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 HF: Microfluidics: Particles |
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Chair: Michael Brenner, Harvard University Room: 101F |
Monday, November 23, 2009 10:30AM - 10:43AM |
HF.00001: Directed assembly in designed flow fields Tobias M. Schneider, Shreyas Mandre, Michael P. Brenner Various approaches aim at the controlled assembly of mesoscopic objects intro predefined shapes. While self-assembling strategies are conceptually difficult and often hard to implement, we present a new concept for directed assembly. We show that by controlling the fluxes on the boundary of a Hele-Shaw cell the flow field in the cell can be tailored such that many particles are simultaneously advected along prescribed trajectories and thereby assembled in the desired shape. Optimizing the particle paths allows to reduce the required fluxes such that they should be experimentally accessible using microfluidic techniques. [Preview Abstract] |
Monday, November 23, 2009 10:43AM - 10:56AM |
HF.00002: Concentration distributions of arbitrary shaped particles in microfluidic channel flows Arvind Saibaba, Eric Shaqfeh, Eric Darve We are interested in the study of the transient and steady state concentration distribution of orientable Brownian particles across channels at low Reynolds numbers. This is important in understanding margination of blood ``particles'' including platelets as well as new drug delivery and cancer nanotechnology particles which are involved in hemostasis as well as delivering drugs to the vascular endothelial cells. Although our formulation is general, the particles we consider are rigid Brownian ``surfboards'' which have been found to be effective in drug delivery since they are resistant to leukocyte attack [1]. The Stokes flow in the channel around the particles, driven by a mean pressure gradient, is computed using the Boundary Element method within the single layer formulation. The particle motion is calculated using rigid body dynamics with a contribution due to Brownian motion that satisfies the Fluctuation-Dissipation theorem. Finite concentrations are considered, and all hydrodynamic interactions are included. The concentration distribution is computed and interpreted as a balance between the concentration dependent variation in the non-equilibrium particle osmotic pressure and the cross stream particle normal stresses. \\[4pt] [1] J. A. Champion, S. Mitragotri, ``Role of target geometry in phagocytosis'', PNAS 103, 4930-4934, (2006) [Preview Abstract] |
Monday, November 23, 2009 10:56AM - 11:09AM |
HF.00003: Transmitting chemical and mechanical signals via a cluster of microcapsules Amitabh Bhattacharya, German V. Kolmakov, Anna C. Balazs Biological cells often perform tasks collectively by sensing the local density of cells and then performing a particular task in concert (e.g. emitting light) when this cell density increases above a certain threshold. Using an approach based on the Lattice-Boltzmann method, we simulate a similar synthetic system consisting of primarily two kinds of signaling microcapsules, immersed in a fluid, and sitting on an adhesive surface. The first kind constantly releases ``agonist'' molecules, and the second kind release nanoparticles above a certain threshold concentration of ``agonist'' molecules. The nanoparticles adsorb onto the surface and decrease the capsule-surface adhesion strength at the point of adsorption. The resulting gradients in adhesion strength along the surface induces motion in the microcapsules. We examine arrangements of these microcapsules in which mechanical and chemical signals can cascade through a cluster of microcapsules, and comment on the robustness of this system. [Preview Abstract] |
Monday, November 23, 2009 11:09AM - 11:22AM |
HF.00004: Collective behavior and quorum sensing in a system of communicating microcapsules German Kolmakov, Amitabh Bhattacharya, Anna Balazs We report the results on collective motion of polymeric microcapsules in a fluid-filled microchannel. We consider the case where motion of the nanoparticle-filled microcapsules is controlled by adhesion at the channel's wall and hydrodynamic coupling between the capsules. Using the hybrid Lattice Boltzmann method for fluid dynamics and Lattice spring model for the micromechanics of elastic solid, we determined how the characteristics of the substrate, the polymeric shell, encapsulated fluid and the surrounding solution affect the capsule's velocity and ``gait'' of the capsule within the system. In numerical computations we find the conditions under which microcapsules communicating through modification of the microchannel surface by released nanoparticles exhibit collective motion, thereby mimicking behavior of the colony of living cells. In particular, we show that this system demonstrates a quorum sensing. That is, the capsules motion depends on population and behavior of neighboring groups of capsules. Finally, the design of a repair-and-go system is presented, in which we show that deposition of nanoparticles from moving microcapsules onto a damaged substrate can be used as an effective tool for selective repair of defects or cracks on the substrate. [Preview Abstract] |
Monday, November 23, 2009 11:22AM - 11:35AM |
HF.00005: Microfluidic Manipulation of Suspended Single Cells: Cell Deformation and Mechanical Stress Analysis Nathalie Neve, Sean Kohles, Derek Tretheway Bone and cartilage cells experience multiple stresses in vivo. The optimum mechanical conditions for cell health are not fully understood. With the recent development of an integrated optical tweezer with micron resolution particle image velocimetry, the opportunity to apply and measure controlled multiaxial stresses to suspended single cells is possible. In this work, we examine optically suspended cells in uniform and extensional flow fields. The cellular deformation and applied fluid induced stresses are determined. Maximum applied stresses for uniform flows are substantially smaller than typical fluid stresses in cell monolayer studies. Extensional flows enable potentially higher applied stresses. For extensional flows, bone cells show no deformation for shear stresses up to 250 mPa, while significant deformation of muscle cells is observed. [Preview Abstract] |
Monday, November 23, 2009 11:35AM - 11:48AM |
