Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session G9: Nanoscale Flows: Basic Flow Physics |
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Chair: J Walther, Technical University of Denmark Room: 109 |
Monday, November 23, 2015 8:00AM - 8:13AM |
G9.00001: Continuum Navier-Stokes modelling of water flow past fullerene molecules J. H. Walther, A. Popadic, P. Koumoutsakos, M. Praprotnik We present continuum simulations of water flow past fullerene molecules. The governing Navier-Stokes equations are complemented with the Navier slip boundary condition with a slip length that is extracted from related molecular dynamics simulations. We find that several quantities of interest as computed by the present model are in good agreement with results from atomistic and atomistic-continuum simulations at a fraction of the computational cost. We simulate the flow past a single fullerene and an array of fullerenes and demonstrate that such nanoscale flows can be computed efficiently by continuum flow solvers, allowing for investigations into spatiotemporal scales inaccessible to atomistic simulations. [Preview Abstract] |
Monday, November 23, 2015 8:13AM - 8:26AM |
G9.00002: Nanofluidic Brownian Ratchet via atomically-stepped surfaces Amir Rahmani, Carlos Colosqui Theoretical analysis and fully atomistic molecular dynamics simulations reveal a Brownian ratchet mechanism by which thermal motion can drive the directional displacement of liquids confined in micro- or nanoscale channels and pores. The particular systems discussed in this talk consist of two immiscible liquids confined in a slit-like nanochannel with atomically-stepped surfaces. Mean displacement rates reported in molecular dynamics simulations are in close agreement with theoretical predictions via analytical solution of a Smoluchowski equation for the probability density of the position of the liquid-liquid interface. The direction of the thermally-driven displacement of liquid is determined by the nanostructure surface geometry and thus imbibition or drainage can occur against the direction of action of capillary forces. The studied surface nanostructure with directional asymmetry can control the dynamics of wetting processes such as capillary filling, wicking, and imbibition in porous materials. The proposed physical mechanisms and derived analytical expressions can be applied to design nanofluidic and microfluidic devices for passive handling and separation. [Preview Abstract] |
Monday, November 23, 2015 8:26AM - 8:39AM |
G9.00003: Modeling anomalous diffusion of dense fluids in carbon nanotubes Gerald Wang, Nicolas Hadjiconstantinou Molecular diffusive mechanisms exhibited under nanoconfinement can differ considerably from the Fickian self-diffusion expected in a bulk fluid. We propose a theoretical description of this phenomenon in a nanofluidic system of considerable interest - namely, a dense fluid confined within a carbon nanotube (CNT). We show that the anomalous diffusion reported in the literature is closely related to the fluid layering widely observed in this system and recently theoretically described [Wang and Hadjiconstantinou, Physics of Fluids, 052006, 2015]. In particular, we find that the key to describing the anomalous molecular diffusion (within sufficiently large CNTs) lies in recognizing that the diffusion mechanism is spatially dependent: while fluid in the center of the nanotube (at least three molecular diameters away from the wall) exhibits Fickian diffusion, fluid near the CNT wall can demonstrate non-Fickian diffusive behavior. The previously reported anomalous diffusive behavior can be reproduced, to a good approximation level, by appropriately combining the bulk and near-wall behavior to form a model for the {\it overall} diffusion rate within the nanotube. Such models produce results in quantitative agreement with molecular dynamics simulations. [Preview Abstract] |
Monday, November 23, 2015 8:39AM - 8:52AM |
G9.00004: Water transport in graphene nano-channels Enrique Wagemann, Elton Oyarzua, J. H. Walther, Harvey Zambrano The transport of water in nanopores is of both fundamental and practical interest. Graphene Channels (GCs) are potential building blocks for nanofluidic devices due to their molecularly smooth walls and exceptional mechanical properties. Numerous studies have found a significant flow rate enhancement, defined as the ratio of the computed flow rate to that predicted from the classical Poiseuille model. Moreover, these studies point to the fact that the flow enhancement is a function of channel height and the fluid-wall physical-chemistry. In spite of the intensive research, an explicit relation between the chirality of the graphene walls and the slip length has not been established. In this study, we perform non-equilibrium molecular dynamics simulations of water flow in single- and multi-walled GCs. We examine the influence on the flow rates of dissipating the viscous heat produced by connecting the thermostat to the water molecules, the CNT wall atoms or both of them. From the atomic trajectories, we compute the fluid flow rates in GCs with zig-zag and armchair walls, heights from 1 to 4 nm and different number of graphene layers on the walls. A relation between the chirality, slip length, and flow enhancement is found. [Preview Abstract] |
