Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session L36: Nano Flows I |
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Chair: Ali Beskok, Southern Methodist University Room: Alcove A |
Monday, November 24, 2014 3:35PM - 3:48PM |
L36.00001: Ultra-sensitive flow measurement in nanopores through pressure-driven particle translocation Alessandro Siria, Alessandro Gadaleta, Anne-Laure Biance, Lyderic Bocquet The field of nanofluidics is of growing interest, both for applications and fundamental research. Nevertheless, this discipline still lacks a fundamental tool, i.e. the ability of measure the extremely small liquid flows in nanometric systems. This is especially aggravating, considering that one of the most interesting open problems in the field is the deviation of hydraulic permeability, in some systems from the values predicted by classical fluid mechanics. We propose a novel method for the measurement of pressure-driven flows in nanometric systems to characterize the translocation rate and dwell time of nanoparticles contained in a colloidal suspension. We are able to detect the passage of each nanoparticle across a nanopore by observing the sudden change in ionic current, and by analyzing the statistics of translocation events we can measure the permeability of the pore with high sensitivity and good accuracy. [Preview Abstract] |
Monday, November 24, 2014 3:48PM - 4:01PM |
L36.00002: Experimental investigation of flow and slip transition in nanochannels Zhigang Li, Long Li, Jingwen Mo Flow slip in nanochannels is sought in many applications, such as sea water desalination and molecular separation, because it can enhance fluid transport, which is essential in nanofluidic systems. Previous findings about the slip length for simple fluids at the nanoscale appear to be controversial. Some experiments and simulations showed that the slip length is independent of shear rate, which agrees with the prediction of classic slip theories. However, there is increasing work showing that slip length is shear rate dependent. In this work, we experimentally investigate the Poiseuille flows in nanochannels. It is found that the flow rate undergoes a transition between two linear regimes as the shear rate is varied. The transition indicates that the non-slip boundary condition is valid at low shear rate. When the shear rate is larger than a critical value, slip takes place and the slip length increases linearly with increasing shear rate before approaching a constant value. The results reported in this work can help advance the understanding of flow slip in nanochannels. [Preview Abstract] |
Monday, November 24, 2014 4:01PM - 4:14PM |
L36.00003: Droplets and the three-phase contact line at the nano-scale. Statics and dynamics Petr Yatsyshin, David Sibley, Nikos Savva, Serafim Kalliadasis Understanding the behaviour of the solid-liquid-vapour contact line at the scale of several tens of molecular diameters is important in wetting hydrodynamics with applications in micro- and nano-fluidics, including the design of lab-on-a-chip devices and surfaces with specific wetting properties. Due to the fluid inhomogeneity at the nano-scale, the application of continuum-mechanical approaches is limited, and a natural way to remedy this is to seek descriptions accounting for the non-local molecular-level interactions. Density Functional Theory (DFT) for fluids offers a statistical-mechanical framework based on expressing the free energy of the fluid-solid pair as a functional of the spatially varying fluid density. DFT allows us to investigate small drops deposited on planar substrates whilst keeping track of the microscopic structural details of the fluid. Starting from a model of intermolecular forces, we systematically obtain interfaces, surface tensions, and the microscopic contact angle. Using a dynamic extension of equilibrium DFT, we investigate the diffusion-driven evolution of the three-phase contact line to gain insight into the dynamic behaviour of the microscopic contact angle, which is still under debate. [Preview Abstract] |
Monday, November 24, 2014 4:14PM - 4:27PM |
L36.00004: Two phase flow of helium in single nanopipes Angel Velasco, Crystal Yang, Zuzanna Siwy, M.E. Toimil-Molares, Peter Taborek We report measurements of pressure driven flow of liquid helium entering vacuum through a single pipe of nanometer scale diameter. Nanopores were fabricated by etching a single ion track in either PET or mica. A calibrated mass spectrometer was used to measure the flow rates of liquid helium through pipes with diameter ranging from 80 nm to 31 nm. The liquid evaporates inside or near the exit of the nanopipe. The flow of helium was studied from 0.5 K to the lambda point (2.18 K) and from the lambda point to above the critical point (5.2 K). Flow rates were controlled by changing the pressure drop across the pipe in the range 0-5 Atm. When the pressure in the pipe reached the saturated vapor pressure, an abrupt flow transition was observed. For normal helium a viscous flow model accounting for interfacial forces is used to determine its position inside the pipe [1]. The observed mass flow rates are consistent with no slip boundary conditions. In superfluid the flow is essentially independent of the pressure drop with a maximum critical velocity of 11 m/s. The critical velocity has temperature dependence consistent with the homogeneous nucleation of vortices. \\[4pt] [1] A. E. Velasco, C. Yang, Z. S. Siwy, M. E. Toimil-Molares, and P. Taborek, Applied Physics Letters \textbf{105} (2014) [Preview Abstract] |
