Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session EC: Microfluidics: Complex Fluids/Suspensions |
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Chair: Michael Graham, University of Wisconsin, Madison Room: Hilton Chicago Grand Ballroom |
Sunday, November 20, 2005 4:10PM - 4:23PM |
EC.00001: Liquid crystal (LC) droplet formation and anchoring dynamics in a microfluidic device Ben Hamlington, Amy Shen, James Feng, Darren Link Liquid crystal drops dispersed in a continuous phase of silicon oil are generated with a narrow distribution in droplet size in microfluidic devices both above and below the nematic to isotropic transition temperature. For these two cases, with fixed shear viscosity at both phases and altering the channel surface property, we observe not only the different LC droplet generation and coalescence dynamics, but also distinct droplet morphology. Our experiments show that the nematic liquid crystalline order is important for the LC droplet formation and anchoring dynamics. [Preview Abstract] |
Sunday, November 20, 2005 4:23PM - 4:36PM |
EC.00002: Migration of polymer molecules near flowing fluid interfaces Hongbo Ma, Juan J. de Pablo, Michael D. Graham Migration of polymer molecules near flowing fluid interface is explored. First, a simplified system, a bead-spring dumbbell above a planar solid wall is considered. In shear flow, the dumbbell is stretched and aligned in the flow direction. The perturbation flow generated by the motion of beads can be calculated using the full hydrodynamic interaction tensor, and it is shown that the center of mass of the dumbbell is driven away from the wall by this perturbation flow. At the same time, Brownian diffusion always tries to smooth out the concentration gradient. As a direct result of balancing these two competing factors, a depletion layer appears. At steady state, the thickness of the depletion layer can be much larger than the size of the molecule, and depends on the normal stress differences and the molecular diffusivity, which in turn depend on the shear rate and molecular weight. Theoretical predictions are compared to detailed computer simulations for a more realistic system, a bead-spring chain model of a DNA molecule and good agreement is obtained. The migration near the interface between two fluids with different viscosities is also investigated. Finally, a novel separation mechanism based on the migration phenomena is proposed and a prototype device with wavy-shape channel is explored. [Preview Abstract] |
Sunday, November 20, 2005 4:36PM - 4:49PM |
EC.00003: Segregation Dynamics of Suspensions in Micro-Vortices Diana Hou, Hsueh-Chia Chang Cylindrical vortices generated by AC electro-osmotic (AC-EO) flows on microelectrodes have been seen to attract and concentrate micro-particles. The particles are attracted to the vortex by dielectrophoretic (DEP) forces of the underlying micro-electrodes. However, shear-induced particle migration mechanisms can counter the DEP transport resulting in a filled vortex or an annulus. A dimensionless parameter $\chi $ relates the two opposing forces and defines the extent of particle invasion. The cylindrical vortices suffer a symmetry-breaking instability, break up into band structures along the axis and coalesce into more concentrated slugs. The instabilities are analyzed theoretically and attributed to longitudinal shear-induced migration due to dependence of the vortex shear rate on the local particle concentration. The concentrated funnel-shaped slugs exhibit fore-aft asymmetry along their axis and propagate in a specific direction at a constant speed. They are shown to be a solitary traveling wave solution of the Kuramoto-Sivashinsky equation. [Preview Abstract] |
Sunday, November 20, 2005 4:49PM - 5:02PM |
EC.00004: Experimental investigation of particle dynamics and blockage at microchannel inlet Eiichiro Yamaguchi, Ronald Adrian Experimental investigation and visualization of particle motion around microchannel inlet have been conducted over range of $0.2 \leq \mathrm{R} \leq 0.67$ and $0.32 \leq \mathrm{Re} \leq 14.43$, where $\mathrm{R}$ is ratio of particle and channel hydraulic diameter. The experimental results indicate that probability of channel blockage by the particles depends largely on the inlet configuration and flow condition. Especially for shear-induced blockage formation with relatively large $\mathrm{R}$, flow condition and particle dynamics in developing flow region around the channel inlet is responsible for maximum $70\%$ of total blockage occurrence. Optical measurement of flow and particles support the experimental data, and indicate that collision of particle against the channel wall around inlet increases collision rate between particles. The collision rate of particle and the inlet is defined by $\mathrm{R}$, relative motion of flow and particle at developing region and approaching velocity toward the wall can be characterized by particle/fluid density ratio and $\mathrm{Re}$, and geometrical configuration of inlet defines how all parameters are combined. Flow visualization of inlet with various shapes and experimental results of the blockage probability will be presented. [Preview Abstract] |
Sunday, November 20, 2005 5:02PM - 5:15PM |
