Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session MG: Nano-Fluids II |
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Chair: Joel Koplik, City College of New York Room: 101G |
Tuesday, November 24, 2009 8:00AM - 8:13AM |
MG.00001: A nonlinear equation for ionic diffusion in a strong binary electrolyte Sandip Ghosal, Zhen Chen The problem of the one dimensional electro-diffusion of ions in a strong binary electrolyte is considered. The mathematical description consists of a diffusion equation for each species augmented by transport due to a self consistent electrostatic field determined by the Poisson equation. If concentrations do not vary appreciably over distances of the order of the Debye length, the Poisson equation can be replaced by the condition of local charge neutrality first introduced by Planck. It can then be shown that both species diffuse at the same ``ambipolar'' rate with a common diffusivity.Here we derive a more general theory by exploiting the ratio of Debye length to a characteristic length scale as a small asymptotic parameter. It is shown that the concentration of either species may be described by a nonlinear integro-differential equation which replaces the classical linear equation for ambipolar diffusion but reduces to it in the appropriate limit. Through numerical integration of the full set of equations it is shown that this nonlinear equation provides a better approximation to the exact solution than the linear equation it replaces. [Preview Abstract] |
Tuesday, November 24, 2009 8:13AM - 8:26AM |
MG.00002: Porescale transport phenomena in charge-selective hemofilters Subhra Datta, Albert Conlisk Theoretical models for hindered transport of biomolecules and electrostatic and electrokinetic phenomena in the pressure driven flow of blood simulants in structured nanoporous membranes are developed, motivated by the design requirements for a hemofilter for an implantable artificial kidney. In particular, the selectivity of charged membrane to charged biomolecules of biological interest, the inference of the pore wall surface charge density from streaming potential measurements, when electrical double layers overlap and the pore wall surface charge density is heterogeneous (e.g. due to nonuniformities in the applied surface coatings) and the coupling of intrapore phenomena with mass transfer and fluid flow upstream and downstream of the membrane are discussed. The developed theory is applied to the problem of choosing a hemofilter pore size that provides adequate retention/clearance of desirable/undesirable solutes from blood. [Preview Abstract] |
Tuesday, November 24, 2009 8:26AM - 8:39AM |
MG.00003: Micropancake Growth James Seddon, Olesya Bliznyuk, Stefan Kooij, Harold Zandvliet, Bene Poelsema, Detlef Lohse Micropancakes are thin fluidic layers that form at the interface between a hydrophobic substrate and bulk liquid. To date, details of how they differ from the bulk liquid has been the subject of speculation. The current consensus is that they are gaseous, with typical heights of 1-2nm and lateral extents of microns. Here we present results of an experimental investigation into the dynamics of micropancakes. Atomic force microscopy is used to firstly confirm the existence of micropancakes before monitoring long-time effects. We find that the micropancakes grow laterally with time, thus tending to reduce the area of contact between the bulk liquid and substrate. The growth is initially directional, mediated by substrate pinning, however a surprising transition from this growth behaviour to pancake rearrangement then occurs. [Preview Abstract] |
Tuesday, November 24, 2009 8:39AM - 8:52AM |
MG.00004: Applications of Needle-Like Magnets in Opto-Fluidics Alexander Tokarev, Meena Mirdamadi, Matthieu Bardet, Serhiy Malynych, George Chumanov, John Ballato, Konstantin Kornev Imaging in micro and nanofluidics is a challenge: the sizes of micro and nanochannels are so small that the installment of additional optical and mechanical switches is almost impossible. Another constrain is the size of the device and associated increase in viscous dissipation. We suggest manipulating the light by using the existing fiber optics and adding a functional lens which would expand/contract the light on demand. We showed that the shape of the laser beam passing through the colloid with suspended magnetic nanoparticles can be altered by varying the applied magnetic field. When the propagation of light is perpendicular to the magnetic field, this lens filled with a magnetic fluid works as a cylindrical lens focusing the light onto a line instead of onto a point. In the paper we report the experimental results on kinetics of chain formation as observed through dynamic light absorption and multiple scattering. Magnetic nanoneedles are also attractive candidates for making deformable optical lenses. We show that optically transparent solutions of nickel nanowires in ethylene glycol are responsive to weak magnetic fields. When magnetic field is applied from the top, the droplet of this solution changes its shape making a central spike at some critical field. A theoretical analysis and experimental data on this unusual effect will be presented in the paper. [Preview Abstract] |
Tuesday, November 24, 2009 8:52AM - 9:05AM |
