Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session T15: Fluidic Devices: Micro and Nano |
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Sponsoring Units: DFD Chair: Thomas Cubaud, Stony Brook University Room: 316 |
Wednesday, March 18, 2009 2:30PM - 2:42PM |
T15.00001: Time evolution of distributive entropy in rectangular microchannel mixers Miron Kaufman, Petru Fodor Patterning ridges on the surface of microchannels has been found to be a viable strategy to induce mixing in straight channels, despite the characteristically small Reynolds numbers. In this work we evaluate the time evolution of the R\'{e}nyi entropy associated with the spatial distribution of tracers advected by an incompressible fluid moving in several straight rectangular channels: staggered herring bone [1], fractal surface patterning [2]. The steady state flow fields are obtained by solving the Navier -- Stokes and continuity equations using a finite element analysis package. The R\'{e}nyi entropy is then evaluated at different times using the spatial distribution of the tracers. The entropy increases with time as lnt with a slope approximately equal to unity. The slope quantifies the rate of distributive mixing. The rate of increase in the entropy is found to be independent of the Renyi beta parameter. This is qualitatively different than the distributive mixing in channels with moving walls [3] where the rate of distributive mixing changes with the beta parameter. We also study the dependence of the distributive entropy on the Reynolds number. [1] A.D. Stroock et al., Science 295, 647 (2002); [2] M. Camesasca, M. Kaufman, I. Manas-Zloczower, J. Micromech. Microeng. 16, 2298 (2006); [3] W. Wang, I. Manas-Zloczower, M. Kaufman, Chemical Engineering Communications, 192(4), 405-423 (2005). [Preview Abstract] |
Wednesday, March 18, 2009 2:42PM - 2:54PM |
T15.00002: Capillary absorption of metal nanodroplets by carbon nanotubes Shaun Hendy, Dmitri Schebachov We present a simple model that demonstrates the possibility of capillary absorption of non-wetting liquid nanoparticles by carbon nanotubes assisted by the action of the Laplace pressure due to the droplet surface tension. We test this model with molecular dynamics simulation and find excellent agreement with the theory, which shows that for a given nanotube radius, there is a critical size below which a metal droplet will be absorbed. We then consider the dynamics of capillary absorption using the steady-state flow model due to Marmur, which is based on the Lucas-Washburn model with the addition of a driving force due to the Laplace pressure of the droplet. We find an exact solution to Marmur's evolution equation for the height of the absorbed liquid column as a function of time, and show that this reproduces the dynamics observed in the simulations well. The simulations show that the flow of the metal exhibits a large degree of slippage at the tube walls, with slip lengths of up to 10nm. These findings suggest new methods for fabricating composite metal-CNT materials, and have implications for our understanding of the growth of CNTs from metal catalyst particles. The results also explain the recent observations of the absorption of Cu nanodroplets by carbon nanotubes. [Preview Abstract] |
Wednesday, March 18, 2009 2:54PM - 3:06PM |
T15.00003: Dynamic Pattern Formation In a Bubble-Generating Concentric Microfluidic Device Keng-hui Lin, Kuo-yuan Chung We observe rich spatiotemporal patterns of bubbles inside liquid droplets through a concentric microfluidic device made by two capillary tubes flown with gas and liquid respectively. When the gas pressure increases, the bubbles change from mondisperse, bidisperse to polydisperes. When the liquid flow rate to the gas flow rate is small, the bubble can not be stabilized inside the liquid droplet. The diameter of the bubbles can be scaled with the ratio of gas flow rate to the liquid flow rate. Our device offers different geometry to understand the bubble breakup in the microfluidic device. [Preview Abstract] |
Wednesday, March 18, 2009 3:06PM - 3:18PM |
T15.00004: Flow-Based Organization of Soft Matter in Three Dimensions Lian Leng, Siavash Aslanbeigi, Axel Guenther Flows of miscible and immiscible liquids through microchannel networks have been previously used to achieve spatial organization within one plane. However, extending this approach to three dimensions, an essential requirement to create synthetic bulk materials with a regular microstructure, is not straightforward. To our knowledge for the first time, we demonstrate microfluidic strategy for the three-dimensional organization of soft bulk materials. The approach is enabled by a massively scaled microfluidic architecture that distributes two miscible or immiscible fluid streams through an array of parallel channels. The soft-lithographic fabrication process was adapted to consistently define microfluidic channel networks in elastomer substrates that are only 500 microns thin; followed by subsequent bonding of up to ten such layers in the vertical direction. The chip was connected with fluidic inlets, completely immersed in water and continuously extruded the organized material at its exit. Upon leaving the chip, neighbouring fluid streams formed a hydrogel retaining the desired regular microstructure. The material microstructure was controlled by adjusting the flow rates of the interdiffusing fluid streams (e.g. aqueous alginate and calcium chloride solutions). [Preview Abstract] |