EC.00005: Dynamic self-assembly of paramagnetic beads for microdevices Eric Keaveny, Martin Maxey Paramagnetic beads, about 1 micron in diameter, suspended in a liquid will aggregate to form chains when an initially random dispersion is subject to a steady magnetic field. An important issue is to accurately calculate the particle-particle forces as the chain forms, and within the chain. This governs the stability of the chain in shear flows or rotating magnetic fields and determines conditions under which the chain fragments. We have developed new methods to efficiently calculate the magnetic dipole interactions of the particles and to compute the forces between particles accurately. Examples will be given of the hydrodynamic interactions between chains in response to these forces. For a rotating field the chains tend to deform and depending on rotation rates will form S-shaped chains or aggregate clusters. [Preview Abstract] |
Sunday, November 20, 2005 5:15PM - 5:28PM |
EC.00006: Assessing the applicability of Brownian Dynamics to simulation of nanoparticle clustering in liquid suspensions Sergiy Markutsya, Shankar Subramaniam, Monica Lamm, Dennis Vigil, Rodney Fox Brownian Dynamics (BD) is an attractive approach to simulate solute nanoparticles in a suspension of solvent molecules because it exploits the separation of time scales between solvent and solute dynamics to avoid explicit representation of the solvent molecules. In BD a nanoparticle's velocity evolves because of interaction with other nanoparticles through a systematic pairwise interaction force, while the effect of the solvent molecules on the nanoparticles is represented by a combination of frictional and random forces (Langevin part). This Langevin equation for the velocity is solved along with the nanoparticle position evolution, and we denote this the Position-Velocity Langevin (PVL) system. If the momentum relaxation time of the nanoparticles is itself rapid and long-time configurational dynamics are of interest, then Ermak and McCammon showed that it is possible to reduce PVL to a Position Langevin (PL) equation system. The reduction from PVL to PL gives a significant gain in computational time, however, it was not clear if PL approach describes clustering adequately. Preliminary analysis indicates that the clustering regime does not admit the time scale separation for latex nanoparticles in water. While BD has been used to study such phenomena as diffusion, the purpose of this study is to ascertain whether BD can be used to model the clustering of nanoparticles in liquid suspension. [Preview Abstract] |
Sunday, November 20, 2005 5:28PM - 5:41PM |
EC.00007: Shear-induced migration of dilute Brownian suspensions Jinhua Cao, Steve Wereley A rigid spherical particle translating at small tube Reynolds number in a shearing flow experiences lateral migration due to inertial and wall effects, even in the limit of vanishingly small Reynolds numbers. In a suspension flow of sufficiently small particles, Brownian motion competes with this migration phenomenon. We investigate the migration phenomenon of Brownian particles in a pressure-driven flow for a range of particle volume fractions much less than 0.01 using epi-fluorescent microscopy and micro particle image velocimetry. The flow velocity and particle size are both varied, resulting in the bulk Pecl\'{e}t number (Pe) ranging over four orders of magnitude. Both the velocity and the particle distributions were measured. When Pe is smaller than 1000, particles migrate away from the channel wall due to the wall effect while the particle concentration in regions remote from the wall (more than 10 particle diameters) remains nearly uniform. When Pe increases beyond 1000, all particles, whether initially near the wall or near the center of the channel, move toward a preferred radial position of 0.5 to 0.7 times the channel radius, with the migration effect becoming progressively stronger as Pe increases. [Preview Abstract] |
Sunday, November 20, 2005 5:41PM - 5:54PM |
EC.00008: Alignment of Carbon Nanotubes in Liquid Suspension by Electric Fields Matthew Brown, Jerry W. Shan, Frank M. Zimmermann We present a study of the electric-field-induced spatial alignment of single-wall carbon nanotubes (SWNTs) in ethanol suspension. Field-induced optical dichroism due to preferentially aligned nanotubes in the suspension affects the state of polarization of light passing through the sample. The change in polarization angle of a linearly polarized laser beam transmitted through the sample was used to measure the nematic order parameter characterizing the degree of alignment. Electric-field-induced alignment was measured for varying electric-field strengths, field frequencies, and temperatures of the sample. The observed alignment approached a steady-state value after a transient response on the order of seconds. The transient time scale is compared to that expected from viscous fluid dynamics. The dependence of the steady-state alignment on applied field strength and temperature is explained in terms of an equilibrium statistical mechanical model. The analysis allows us to determine the average electric polarizability of the nanotobes in the axial direction. The obtained polarizability is compared with theoretical values from the literature. [Preview Abstract] |
Sunday, November 20, 2005 5:54PM - 6:07PM |
