Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session PK: Nano-fluids III |
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Chair: Xiangchun Xuan, Clemson University Room: 102B |
Tuesday, November 25, 2008 11:35AM - 11:48AM |
PK.00001: The decay of thermal capillary waves on thin liquid films Adam Willis, Jonathan Freund Thermal fluctuations are expected to excite capillary waves on free surfaces of liquids. For a liquid film on a solid wall, continuum models predict that waves of wavesnumber $q$ will decorrelate at rate $\omega$ that scales as $\omega \sim q^4$ for thin films (the lubrication limit with a no-slip boundary condition) and as $\omega \sim q$ for thick films (flow in a viscous half-space). Atomistic simulations of model polymeric fluids are employed to confirm these expected scalings and probe how this behavior fails as the atomic granularity of the fluid becomes important. These expected scalings are indeed found, but an unexpected $\omega \sim q^2$ powerlaw is also evident at shorter wavelengths than the $\omega \sim q$ region. For these same $q$ values, the capillary waves still seem to obey equipartition with energies defined simply by surface curvature, suggesting that there is not a complete failure of a continuum description of the fluid for these $q$. A $\omega \sim q^2$ would be expected for a slip boundary condition at the wall boundary, but no such slip is observed. Results for different polymer lengths collapse in this $q^2$ region when scaled with the radius of gyration of the polymer, suggesting that the anomaly is the result of a breakdown of the constitutive model. [Preview Abstract] |
Tuesday, November 25, 2008 11:48AM - 12:01PM |
PK.00002: Ionic separation in nanofluidic channels Xiangchun Xuan Ionic species of equal electrophoretic mobility (or charge-to-size ratio) may not be separated in electroosmotic or pressure-driven flow along microfluidic channels. In nanofluidic channels, however, the enormous electric fields inside electrical double layers cause transverse species distributions yielding charge-dependent species speeds in the flow. Those species of equal mobility can thus be separated solely by charge (or equivalently, size). Here we develop an analytical model to optimize and compare the separation of such ionic species in electroosmotic (termed nanochannel electrophoresis) and pressure-driven (termed nanochannel chromatography) flows along nanochannels in terms of selectivity, plate height and resolution. Both planar and cylindrical geometries are considered. It is found that nanochannel chromatography offers a larger selectivity (good) while a larger plate height (bad) than nanochannel electrophoresis does. The maximum resolution of ionic separation is therefore comparable between the two nanofluidic approaches. The optimal channel half-height or tube radius is found within the range of 1 to 10 times the Debye length. [Preview Abstract] |
Tuesday, November 25, 2008 12:01PM - 12:14PM |
PK.00003: The Study of Solvation Effects on Thermodynamic Properties of Nanofluids Using Molecular Dynamics Gianluca Puliti, Samuel Paolucci, Mihir Sen, Daniel Gezelter Liquid layering around nano-particles is proposed to be a major contributor in the surprisingly unpredictable properties of nanofluids. Equilibrium molecular dynamic simulations are presented for a water-based nanofluid with gold nano-particles. It makes use of state-of-the-art force fields to capture a broad spectrum of realistic physical phenomena. Thermodynamic properties, such as internal energy, heat capacity, enthalpy and entropy of the nanofluid are analyzed for different particle configurations. The understanding of basic thermodynamic effects in nanofluids is a stepping stone for further studies. [Preview Abstract] |
Tuesday, November 25, 2008 12:14PM - 12:27PM |
PK.00004: Equilibrium statistical mechanics of films on a substrate Antonio Pereira, Alexandr Malijevsky, Serafim Kalliadasis We investigate the small-scale behavior of a fluid (liquid or gas) film in contact with a substrate by using a density-functional-theory approach in the context of the description of the statics and dynamics of interphase boundaries. The fluid-fluid interaction potential is divided into a short-range repulsive component and a long-range attractive one. Different types of interaction potentials are considered as well as the influence of the wall potential onto the fluid density profile and a comparison with a gradient theory obtained from the density-functional approach is also made. Emphasis is then put on examining the case of a three-phase conjunction and the connection between the micro- and the meso-scale. [Preview Abstract] |
Tuesday, November 25, 2008 12:27PM - 12:40PM |
PK.00005: Biomolecular transport through hemofiltration Membranes A.T. Conlisk, Subhra Datta, William H. Fissell, Shuvo Roy A theoretical model for filtration of large solutes through a nanopore in the presence of transmembrane pressures, applied/induced electric fields, and dissimilar interactions at the entrance and exit to the nanopore is developed to characterize the experimental performance of a hemofiltration membrane designed for a proposed implantable Renal Assist Device (RAD). The model reveals that the sieving characteristics of the nanopore membrane can be improved by applying an external electric field, and ensuring a smaller ratio of the pore-feed and pore-permeate equilibrium partitioning coefficients when diffusion is present. The model is then customized to study filtration of both charged and uncharged solutes in the slit-shaped nanopores of the hemofilter for the RAD. Experimental data on the sieving coefficient of serum proteins are reported and compared with the theoretical predictions. Both steric and electrostatic partitioning are considered and the comparison suggests that in general electrostatic effects are present in the filtration of proteins though some data, particularly those recorded in a strongly hypertonic solution (10$\times$PBS), show better agreement with the steric partitioning theory. [Preview Abstract] |
Tuesday, November 25, 2008 12:40PM - 12:53PM |
PK.00006: Selection of Non-Equilibrium Over-Limiting Currents: Universal Depletion Layer Formation Dynamics and Vortex Instability Gilad Yossifon, Hsueh-Chia Chang We report the first direct experimental proof for Rubinstein's instability [1] by using an applied AC electric field across a straight nano-slot, whose transverse dimension is at least 10 times larger than the depletion layer, EO convective flow is completely arrested and ion transport is dominated by diffusion and electro-migration. The ion flux dynamics is imaged using fluorescent dye molecules in combination with confocal microscopy, to understand the non-equilibrium phenomenon of over-limiting current density across a nanoporous membrane. With a slow AC field, an ion depletion front is generated intermittently from one end of the nano-slot and a vortex instability is found to arrest the self-similar diffusive front growth. This electrokinetic instability evolves into a stationary interfacial vortex array that specifies the over-limiting current, independent of external stirring or convective flow. [1] I. Rubinstein, E. Staude and O. Kedem, Desalination 69, 101 (1988). [Preview Abstract] |
Tuesday, November 25, 2008 12:53PM - 1:06PM |
PK.00007: Rotational-Translational Coupling in Nanopores Sony Joseph, N.R. Aluru A rare combination of molecularly smooth walls and hydrophobicity of the surface make carbon nanotube (CNT) membranes fast transporters of water. Though bi-directional single file water transport in ``bursts'' through a (6,6) CNT and collective intermittent reversing of water dipolar orientations has been observed in molecular dynamics simulation for short tubes, the molecular mechanism governing the relation between the dipole orientation and the flow direction has not been elucidated. Here we show that when the orientation of the water molecules is maintained along one direction in longer tubes, a net water transport along that direction can be attained due to coupling between rotational and translational motions. The rotations of the water molecules are correlated more with the translation of the neighboring water molecule with the acceptor oxygen than the neighbor with the donor hydrogen. By applying an electric field or by attaching chemical functional groups at the tube ends, the orientations can be maintained and this mechanism of rotational-translational coupling can be used to pump confined water through nanotubes. [Preview Abstract] |
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