Monday, November 23, 2015 8:52AM - 9:05AM |
G9.00005: Flow enhancement of water flow through silica slit pores with graphene-coated walls Harvey Zambrano, Enrique Wagemann, Elton Oyarzua, J. H. Walther Nanofluidic devices such as Lab-On-a-Chip often are designed to transport water solutions through hydrophilic nano-conduits. In these systems with narrow confinement, the viscous forces dominate the flow and as a result, the hydrodynamic friction drag is very high. Moreover, the drag and the amount of energy required for pumping a fluid are directly related. Therefore, it is desirable to explore drag reduction strategies in nanoconfined flows. Liquids are known to slip past non-wetting surfaces. Graphene is a single-atom-thick sheet of carbon atoms arranged in a hexagonal honeycomb lattice, which features a unparalleled combination of high specific surface area, chemical stability, mechanical strength and flexibility. Recently, the wettability of water droplets on multilayer graphene sheets deposited on a silica substrate has been investigated. In this study, we investigate the role of graphene coatings to induce flow enhancement in silica channels. We conduct molecular dynamics simulations of pressurized water flow inside silica channels with and without graphene layers covering the walls. In particular, we compute density and velocity profiles, flow enhancement and slip lengths to understand the drag reduction capabilities of multilayer graphene coatings. [Preview Abstract] |
Monday, November 23, 2015 9:05AM - 9:18AM |
G9.00006: Experimental Study of Water Transport through Hydrophilic Nanochannels Mohammad Amin Alibakhshi, Quan Xie, Yinxiao Li, Chuanhua Duan In this paper, we investigate one of the fundamental aspects of Nanofluidics, which is the experimental study of water transport through nanoscale hydrophilic conduits. A new method based on spontaneous filling and a novel hybrid nanochannel design is developed to measure the pure mass flow resistance of single nanofluidic channels/tubes. This method does not require any pressure and flow sensors and also does not rely on any theoretical estimations, holding the potential to be standards for nanofluidic flow characterization. We have used this method to measure the pure mass flow resistance of single 2-D hydrophilic silica nanochannels with heights down to 7 nm. Our experimental results quantify the increased mass flow resistance as a function of nanochannel height, showing a 45{\%} increase for a 7nm channel compared with classical hydrodynamics, and suggest that the increased resistance is possibly due to formation of a 7-angstrom-thick stagnant hydration layer on the hydrophilic surfaces. It has been further shown that this method can reliably measure a wide range of pure mass flow resistances of nanoscale conduits, and thus is promising for advancing studies of liquid transport in hydrophobic graphene nanochannels, CNTs, as well as nanoporous media. [Preview Abstract] |
Monday, November 23, 2015 9:18AM - 9:31AM |
G9.00007: Kinetic Limited Water Evaporation in Hydrophilic Nanofluidic Channels Yinxiao Li, Mohammad Amin Alibakhshi, Quan Xie, Chuanhua Duan Capillary evaporation is one of the most efficient approaches for heat and mass transfer, but the interfacial resistance in capillary evaporation governed by the kinetic theory has remained poorly understood. Here we report experimental studies of the kinetic-limited water capillary evaporation in 2-D hydrophilic nanochannels. A novel hybrid nanochannel design is employed to guarantee sufficient water supply to the liquid/vapor evaporation interface and to enable precise evaporation rate measurements. We study the effects of confinement (16 $\sim$ 105nm), temperature (20 $\sim$ 40 $^{\circ}$C), and relative humidity (0{\%} $\sim$ 60{\%}) on the evaporation rate and the evaporation coefficient. A maximum evaporation flux of 21287 micron/s is obtained in 16-nm nanochannels at 40$^{\circ}$C and RH$=$0{\%}, which corresponds to a heat flux of 4804 W/cm$^{\circ}$. The evaporation coefficient is found to be independent on geometrical confinement, but shows a clear dependence on temperature, decreasing from 0.55 at 20$^{\circ}$C to 0.5 at 40 $^{\circ}$C. These findings have implications for understanding heat and mass transport in nanofluidic devices and porous media, and shed light on further development of evaporation-based technologies for thermal management, membrane purification and lab-on-a-chip devices. [Preview Abstract] |