EC.00009: Direct measurements of DNA-laden flows in microfluidic devices Shelly Gulati, Susan J. Muller, Dorian Liepmann The characterization of flows containing macromolecules such as DNA is critical for the optimal design of microfluidic systems for biochemical analyses. The effects on $\lambda $-DNA transport in microscale flows are significant because the flow behavior may be influenced by molecular interactions, both viscous and elastic forces dominate inertial forces at this length scale, and the macromolecular length scale L approaches the device length scale D. Planar micro-contraction geometries (gradual and 2:1 abrupt) are used as test structures because they are canonical microfluidic components and a viscoelastic benchmark. The DNA solution is subjected to regions of elongation along the channel centerline and shear at the walls and L/D $\sim $ 0.12 and $\sim $ 0.22 for the 2:1 abrupt and gradual contraction, respectively. Digital Particle Image Velocimetry (DPIV), pressure measurements, and flow visualization are used to characterize the flows of water and semi-dilute DNA solutions over the range 0.5 $<$ De $<$ 180 and 0.0001 $<$ Re $<$ 0.9. Recirculation regions observed upstream of the contraction for semi-dilute DNA flows indicate strong elastic flow behavior. Conformational studies of DNA flows in these geometries relate molecular conformation to the velocity fields across a similar parameter range. [Preview Abstract] |
Sunday, November 20, 2005 6:07PM - 6:20PM |
EC.00010: Flow-induced scission of macromolecules H. Sim, R. Sureshkumar, B. Khomami Mechanical scission of long chain synthetic and bio- polymers in strong flows is pertinent to applications ranging from genomics to polymer-induced turbulent drag reduction. Experiments generally differentiate between two types of fracture mechanisms in extensional flows depending on whether the polymer experiences a steady (e.g. cross slot flow) or transient (e.g. contraction flow) field. Theories based on ``mid-point scission hypothesis'' as well as computer simulations using bead-spring models that use ad hoc energy-based criteria have been used to explain experimental observations. We present the results of a study that for the first time couples Brownian Dynamics Simulation (BDS) using bead-rod (Kramers) chains with a novel algorithm for the determination of scission events which themselves are stochastic processes. Flexible lambda-phage DNA is selected as a model molecule. We will discuss effect of molecular weight (MW), flow type and hydrodynamic interactions on chain scission and the MW distribution. [Preview Abstract] |
Sunday, November 20, 2005 6:20PM - 6:33PM |
EC.00011: Multiparticle hydrodynamic interactions in parabolic creeping flow between two parallel planar walls Jerzy Blawzdziewicz, Sukalyan Bhattacharya, Eligiusz Wajnryb Hydrodynamic interactions of spherical particles in incident Poiseuille flow in a channel with infinite planar walls are investigated. The particles are suspended in a Newtonian fluid, and creeping-flow conditions are assumed. We consider the motion of freely suspended particles as well as the forces and torques acting on particles adsorbed at a wall. Using our highly accurate Cartesian-representation algorithm, we find that the pair hydrodynamic interactions in this wall-bounded system have a complex dependence on the lateral interparticle distance due to the combined effects of the dissipation in the gap between the particle surfaces and the backflow associated with the presence of the walls. For immobile particle pairs we have examined the crossover between several far-field asymptotic regimes corresponding to different relations between the particle separation and the distances of the particles from the walls. We have also shown that the cumulative effect of the far-field flow substantially influences the force distribution in arrays of immobile spheres. Therefore, the far-field contributions must be included in any reliable algorithm for evaluating many-particle hydrodynamic interactions in the parallel-wall geometry. [Preview Abstract] |
Sunday, November 20, 2005 6:33PM - 6:46PM |
EC.00012: Mapping potential energy landscapes of templated substrates using diffusing colloidal particles Pradipkumar Bahukudumbi, Michael Bevan, Ali Beskok Manipulating self and directed colloidal assembly on templated substrates requires the ability to measure and reversibly tune interactions on the order of kT. Accurate measurements of small energy differences in particle-substrate interactions are necessary to control the equilibrium self assembly processes. In this work, we report measurements of colloidal particle interactions and assembly dynamics at patterned substrate interfaces using optical microscopy techniques. Standard photolithography techniques were used to fabricate arrays of different size physical features on an Indium-Tin-Oxide (ITO) electrode surface. Electric fields were used in conjunction with gravitational effects to control inherent competitive transport mechanisms to produce ordered colloidal structures. A novel method to map potential energy surfaces that utilizes a diffusing colloidal particle as a sensitive ``probe'' to measure \textit{kT} interactions will also be described. The 2D diffusion of the colloidal probe was monitored using Video microscopy (VM), and the x, y center coordinates and the particle trajectories were tracked using standard image processing algorithms. Consequently, the potential energy landscape probed by the particle can be obtained by inverting the 2D histogram of particle positions using Boltzmann's equation. [Preview Abstract] |
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