Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session E6: Microfluids: Particles I - Orientation and Self-Assembly |
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Chair: Jerry Shan, Rutgers University Room: 328 |
Sunday, November 24, 2013 4:45PM - 4:58PM |
E6.00001: Hybrid Opto-electric Manipulation of Macromolecules Steve Wereley Recently our research group has developed an innovative method for capturing, concentrating, manipulating and sorting populations of micro- and nanometer-scaled entities-particles, cells, macro-molecules, etc. These populations range from individual particles to thousands of particles (Lab-on-a-Chip, 2008; Microfluidics and Nanofluidics, 2008) while the sizes range from microns to nanometers. This novel technique combines features of optical trapping and dielectrophoresis in an innovative, dynamic way using a simple parallel plate electrode configuration. Transparent electrodes comprised of Indium Tin Oxide (ITO) on glass substrates are used to generate an electric field in the fluid while at the same time allowing light into and out of the fluid. Near-IR optical illumination causes subtle localized heating, creating an electric permittivity gradient that in turn drives a microscopic toroidal vortex. The vortex efficiently transports particles to a preferred location, usually the surface of the electrode. Recent advances have extended have allowed us to apply this technique to macromolecules (DNA, proteins) as well as nanoscale particles (quantum dots, nanowires and PSL particles). [Preview Abstract] |
Sunday, November 24, 2013 4:58PM - 5:11PM |
E6.00002: Customizing mesoscale self-assembly with 3D printing Nicolas Vandewalle, Martin Poty, Geoffroy Lumay Self-assembly due to capillary forces is a common method for generating 2D mesoscale structures from identical floating particles at the liquid-air interface. Designing building blocks to obtain a desired mesoscopic structure is still a challenge. We show herein that it is possible to shape the particles with a low cost 3D printer, for composing specific mesoscopic structures. Since capillary interactions can be downscaled, our method, for producing capillary multipoles, opens new ways to low cost microfabrication. [Preview Abstract] |
Sunday, November 24, 2013 5:11PM - 5:24PM |
E6.00003: Electro-orientation of Carbon Nanotubes in Polymer Suspensions Richard Castellano, Gabriel Giraldo, Cevat Akin, Jerry Shan Carbon nanotube (CNT) membranes have been of recent interest due to experiments and simulations that have found flow rates through nanotubes to be 2 to 3 orders of magnitude faster than predicted by viscous-flow theory. As such, they offer promise as highly permeable membranes for a variety of filtration and separation processes. However, current CNT-membrane fabrication techniques utilize CVD growth of CNTs which is costly and difficult to scale up. Electro-orientation of post-growth CNTs in polymer suspension is a possible cost-effective and scalable approach to producing aligned CNT membranes and composites. An electric field ($E$-field) applied to a prolate particle induces a dipole in the direction of the particle's major axis which causes the particle to rotate into alignment with the $E$-field. The alignment rate of CNTs in various polymer suspensions is experimentally studied here as a function of the applied $E$-field strength and frequency. When dealing with CNTs, thermal energy is a significant component of the particle motion, causing misalignment from the applied $E$-field. In order to quantify the significance of Brownian motion, we measure the probability distribution of alignment angles for CNTs at various field strengths. We compare our experimental results with theoretical predictions and discuss the implications for producing membranes of aligned CNTs by electro-orientation. [Preview Abstract] |
Sunday, November 24, 2013 5:24PM - 5:37PM |
E6.00004: A hexatic-to-disorder transition in colloidal crystals near electrodes: Rapid annealing of polycrystalline domains Cari Dutcher, Taylor Woehl, Nicholas Talken, William Ristenpart Colloids are known to form planar, hexagonal closed packed (HCP) crystals near electrodes in response to electrohydrodynamic (EHD) flow. Previous work has established that the EHD velocity increases as the applied AC frequency decreases. Here we report the existence of an order-to-disorder transition at sufficiently low frequencies, despite the increase in the attractive EHD driving force. At large frequencies ($\sim$500 Hz), spherical micron-scale particles form HCP crystals; as the frequency is decreased below $\sim$250 Hz, however, the crystalline structure transitions to randomly closed packed (RCP). The transition is reversible and second order with respect to frequency, and independent measurements of the EHD aggregation rate confirm that the EHD driving force is indeed higher at the lower frequencies. We present evidence that the transition is instead caused by an increased particle diffusivity due to increased particle height over the electrode at lower frequencies, and we demonstrate that the HCP-RCP transition facilitates rapid annealing of polycrystalline domains. [Preview Abstract] |
Sunday, November 24, 2013 5:37PM - 5:50PM |
E6.00005: Electrokinetically driven reversible self-assembly of colloidal particle bands near the wall Necmettin Cevheri, Minami Yoda Recent studies in microchannels have shown that the near-wall dynamics of neutrally buoyant dielectric colloidal (radii $a <$ 1 $\mu $m) suspended particles are affected by an electric field of magnitude $E$ applied parallel to the wall. Evanescent-wave particle velocimetry was used to study $a = 245$ nm fluorescent polystyrene particles suspended at volume fractions of $O(10^{-4})$ in combined electroosmotic (EO) and Poiseuille flow of an aqueous electrolyte solution, which is effectively the superposition of simple shear and uniform flows within 0.5 $\mu $m of the wall. In ``counterflow,'' where the EO opposes the shear flow through fused-silica microchannels, at a large enough value of $E$ so that flow reversal occurs in the near-wall region, the particles self-assemble into concentrated bright ``stripes'' along the streamwise direction alternating with dark stripes containing almost no particles with a consistent cross-stream spatial frequency. These stripes are only observed within $\sim$ 1 $\mu $m of the wall, and disappear in the absence of an electric field. These observations suggest the existence of a novel electrokinetic instability, and could lead to new methods for controlled self-assembly of particles into anisotropic colloidal crystals. [Preview Abstract] |
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