Monday, November 24, 2014 4:27PM - 4:40PM |
L36.00005: Gas Flows in Nano-Scale Confinements Ali Beskok, Murat Barisik Most studies on gas transport in nano-scale confinements assume dynamic similarity with rarefied gas flows, and employ kinetic theory based models. This approach is incomplete, since it neglects the van der Waals forces imposed on gas molecules by the surfaces. Using 3D molecular dynamics (MD) simulations of force driven gas flows, we show the significance of the wall force field in nano-scale confinements by defining a new dimensionless parameter ($B)$ as the ratio of the wall force-penetration length to the channel height. Investigation of gas transport in different nano-channels at various Knudsen numbers show the importance of wall force field for finite $B$ values, where the dynamic similarity between the rarefied and nano-scale gas flows break down. Molecularly structured walls determine the bulk flow physics by setting a proper tangential momentum accommodation coefficient, and also determine transport in the near wall region. Gas nano-flows with finite $B$ exhibit significant differences in the local density and velocity profiles, affecting the mass flow rate and the behavior of Knudsen's minimum in nano-channels. [Preview Abstract] |
Monday, November 24, 2014 4:40PM - 4:53PM |
L36.00006: Predicting the Anomalous Density of a Dense Fluid Confined within a Carbon Nanotube Gerald Wang, Nicolas Hadjiconstantinou The equilibrium density of fluids under nanoconfinement can be substantially smaller than their bulk density. Understanding the physical basis for and magnitude of these anomalous densities is key to many nanoengineering applications, such as constructing a sub-continuum model of nanoscale fluid flow. We provide here a theoretical description of this phenomenon in the most frequently, perhaps, studied system - a dense fluid confined within a carbon nanotube (CNT). We show that the reduced density is primarily due to repulsive interactions between the fluid and the CNT, which modify the fluid structure near the fluid-CNT interface and lead to a ``stand-off'' distance between the two materials. Using a mean-field approach to describe the energetic landscape near the CNT wall, we obtain closed-form analytical results describing the length scales associated with the layered fluid. Combined with empirical knowledge of the layered-fluid density, these results allow us to derive a prediction for the equilibrium fluid density as a function of the CNT radius that is in excellent agreement with molecular dynamics simulations. We also show how aspects of this theory can be extended to describe water confined within CNTs and find good agreement with results from the literature. [Preview Abstract] |
Monday, November 24, 2014 4:53PM - 5:06PM |
L36.00007: Molecular dynamics simulation of oxygen flows in graphene nanochannels Haruka Yasuoka, Ryo Takahama, Masayuki Kaneda, Kazuhiko Suga MD simulations are performed to investigate the flow characteristics of oxygen flows in graphene nanochannels. For comparison, flows of argon molecules which have relatively similar values of mass and diameter are also simulated. The L-J potential is used for the fluid-fluid interaction and the wall-fluid interaction. For the bond of the carbon molecules for the channel walls, the Brenner potential is used. For all the cases, the normalized number density, pressure and temperature are set as $\rho$ = 0.2, P = 0.4 T = 2.0, Two channel height cases H = 20$\sigma$ and H = 50$\sigma$, where $\sigma$ is the argon molecule diameter, are considered. In those conditions, Knudsen numbers are estimated to be about 0.056 and 0.023. In both channel height cases, it is found that the oxygen flow rates are larger than those of the argon flows even though acceleration acting on fluid molecules is constant. This is because the wall-fluid interaction between oxygen and carbon molecules is weaker than that of argon flow cases. It is found that the normalized velocity profiles are indifferent of the fluid molecules. Therefore, it can be said that the diatomic molecular structure of the fluid molecules does not have significant effects on the flow characteristics in the graphene nanochannels. [Preview Abstract] |
Monday, November 24, 2014 5:06PM - 5:19PM |