Wednesday, March 18, 2009 3:18PM - 3:30PM |
T15.00005: Viscous droplet deformation and breakup in microfluidic cross-flows Thomas Cubaud The dynamic response of translating high-viscosity droplets is experimentally investigated by means of a sharp increase of the flow velocity in a microchannel junction. The additional local injection of the continuous phase from symmetric side-channels into a square microchannel produces a broad range of time-dependent deformations and breakup. In particular, due to microscale wall confinement, the system displays a non-linear behavior with the initial droplet size. Deformations, relaxation times, and fragmentation processes are examined as a function of flow and fluids properties with a particular emphasis on the formation of slender viscous structures and spoon-like droplets, i.e., asymmetrical droplets. [Preview Abstract] |
Wednesday, March 18, 2009 3:30PM - 3:42PM |
T15.00006: Morphology of liquids spreading along open nanofluidic channels Antonio Checco Dynamic atomic force microscopy (AFM) in the non-contact regime is used to study the morphology of a non-volatile liquid (squalane) as it spreads along wettable nanostripes embedded in a non-wettable surface. AFM allows the direct observation of the microscopic contact line of spreading nanoliquids with unprecedented spatial resolution. Results show that the liquid profile depends on the amount of lateral confinement imposed by the nanostripes and it is truncated at the microscopic contact line in good qualitative agreement with classical mesoscale hydrodynamics. However, the width of the contact line is found to be significantly larger than expected theoretically. This behavior may originate from small chemical inhomogeneity of the patterned stripes as well as from thermal fluctuations of the contact line. [Preview Abstract] |
Wednesday, March 18, 2009 3:42PM - 3:54PM |
T15.00007: Separation of chiral objects by shear flow in microfluidic channels - Theory Henry Fu, Marcos, Thomas Powers, Roman Stocker Motivated by the desire to separate chiral molecules, we investigate the motion of helices in shear flow generated by a microfluidic channel. We present a model based on resistive force theory to show that hydrodynamic forces on a helix in shear flow produce a drift perperdicular to the shear plane. The drift depends on the sign of the shear rate and the chirality of the helix. Net drift results from preferential alignment with streamlines. For large ($>$ 1 micron), elongated particles, alignment is a consequence of the deterministic tumbling trajectories (Jeffery orbits) in shear flow. For smaller particles, we estimate the effect of Brownian rotational diffusion on chirality-sensitive drift. We deduce a lower size limit for separation of chiral objects by shear flow in microfluidic channels. [Preview Abstract] |
Wednesday, March 18, 2009 3:54PM - 4:06PM |
T15.00008: Separation of chiral objects by shear flow in microfluidic channels - Experiment Marcos, Henry Fu, Thomas Powers, Roman Stocker We use microfluidics to test the prediction that a helix in shear flow drifts across streamlines. We use the non-motile, helical-shaped bacterium Leptospira biflexa as our model chiral object. As the shear in the top and bottom halves of the microchannel has opposite sign, we predict and observe the bacteria in these two regions to drift in opposite directions. The magnitude of the separation is in good agreement with theory. [Preview Abstract] |
Wednesday, March 18, 2009 4:06PM - 4:18PM |
T15.00009: Tunable liquid optics: electrowetting-controlled liquid mirrors based on self-assembled Janus tiles Tom Krupenkin, Mike Bucaro, Paul Kolodner, Ashley Taylor, Alex Sidorenko, Joanna Aizenberg In this work we describe a tunable, high-reflectivity optofluidic device based on self-assembly of anisotropically-functionalized hexagonal micromirrors (Janus tiles) on the surface of an oil droplet to create a concave liquid mirror. The liquid mirror is deposited on a patterned transparent electrode that allows the focal length and axial position to be electrically controlled. The mirror is mechanically robust and retains its integrity even at high levels of vibrational excitation of the interface. The use of reflection instead of refraction overcomes the limited available refractive-index contrast between pairs of density-matched liquids, allowing stronger focusing than is possible for a liquid lens of the same geometry. This approach is compatible with optical instruments that could provide novel functionality - for example, a dynamic 3D projector; i.e., a light source which can scan an image onto a moving, non-planar focal surface. Janus tiles with complex optical properties can be manufactured using our approach, thus potentially enabling a wide range of novel optical elements. [Preview Abstract] |