HF.00006: Directed locomotion of bimetallic synthetic nanomotors Jonathan Posner, Philip Wheat, Jeffrey Moran, Christine Burdett, J. Burdick, R. Laocharoensuk, P. Calvo-Marzal, K. Manesh, D. Kagan, S. Balasubramanian, M. Cardona, G.-U. Flechsig, Joe Wang Controlled motion of synthetic nanoscale motors may represent a major step towards the development of practical nanomachines and autonomous microsystems. Bimetallic nanorods can autonomously propel themselves at hundred of body lengths per second through aqueous solutions by using hydrogen peroxide as a fuel. The magnetic and chemical controlled motion of Pt-Ni-Au nanorods is presented. The magnetic properties of nickel-loaded nanomotors offer controlled cargo manipulations, including en-route load, drag and release of spherical cargo that have volumes two orders of magnitude larger than the nanomotors itself. Nanomotors can be directed through microfluidic channel networks and motion triggered using locally generated chemical species. The nanomotor locomotion force is determined by measuring the velocity of motors towing spherical cargo. The nanomotors approximately generate 0.16 picoNewtons of force and can transport microscale cargo with a coefficient of Stokes drag that is eight times their own. [Preview Abstract] |
Monday, November 23, 2009 11:48AM - 12:01PM |
HF.00007: Locomotion of a rod-shaped nanomotor propelled by heterogeneous catalytic reactions Jeffrey Moran, Jonathan Posner Bimetallic nanorods can autonomously propel themselves through aqueous solutions by using hydrogen peroxide as a fuel. Several physical arguments have been proposed to describe the physics underlying the chemically-powered locomotion of these synthetic nanomotors, but there is no accepted or detailed theory on the propulsion mechanism. A computational simulation and scaling analysis of rod-shaped nanoparticles with asymmetric surface fluxes is presented. The model shows that locomotion is driven by electric body forces that arise due to finite space charge and internally generated electric fields surrounding the rod. The electric fields and charge density are generated by dipolar cation fluxes, such as those generated by heterogeneous electrochemical reactions. The scaling analysis and detailed simulations predict that the nanomotor velocity depends on the reaction flux, nanorod electrical surface potential, solvent viscosity, and rod geometry. Strong agreement is observed between the scaling analysis and simulations. [Preview Abstract] |
Monday, November 23, 2009 12:01PM - 12:14PM |
HF.00008: Confinement-induced partial screening of intra-chain hydrodynamic interactions Yeng-Long Chen, Jen-Fang Chang, Po-keng Lin In studies of polymer dynamics in a confined environment, one of the most commonly invoked assumptions is that intra-chain hydrodynamic interactions are completely screened when the confinement length is smaller than the polymer radius of gyration (Rg). However, recent experiments of single DNA molecules confined in nanoslits have observed that chain diffusivity (D) dependence on slit height (H) does not follow the Rouse scaling exponent of 2/3. Rather, the scaling exponent is found to be around 0.5 for Lk $<$ H $<$ Rg, where Lk is the DNA Kuhn length. This suggests that intra-chain hydrodynamic interactions are not completely screened. In this work, we carried out experiments and complementary simulations to characterize the effect of screening on polymer dynamics. To model the polymer dynamics, we employ a method of Brownian dynamics, combined with the lattice Boltzmann method, to simulate polymer dynamics confined in nanochannels. We find that hydrodynamic screening occurs gradually as the channel height decreases, and also that complete screening in H $<$ Lk channels leads to topological independence of chain diffusivity. [Preview Abstract] |
Monday, November 23, 2009 12:14PM - 12:27PM |
HF.00009: Three-dimensional x-ray imaging of macro-clusters in ferrofluids Wah-Keat Lee Ferrofluids are a class of magnetic fluids where nano-sized ($\sim $ 10 nm) magnetic particles are dispersed in a carrier fluid. Ferrofluids have long been used for vacuum seals, but lately, has been proposed for a multitude of new applications including heat transfer and biomedicine. It has been known for some time that the magnetic particles tend to align with an applied magnetic field and that the individual chains can coalesce and form thick and long macro-sized structures whose shapes depend on the properties of the ferrofluid and the applied field. However, due to their opacity to visible light, ferrofluid experiments have been mainly limited to very thin films ($\sim $ 10s of microns). Since the macro-structures can be in the 10-100 micron range, thin film measurements are susceptible to wall effects. TEM and resin techniques have been used to study the structure of these clusters. However, it is doubtful if these frozen or dried structures reflect the natural fluid state. Here, we present x-ray microtomography measurements on a mm-sized tube of ferrofluid under an applied magnetic field. We show the three-dimensional nature of the columns and labyrinth structures. The measurements also allow us to provide estimates on the local magnetic particle concentration within the ferrofluid. [Preview Abstract] |
Monday, November 23, 2009 12:27PM - 12:40PM |
HF.00010: Direct measurement of shear-induced cross-correlation of Brownian motion Andreas Ziehl, Christian Wagner Shear-induced cross-correlations between particle fluctuations perpendicular and along streamlines are investigated experimentally and theoretically in a linear shear flow. We used optical tweezers to localize one or two particles, each in a harmonic potential, and to detect the positions of the particles as a function of time with a high spatial precision below 8nm. These positions are recorded via a high speed camera with 15kHz resolution. In contrast to measurements in a quiescent fluid, we find that in shear flow, generated in a special designed micro fluidic device, orthogonal movements of a bead in stream- and gradient- directions are correlated and the time reversal symmetry is broken. Again in a quiescent fluid, fluctuations of two particles, separated by a few microns, are known to be anti-correlated along their connecting vector due to hydrodynamic coupling. In linear shear flow, we found a coupling process that correlates the orthogonal directions of the two particles. The correlation exhibits a minimum in time and again the time reversal symmetry is broken. [Preview Abstract] |
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