MG.00005: Particle dynamics and rheology of single-wall-carbon-nanotube suspensions under shear and electric fields Jerry Shan, Chen Lin The particle orientation and electrorheology of dilute single-wall-carbon-nanotube suspensions was experimentally investigated. Ensemble-averaged nanotube orientation angles were measured with an optical polarization-modulation technique simultaneously with macroscopic electrorheological measurements. The results were compared with theoretical predictions of Mason. The time scales of the particle-orientation and electrorheological responses differ by an order of magnitude, indicating that nanotube alignment under the external electric field does not directly affect the rheology of the suspension at low nanotube concentrations. Equilibrium particle-orientation angles for various shear rates and electric fields were found to collapse when plotted against the ratio of shear-flow to electrostatic forces, as predicted by classical theory. However, there were significant discrepancies between the measured and predicted orientation angles of the nanotubes. It is shown that the discrepancy is due to both hydrodynamic and electrostatic interactions between particles. The significance of such interactions in dilute suspensions of highly anistropic particles under shear and electric fields is discussed. [Preview Abstract] |
Tuesday, November 24, 2009 9:05AM - 9:18AM |
MG.00006: Flow Velocity Profile in a Nanocapillary Guiren Wang, Cuifang Kuang, Wei Zhao Although there has been considerable research performed in the area of nanofluidics that is focused on the study of the velocity, only theoretical or simulation results exist. The reason for the lack of experimental evidence could be because that it is very difficulty to measure the flow velocity in nanochannel, where the transverse dimension is between 1--500 nm. We have developed a far field nanoscopic optical velocimetry that can potentially measure the velocity in nanofluidics with nanoscale spatial resolution. It is based on laser induced fluorescence photobleaching and stimulated emission depletion. In this presentation, we report our initial experimental data using water solution in a quartz nanocapillary with an inner diameter of 360 nm. The spatial resolution is better than 70 nm. First, no clear slip-flow is observed. This could probably be due to that our measurement is not accurate enough within 35 nm from the wall. Second, compared with the conventional Hagen-Poiseuille equation, the velocity profile is not purely parabolic. Instead, there seems to be a region, where the velocity profile is not smooth along radial direction and the corresponding velocity gradient is very small. Near the axial region, the velocity gradient is increased again. [Preview Abstract] |
Tuesday, November 24, 2009 9:18AM - 9:31AM |
MG.00007: Surface nanobubble contact angles Bram Borkent, Sissi de Beer, Frieder Mugele, Detlef Lohse Previous AFM experiments on surface nanobubbles have suggested an anomalously large contact angle $\theta$ of the bubbles (typically $\sim160\,^{\circ}$ measured through the water) and a possible size dependence $\theta(R)$. Here we determine $\theta(R)$ for nanobubbles on smooth highly orientated pyrolytic graphite (HOPG) with a variety of different cantilevers. It is found that $\theta(R)$ is constant within the experimental error, down to bubbles as small as $R=20\,$nm, and its value is equal to $119\pm4\,^{\circ}$. This result, which is the lowest contact angle for surface nanobubbles found so far, is very reproducible and independent of the cantilever type used, provided that the cantilever is clean and the HOPG surface is smooth. In contrast we find that, for a particular set of cantilevers, the surface can become relatively rough due to precipitated matter from the cantilever onto the substrate, in which case larger nanoscopic contact angles ($\sim150\,^ {\circ}$) show up. [Preview Abstract] |
Tuesday, November 24, 2009 9:31AM - 9:44AM |
MG.00008: Shear rate threshold for the boundary slip in dense polymer films Nikolai Priezjev The shear rate dependence of the slip length in thin polymer films confined between atomically flat surfaces is investigated by molecular dynamics simulations. The polymer melt is described by the bead-spring model of linear flexible chains. We found that at low shear rates the velocity profiles acquire a pronounced curvature near the wall and the absolute value of the negative slip length is approximately equal to thickness of the viscous interfacial layer. At higher shear rates, the velocity profiles become linear and the slip length increases rapidly as a function of shear rate. The gradual transition from no-slip to steady-state slip flow is associated with faster relaxation of the polymer chains near the wall evaluated from decay of the time autocorrelation function of the first normal mode. We also show that at high melt densities the friction coefficient at the interface between the polymer melt and the solid wall follows power law decay as a function of the slip velocity. At large slip velocities the friction coefficient is determined by the product of the surface induced peak in the structure factor, temperature and the contact density of the first fluid layer near the solid wall. (Reference cond-mat/0906.2771). [Preview Abstract] |
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