Monday, November 23, 2015 9:31AM - 9:44AM |
G9.00008: Insights from plants: tunable nano-flows induced by drying Olivier Vincent, Antoine Robin, Alexandre Szenicer, Abraham Stroock Moving fluids through nanoscale confinements is a difficult process due to high friction with the walls. Pushing fluids to achieve significant (or even measurable) flows requires very large pressures, which can be inconvenient and costly. Inspired by plants, we used evaporation to generate controlled steady-state nano-flows in pores $\sim 3$ nm in diameter embedded in a silicon-based micro-platform. The capillary negative pressure that drives the flow, on the order of tens to hundreds of MPa in magnitude, develops spontaneously upon drying and can be externally tuned by changing the relative humidity (vapor saturation) outside of the sample. We show that the analysis of the dynamic drying response allows to get precise measurements of the behavior of highly confined liquids and could be used both as tool for the study of nanoscale fluid physics and as a method to handle liquids in a controlled way for lab-on-chip applications. We also discuss flow enhancement possibilities based on ideas from the vascular anatomy of plants. [Preview Abstract] |
Monday, November 23, 2015 9:44AM - 9:57AM |
G9.00009: Landau-Squire jet as a versatile probe to measure flow rate through individual nanochannel and nanotubes Eleonora Secchi, Sophie Marbach, Alessandro Siria, Lyderic Bocquet Over the last decade, nanometric sized channels have been intensively investigated since new model of fluid transport are expected due to the flow confinement at the nanometric scale. Nanoconfinement generates new phenomena, such as superfast flows in carbon nanotubes and slippage over smooth surfaces. However, a major challenge of nanofluidics lies in fabricating nanoscale fluidic devices and developing new velocimetry techniques able to measure flow rates down to femtoL/s. In this work we report the experimental study of the velocity fields generated by pressure driven flow from glass nanochannel with a diameter ranging from 1$\mu $m to 100nm. The flow emerging from these channels can be described by the classical Landau-Squire solution of the Navier-Stokes equation for a point jet. We show that due to the peculiarity of this flow, it can be used as an efficient probe to characterize the permeability of nanochannels. Velocity field is measured experimentally seeding the fluid in the reservoir with 500 nm Polystyrene particles and measuring the velocity with a standard PIV algorithm. Predictions are tested for nanochannels of several dimensions and supported by ionic current measurement. This demonstrates that this technique is a powerful tool to characterize the flow through nanochannels. We finally apply this method to the measurement of the flow emerging from a single carbon nanotube inserted in the nanochannels and present first data of permeability measurement through a single nanotube. [Preview Abstract] |
Monday, November 23, 2015 9:57AM - 10:10AM |
G9.00010: Second-order Knudsen-layer analysis for the generalized slip-flow theory: Boundary curvature effects Masanari Hattori, Shigeru Takata A systematic asymptotic analysis of the Boltzmann equation shows that the overall behavior of a gas can be described by fluid-dynamic-type equations with the appropriate slip/jump boundary condition when the Knudsen number is small [the generalized slip-flow theory; see Y. Sone, {\it Molecular Gas Dynamics} (Birkh\"{a}user, Boston, 2007)]. Near the boundary, a non-fluid-like correction (the Knudsen-layer correction) to the overall solution is required. Although the theory itself has been established up to the second order of the Knudsen number expansion, the data of the correction have been lacking for a long time for the original Boltzmann equation. Recently, we have obtained the required data, except for the effects of boundary curvature, assuming the hard-sphere molecules and the diffuse reflection boundary condition. In the present work, the effects of boundary curvature have been clarified in details, thereby completing the required numerical data. A local singularity appears at the level of the velocity distribution function. We have developed the numerical method that handles such a singularity safely. [Preview Abstract] |
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