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 BN: Micro Fluids II |
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Chair: Minami Yoda, Georgia Institute of Technology Room: 201 |
Sunday, November 23, 2008 10:30AM - 10:43AM |
BN.00001: A particle-based multiscale model for submicron fluid flows Saumyadip Mukhopadhyay, John Abraham A particle-based multiscale model for submicron fluid flows is proposed in this work. The model is based on a combination of a dissipative-particle dynamics (DPD) model for the mesoscales and molecular dynamics (MD) for the atomistic scales. The coarse-graining procedure involved in deriving DPD from MD is systematically exploited in this work, to transition from the atomistic region to the mesoscale region. Increasing levels of coarse-graining represent increasing length and time scales. The continuity of thermodynamic and transport properties across the interface is facilitated by appropriate selection of model parameters, and the modeling of particle flux across layers. The model is applied to solve Poiseuille and Couette flows, flow over a rough wall, and microscale flows with slip at the wall. Results are compared with analytical/full-scale MD simulations. In the case of the Poiseuille and Couette flows, the results are found to differ from the analytical solutions by less than 10{\%}. The differences with full-scale MD simulation results are within 5{\%} for flow over an obstacle. The reduction in computational cost with increasing coarse-graining is also evaluated. [Preview Abstract] |
Sunday, November 23, 2008 10:43AM - 10:56AM |
BN.00002: Effect of detector exposure time on the apparent diffusion coefficient measured by single particle tracking Shahram Pouya, Manoochehr Koochesfahani, Richard Di Liu We present results from simulation of Brownian motion of nanoparticles where we also take into account the averaging imposed by the exposure period $E$ of the detector. The diffusion coefficient is estimated from the measured displacements of the particles over a prescribed delay time \textit{$\Delta $t.} Results from free diffusion simulations show a clear dependency of the estimated diffusion coefficient on the time-averaging of the detector, decreasing linearly with $E$/\textit{$\Delta $t}. An analytical solution is presented that corroborates this behavior. The simulation is further extended to describe wall-bounded diffusion similar to motion of nanoparticles next to a solid wall. Results show a similar trend to that observed in free diffusion but with an overall reduced diffusion coefficient due to hindered motion imposed by the wall. This study emphasizes the importance of the influence of detector exposure time in measurements using single particle tracking such as near wall velocimetry techniques using quantum dots or other nanoparticles. [Preview Abstract] |
Sunday, November 23, 2008 10:56AM - 11:09AM |
BN.00003: Dielectrophoretic interdigitated electrode arrays in the presence of double layer Sophie Loire, Igor Mezic Uncharged particles in electrolytic solutions can be manipulated using a nonuniform AC electric field which generates a dielectrophoretic (DEP) force, acting on those particles. Nonuniform AC electric fields generated by coplanar microelectrodes also produce steady fluid flow in electrolytic solutions also called AC electroosmosis, ACEO. This fluid flow is explained by the presence of an electrode shielding or double layer where ions from the bulk fluid are distributed above electrodes when an electric field is applied. If the electric field is constant, the distribution of ions can be described by Debye and Huckel. If the electric field is alternating, as is the case in dielectrophoretic, the behavior of the double layer becomes more complex. The presence of this double layer is significant for microfluidic applications and combined use of ACEO and DEP have been used to manipulate micro and nano-particles. DEP force fields have been studied ignoring the presence of the double layer. We study the influence of the electrode shielding on the dielectrophoresis forces. We adopt the simple mathematical model used in previous simulations of ACEO pumps. Neglecting Faradaic reactions, the double layer on each electrode acts like a capacitor with a constant capacitance in the linear regime of small voltages. According to this approach, the DEP force field has interesting properties which could now give an understanding of some previously unexplained experimental observations. [Preview Abstract] |
Sunday, November 23, 2008 11:09AM - 11:22AM |
BN.00004: Shear-induced diffusion of plate-like particles in microchannels Roberto Rusconi, Howard Stone We exploit the recent developments of microfluidic technologies to investigate the collective shear-induced diffusion in suspensions of micron-sized particles. Whereas spherical particles do not diffuse on the time scale of our experiments, the results with plate-like clay particles show a strong cross-stream shear-induced diffusivity at low volume fraction ($\phi_0 \leq 0.01$). Moreover, we find a linear dependence of the collective diffusion coefficient with the average shear rate (in the range $10^2$--$10^4$~$\mathrm{s}^{-1}$) and the particle concentration. These data are in good agreement with previous experimental and theoretical results for spheres when rescaled with the particle number density. [Preview Abstract] |
Sunday, November 23, 2008 11:22AM - 11:35AM |
