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 R6: Nanofluids III |
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Chair: John Shelton, Carnegie Mellon University Room: 328 |
Tuesday, November 26, 2013 1:05PM - 1:18PM |
R6.00001: Towards realistic multiscale molecular-continuum modelling of water flow through nanotube membranes Konstantinos Ritos, Matthew Borg, Duncan Lockerby, Spela Ivekovic, Yonghao Zhang, Jason Reese We present a new hybrid molecular-continuum method for modelling the nano flows inside micrometer-thick membranes. Our aim is to obtain results for practical filtration membranes that are as accurate as molecular dynamics (MD) and at the same time significantly more computationally efficient. Computational savings are obtained by replacing long nanotube sections - that are highly scale-separated - by much smaller but representative MD simulations, without any substantial loss of accuracy. These individual MD simulations are coupled together via standard continuum fluid-dynamics equations that dictate the overall macroscopic flow in the membrane. For this specific problem we use the conservative continuity and momentum equations as we consider the flow isothermal, incompressible and low-speed. Our iterative algorithm computes at each iteration the new constraints on the pressure differences applied to individual micro elements, and enforces overall continuity within the membrane. Validation tests are through direct comparison with full MD simulations of 50 and 150 nm thick membranes. We show results for micrometer-thick membranes and compare our predictions with previously-published experimental data. [Preview Abstract] |
Tuesday, November 26, 2013 1:18PM - 1:31PM |
R6.00002: Molecular dynamics study of instability of nano-liquid column Takeru Yano, Taichi Murakami Molecular dynamics study of argon nano-liquid column in equilibrium with surrounding its vapor is executed to investigate the instability for axisymmetric disturbances. As a result of careful and accurate calculation for nano-liquid columns of radius 1 to 4~nm, we can demonstrate that the classical criterion of stability limit $L_z=2\pi R$ holds for the nano-columns with the appropriate definition of the column radius $R$ for columns of length $L_z$. [Preview Abstract] |
Tuesday, November 26, 2013 1:31PM - 1:44PM |
R6.00003: Study of contact line motion in two phase flow using molecular dynamics Joseph Thalakkottor, Kamran Mohseni Contact line motion is an age old problem. The scale in which the continuum assumption breaks down resulting in the motion of contact line makes it difficult to analyze the problem. Using molecular dynamic simulations we intend to investigate the cause and effects of contact line motion. The results indicates that miscibility between the two fluids, hydrophobicity between fluid and wall, and shear rate of the fluid are some of the key parameters that determine the amount of slip at the triple contact point. Circulation inside a droplet is observed even at nanoscales and is seen to vary inversely with slip length. We also observe non-Newtonian behavior of fluid in the vicinity of the triple contact point. Understanding the affects of these phenomena on contact line motion would help in better understanding the movement of triple contact point in two phase flow. This information would aide in developing a slip model for the triple contact point. [Preview Abstract] |
Tuesday, November 26, 2013 1:44PM - 1:57PM |
R6.00004: CO$_2$ separation using a porous graphene/IL membrane Joonho Lee, Narayana R. Aluru We investigated the separation of CO$_2$ from CO$_2$/O$_2$ mixture using a porous graphene/ionic liquid (IL) membrane. By performing extensive non-equilibrium molecular dynamics (NEMD) simulations, we observed dramatic CO$_2$ separation with a 0.99 nm graphene nanopore. By calculating the density distribution, we show that a strong layering of the ionic liquid is observed near the 0.99 nm graphene nanopore, while such a strong layering is not observed in the larger 2.22 nm diameter pore. The strong layering induces a near perfect blockage of O$_2$ molecules from diffusing into the ionic liquid. Void analysis shows that the layering of ionic liquid serving as a blockage for O$_2$ molecules does not hinder CO$_2$ solvation in the ionic liquid. [Preview Abstract] |
Tuesday, November 26, 2013 1:57PM - 2:10PM |