L36.00008: Shear Viscous Response of Molecularly Thin Liquid Films Charles Tschirhart, Sandra Troian Fluids that exhibit Newtonian response at the macroscale can display interesting deviations at the nanoscale caused by internal fluid microstructure or conformational entropy reduction near an adjacent solid boundary. Such deviations signal the breakdown of the continuum and isotropic fluid approximations at molecular length scales. These effects are particularly pronounced near the interface separating a liquid film from a supporting solid substrate where molecular layering in the fluid can result in inhomogeneity in the shear viscosity. Here we describe ellipsometric measurements of the surface deformation of non-volatile liquid nanofilms subject to a constant interfacial shear stress. For simple Newtonian response, the slope of the deformation can be used to extract the value of the shear viscosity in ultrathin films, which in our experiments range from 2 - 200 nm in thickness. For complex films, we observe deviations from linear deformation which require augmentation of the analytic model normally used to describe the viscous response. These findings may be helpful for improved parametrization of the shear response of supported free surface films as well as course grained models for nanofluidic applications. [Preview Abstract] |
Monday, November 24, 2014 5:19PM - 5:32PM |
L36.00009: Measurement of velocity distribution of fluid flows in nanochannel using evanescent wave-based particle velocimetry Yutaka Kazoe, Yojirou Hiramatsu, Kazuma Mawatari, Takehiko Kitamori The field of nanofluidics for single molecule analysis, ultra filtration and energy conversion has been expanded with recent micro- and nanotechnology. Since liquids in nanospace with dominant surface effects are in a transitional regime from single molecules to continuum, specific fluid properties different from bulk can be expected. Previously, our group has revealed unique properties in size-regulated 10-1000 nm spaces such as higher viscosity, lower dielectric constant and higher proton mobility. However, fluid flows in the nanochannel are still unknown owing to lack of measurement method because nanochannel is smaller than light wavelength. For breaking through the limitation, evanescent wave light, which exponentially penetrates from the surface within 100 nm-order distance, is a key optical phenomenon. In this study, we developed evanescent wave-based particle tracking method for measuring flow profile in nanochannel. 10 nm-order fluorescent tracer materials were used in the measurements, and the position of tracer in the nanochannel was estimated from the brightness. The method was demonstrated in measurements of pressure driven flows in a nanochannel. [Preview Abstract] |
Monday, November 24, 2014 5:32PM - 5:45PM |
L36.00010: Continuum Simulations of Water Flow in Carbon Nanotube Membranes J.H. Walther, A. Popadic, P. Koumoutsakos, M. Praprotnik We propose the use of the Navier-Stokes equations subject to partial-slip boundary conditions to simulate water flows in Carbon NanoTube (CNT) membranes. The finite volume discretisations of the Navier-Stokes equations are combined with slip lengths extracted from Molecular Dynamics (MD) simulations to predict the pressure losses at the CNT entrance as well as the enhancement of the flow rate in the CNT. The flow quantities calculated from the present hybrid approach are in excellent agreement with pure MD results while they are obtained at a fraction of the computational cost. The method enables simulations of system sizes and times well beyond the present capabilities of MD simulations. Our simulations provide an asymptotic flow rate enhancement and indicate that the pressure losses at the CNT ends can be reduced by reducing their curvature. More importantly, our results suggest that flows at nanoscale channels can be described by continuum solvers with proper boundary conditions that reflect the molecular interactions of the liquid with the walls of the nanochannel. [Preview Abstract] |
Monday, November 24, 2014 5:45PM - 5:58PM |
L36.00011: Early regimes of water imbibition in nanoslit silica channels Elton Oyarzua, Harvey Zambrano, Jens Honore Walther, Andres Mejia Capillarity is currently subject to a significant research interest. Attention is mainly paid to the late stage of the imbibition when a developed flow is reached and the Laplace pressure is balanced by the viscosity. Nevertheless, as the miniaturization of devices is reaching the nanoscale a thorough understanding of fluid flow in nanoconfinement is required. In nanofluidics, short timescales and surface characteristics dominate the flows. In this study, molecular simulations are conducted to investigate the early stage of water imbibition in silica nanochannels with heights of 4 to 10 nm. Results indicate that nanoscale imbibition is divided in three regimes. An initial regime with imbibition linearly dependent of time, where the capillary force is mainly balanced by inertia. Thereafter, a period, in which, the balance has contributions from both inertia and viscosity and, subsequently, a final regime, wherein, viscosity dominates the capillary force balance. Velocity profiles confirm the passage from an inviscid flow to a developed Poisseuille flow. The meniscus position as a function of time and air accumulation in front of the advancing meniscus are computed for different air pressures, the results reveal a systematic retarding effect of gas pressurization on the imbibition. [Preview Abstract] |
Monday, November 24, 2014 5:58PM - 6:11PM |
L36.00012: ABSTRACT WITHDRAWN |
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