BN.00005: Microfluidic Rheology of Soft Colloidal Suspensions Kerstin Nordstrom, Paulo Arratia, Emilie Verneuil, Jerry Gollub, Douglas Durian The rheology of a suspension of soft colloidal particles is investigated using a pressure-driven flow in a deep 25 $\mu $m wide microchannel. The system is composed of N-isopropylacrylamide (NIPA), colloidal microgel particles, suspended in aqueous solution. NIPA is temperature-sensitive in that the hydrodynamic radius of a particle decreases as temperature increases [1]. Therefore, colloidal suspensions of different packing fraction can be obtained simply by varying the temperature using a temperature-controlled stage. We determine the velocity profile and the local shear rate of the suspension using particle image velocimetry (PIV). We have developed methods to accurately infer the suspension shear viscosity and shear stress as a function of shear rate. The dynamical range of shear rates probed is approximately 5 orders of magnitude, ranging from 10$^{-3}_{ }$ to 10$^{2}$ s$^{-1}$. Results show that as the packing fraction is increased towards the jamming point, the velocity profiles are markedly non-Newtonian. Further, near the jamming point, the stress versus shear rate curves show yield stress behavior. [1] Alsayed, A.M., Islam, M.F., Zhang, J., Collings, P.J., Yodh, A.J., \textit{Science} \textbf{309}, 1207.-1210 (2005) [Preview Abstract] |
Sunday, November 23, 2008 11:35AM - 11:48AM |
BN.00006: ABSTRACT WITHDRAWN |
Sunday, November 23, 2008 11:48AM - 12:01PM |
BN.00007: Fluidic resistance of confined bubbles in partial wetting regimes Saif A. Khan, Pravien Parthiban, Michiel T. Kreutzer Understanding the fluidic resistance of moving bubbles or droplets is crucial in designing `self-regulating' multiphase microfluidic networks. The resistance of a moving bubble lubricated by a fully wetting liquid is well understood, but its application is highly restricted to a limited number of working liquids, especially in hydrophobic materials such as PDMS that are routinely used to fabricate microfluidic networks. Here we focus our attention on flows where such complete wetting of the walls does not occur, and present a systematic analysis of the regimes that arise out of the interplay between forced wetting and dewetting. We show experimentally that small departures from complete wetting dramatically impact the fluidic resistance while the flow remains ordered and stable. We can thus predict and control the fluidic resistance of a bubble over an order of magnitude by tuning wettability. The results of this work are of significance in the design and operation of bubble train networks in microfluidic devices. [Preview Abstract] |
Sunday, November 23, 2008 12:01PM - 12:14PM |
BN.00008: Dynamics of Linear and Circular DNA in Sub-Micron Channels Yeng-Long Chen, Jen-Fang Chang, Po-keng Lin, Wunshain Fann DNA dynamics in microchannels of height H may be categorized into three regimes: I. H $>$ Rg, II. Rg $>$ H $>$ lp, and III. lp $>$ H, where Rg and lp are the DNA radius of gyration and persistence length, respectively. Dynamics of DNA molecules in regime I has been extensively studied in recent theory and experiments. It has been shown that the intra-chain hydrodynamic interactions (HI) may strongly affect how DNA behaves under a microfluidic flow. In contrast, intra-chain HI is believed to be screened in regime III, as suggested by recent studies of Doyle et al. In this work, we investigate the role of HI in regime II and how it affects DNA of different conformation, specifically for linear and circular chains. In order to capture the intra-chain HI, we employ lattice Boltzmann simulations for the fluid, coupled with coarse-grained dynamics simulations for the DNA. We find that regime II confinement affects the chain conformation and shape differently for linear and circular chains due to the degrees of freedom for chain ends. The different chain conformations also lead to different chain dynamic. [Preview Abstract] |
Sunday, November 23, 2008 12:14PM - 12:27PM |
BN.00009: Wetting Transition Phenomena in Groove-based Superhydrophobic Microchannels with Engineering Parameters Jihoon Kim, Doyoung Byun, Jongin Hong, Hoon Cheol Park We investigated an effect of groove-based superhydrophobic microchannels on a wetting transition from the Cassie-Baxter state to the Wenzel state. Microscale grooves including vertical and overhang structures, on channel walls have been introduced to control an energy barrier between two states. We fabricated groove-based superhydrophobic microchannels by using PDMS-based multilayer soft lithography. Interestingly, air-solid-liquid contact menisci on different grooves were sustained or collapsed depending on engineering parameters, such as the ratio of pitch to width and the shape of the features. If the groove is deep and wide, the transition time of each meniscus increases. It should also be noted that the flow velocity (or Reynolds number) is of importance to maintain a stable interface. Therefore, we observed wetting transition phenomena in differently grooved microchannels and determined pressure distribution by a micro-particle image velocimetry ($\mu$-PIV) technique. Both the transition time and pattern were related to the flow rate in the microchannel as well as the pressure distribution of each interface. [Preview Abstract] |
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