R6.00005: Effect of air on water capillary flow in silica nanochannels Harvey Zambrano, Jens Walther, Elton Oyarzua Capillarity is a classical topic in fluid dynamics. The fundamental relationship between capillarity and surface tension is solidly established. Nevertheless, capillarity is an active research area especially as the miniaturization of devices is reaching the molecular scale. Currently, with the fabrication of microsystems integrated by nanochannels, a thorough understanding of the transport of fluids in nanoconfinement is required for a successful operation of the functional parts of such devices. In this work, Molecular Dynamics simulations are conducted to study the spontaneous imbibition of water in sub 10 nm silica channels. The capillary filling speed is computed in channels subjected to different air pressures. In order to describe the interactions between the species, an effective force field is developed, which is calibrated by reproducing the water contact angle. The results show that the capillary filling speed qualitatively follows the classical Washburn model, however, quantitatively it is lower than expected. Furthermore, it is observed that the deviations increase as air pressure is higher. We attribute the deviations to amounts of air trapped at the silica-water interface which leads to changes in the dynamics contact angle of the water meniscus. [Preview Abstract] |
Tuesday, November 26, 2013 2:10PM - 2:23PM |
R6.00006: Dissimilar viscosity induced sample pre-concentration in elecrokinetic nanofluidic channels Dean Wink, Elijah Shelton, Sumita Pennathur, Brian Storey Nanofluidic analysis systems boast many advantages: portability, small sample handling, short processing times, and potential for integration with mobile electronics. However, such systems face the challenge of detecting increasingly small volumes of sample at low concentrations. In this work, we demonstrate a unique pre-concentration technique in electrokinetic nanofluidic systems based on a viscosity mismatch between two fluids.~ In nanofluidic electrokinetic systems, finite electric double layers (EDL) lead to non-uniform electric potentials and transverse concentration distributions. Therefore, when the EDL is comparable in size to the channel height, negatively charged ions are repelled from negatively charged walls and preferentially populate the channel centerline. Furthermore, an axial piecewise viscosity distribution induces internal pressure gradients within the channel. These force the ions to move at a different average velocities based on the pressure gradient being favorable or adverse, leading to focusing. To experimentally probe this phenomenon, we electrokinetically inject solutions of borate buffer with and without glycerol (to change the viscosity) and use a fluorescent tracer dye to visualize the flow. We perform the injections in cross-geometry channels of 20 micron, 1 micron, and 250 nanometer depths. We measure fluorescence at 5, 10 and 15 mm distances from junction. Enhancement is characterized by comparing intensities to control measurements for systems with uniform viscosity. [Preview Abstract] |
Tuesday, November 26, 2013 2:23PM - 2:36PM |
R6.00007: Overlimiting current through ion concentration polarization layer: Experimental verifications of the surface conduction mechanism Sungmin Nam, Joonseong Heo, Geunbae Lim, Sung Jae Kim The mechanism of overlimiting current which observed in most practical electrochemical membrane systems has become a critical issue that numerous researchers have tried to resolve. Overlimiting current is closely related to the ion concentration polarization phenomenon which represents an imbalance of ion concentrations nearby the membrane. Among a number of studies, a recent theoretical study suggested that surface conduction is the core mechanism, while an electro-convective flow and water dissociation has been regarded as the major mechanism of overlimiting current. In this presentation, we provide rigorous experimental evidences of the role of surface conduction using a micro/nanofluidic platform. Conclusively the surface conduction enhances the overlimiting current characteristics, while the electro-convection retards the initiation of overlimiting current. Thus, this result can contribute to the advances for both understanding a fundamental electrokinetic theory and engineering applications based on the ion concentration polarization phenomena. [Preview Abstract] |
Tuesday, November 26, 2013 2:36PM - 2:49PM |
R6.00008: A Landau-Squire Nanojet Sandip Ghosal, Nadanai Laohakunakorn, Benjamin Gollnick, Fernando Moreno-Herrero, Dirk G.A.L. Aarts, Roel P.A. Dullens, Ulrich F. Keyser Fluid jets are found in nature at all length scales -- microscopic to cosmological. Here we report on what may be the smallest liquid jet ever observed: an electroosmotically driven flow from a single glass nanopore about 75 nm in radius with a maximum flow rate of about 30 pL/s. A novel anemometry technique allows us to map out the vorticity and velocity fields which show excellent agreement with the classical Landau-Squire solution of the Navier Stokes equations for a point jet. We observe a phenomenon that we call flow rectification: an asymmetry in the flow rate with respect to voltage reversal. Such a nanojet could potentially find applications in gene delivery, nano patterning, and as a diode in microfluidic circuits. [Preview Abstract] |
Tuesday, November 26, 2013 2:49PM - 3:02PM |
R6.00009: ABSTRACT WITHDRAWN |
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