orders of magnitude. To better understand how gases permeate through these membranes, we conducted molecular dynamics simulations of gas permeation through different nanopores for four different gases, namely helium, hydrogen, nitrogen and methane. Our results show that in addition to the direct flux, defined as the contribution from molecules crossing directly from the gas-phase on one side of the graphene to the other, in some gases, significant contribution to the flux across the membrane comes from a surface mechanism, in which the molecules cross after being adsorbed onto the graphene surface. Our results quantify the relative contribution of the bulk and surface mechanisms and show that the direct flux can be described reasonably accurately using kinetic gas theory, provided the latter is appropriately modified to account for finite-molecule-size effects by assuming steric interactions between rigid pores and hard-sphere gas molecules of known kinetic diameters. [Preview Abstract] |
Sunday, November 24, 2013 5:11PM - 5:24PM |
E7.00003: Dehydration induced phase transitions in a microfluidic droplet array for the separation of biomolecules Chris Nelson, Shelley Anna Droplet-based strategies for fluid manipulation have seen significant application in microfluidics due to their ability to compartmentalize solutions and facilitate highly parallelized reactions. Functioning as micro-scale reaction vessels, droplets have been used to study protein crystallization, enzyme kinetics, and to encapsulate whole cells. Recently, the mass transport out of droplets has been used to concentrate solutions and induce phase transitions. Here, we show that droplets trapped in a microfluidic array will spontaneously dehydrate over the course of several hours. By loading these devices with an initially dilute aqueous polymer solution, we use this slow dehydration to observe phase transitions and the evolution of droplet morphology in hundreds of droplets simultaneously. As an example, we trap and dehydrate droplets of a model aqueous two-phase system consisting of polyethylene glycol and dextran. Initially the drops are homogenous, then after some time the polymer concentration reaches a critical point and two phases form. As water continues to leave the system, the drops transition from a microemulsion of DEX in PEG to a core-shell configuration. Eventually, changes in interfacial tension, driven by dehydration, cause the DEX core to completely de-wet from the PEG shell. Since aqueous two phase systems are able to selectively separate a variety of biomolecules, this core shedding behavior has the potential to provide selective, on-chip separation and concentration. [Preview Abstract] |
Sunday, November 24, 2013 5:24PM - 5:37PM |
E7.00004: Buckling of a colloid-armored bubble Nicolas Taccoen, Deniz Z. Gunes, Charles N. Baroud We investigate the dissolution of a single air-in-water bubble whose surface is coated with solid particles, as an elementary model of an aging particle-stabilized foam. A microfluidic setup is used to produce a single bubble on demand, force the adsorption of particles to its interface, and hold it stationnary for long-term observation. When the gas dissolves in the surrounding liquid, the particles on the interface eventually jam, thus forming a rigid shell that encloses the bubble. As the temperature and pressure conditons are varied, this armor can either arrest the dissolution of the gas or it can buckle, which leads to the complete disapearance of the bubble. We experimentally demonstrate the existence of a threshold pressure above which the shell is not resistant enough to stabilize the bubble. This is modeled by comparing the mechanical resistance of the hollow shell with the compressive stress due to the dissolution in the liquid, which is controlled through the thermodynamic parameters. These experiments yield the first quantitative measurements of the mechanical resistance of a colloidal shell against ripening. It opens the possibility to study the behavior of more complex armors, by varying the size distribution, the shape and the chemistry of the particles. [Preview Abstract] |
Sunday, November 24, 2013 5:37PM - 5:50PM |
E7.00005: Does the hourglass shape of aquaporins optimize water permeability? Simon Gravelle, Laurent Joly, Fran\c{c}ois Detcheverry, Christophe Ybert, Cecile Cottin-Bizonne, Lyderic Bocquet The ubiquitous aquaporin channels are able to conduct water across cell membranes, combining the seemingly antagonist functions of a very high selectivity with a remarkable permeability. While molecular details are obvious keys to perform these tasks, the overall efficiency of transport in such nanopores is also strongly limited by viscous dissipation arising at the connection between the nanoconstriction and the nearby bulk reservoirs. In this contribution, we focus on these so-called entrance effects and specifically examine whether the characteristic hourglass shape of aquaporins may arise from a geometrical optimum for such hydrodynamic dissipation. Using a combination of finite element calculations and analytical modeling, we show that conical entrances with suitable opening angle can indeed provide a large increase of the overall channel permeability. Moreover, the optimal opening angles that maximize the permeability are found to compare well with the angles measured in a large variety of aquaporins. This suggests that the hourglass shape of aquaporins could be the result of a natural selection process toward optimal hydrodynamic transport. Finally, in a biomimetic perspective, these results provide guidelines to design artificial nanopores with optimal performances. [Preview Abstract] |
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