Wednesday, March 18, 2009 4:18PM - 4:30PM |
T15.00010: A CMOS / Microfluidic Vesicle Based Lab-on-a-Chip Platform David Issadore, Thomas Franke, Keith Brown, Robert Westervelt Droplet based microfluidic systems have proved to be useful tools for performing lab-on-a-chip experiments. Our lab has designed CMOS / microfluidic chips to trap, move, merge, and separate droplets of water in oil using dielectrophoresis (DEP) [1]. Vesicles provide a robust container for cells, bacteria, viruses, fluorescent markers, and can withstand a wide range of chemistries, salinity, and pH. We present a platform for programmable chemical and biological experiments that traps, moves, and merges vesicles suspended in water using DEP on our hybrid chip. Vesicles are loaded with 1-4mM NaCl and rhodamine and are suspended in a 200mM glucose solution. We trap and move individual vesicles along programmable paths at speeds up to 70 micrometers/sec. Two vesicles may be brought together and merged into one when triggered with electric fields that are created by the chip. [1] TP Hunt, D Issadore, RM Westervelt - Lab on a Chip, 2008. [Preview Abstract] |
Wednesday, March 18, 2009 4:30PM - 4:42PM |
T15.00011: Designing actuated cilia pumping fluids in microchannels Alexander Alexeev, Julia Yeomans, Anna C. Balazs Using three-dimensional computational modeling, we examine the motion of actuated cilia in a fluid-filled microchannel. The cilia are modeled as deformable, elastic filaments, which are initially tilted with respect to the channel surface. A sinusoidal force normal to the microchannel wall is applied at the free ends of the tilted cilia and induces periodic oscillations of these flexible filaments. To capture the complex fluid-structure interactions among these filaments, the channel walls and the surrounding solution, we employ our hybrid computational approach that combines a lattice Boltzmann model for hydrodynamics of vicious fluids and a lattice spring model for the micromechanics of elastic solids. We find that the actuated cilia give rise to a unidirectional flow in the microchannel and by simply altering the frequency of the applied force, we can controllably switch the direction of the net flow. The findings suggest that beating elastic cilia could be harnessed to regulate the fluid streams in microfluidic devices. [Preview Abstract] |
Wednesday, March 18, 2009 4:42PM - 4:54PM |
T15.00012: A Deterministic Microfluidic Ratchet Kevin Loutherback, Jason Puchalla, Robert Austin, James Sturm We present a deterministic microfluidic ratchet where the trajectory of particles in a certain size range is not reversed when the sign of the driving force is reversed. This ratcheting effect is produced by employing triangular rather than the conventionally circular posts in a post array that selectively displaces particles transported through the array. The underlying mechanism of this method is shown to to be an asymmetric fluid velocity distribution through the gap between triangular posts that results in different critical particle sizes depending on the direction of the flow. [Preview Abstract] |
Wednesday, March 18, 2009 4:54PM - 5:06PM |
T15.00013: Topological Dependence of ds-DNA Confined in Nanoslits Po-keng Lin, Jen-Fang Chang, Cheng-Hung Wei, Pei-Kuen Wei, Y.-L. Chen Topological constraints are important for the DNA condensation in confinement, such as chromosome in the cell and bacteriophage DNA packaging. We investigated the topological dependence of the size, shape and diffusivity of $\lambda $-DNA confined in a nanoslit with height $h$ = 780 nm (\textbf{$\approx $ }bulk radius of gyration of $\lambda $-DNA) to strong confinement ($h$ = 20 nm $<<$ persistence length $p)$ are systematically investigated. Shape asphericity of both linear and circular DNA increases with decreasing $h$, which indicate the DNA become more anisotropic. Furthermore, we observed the transition from de Gennes to Odijk scaling in the measured extension and diffusivity when $h$ = Kuhn length $L_{k}$. Interestingly, the diffusivity of circular DNA is larger than linear DNA in the blob regime, but they are nearly equal in slits with $h \quad <<$ $L_{k}$. [Preview Abstract] |
Wednesday, March 18, 2009 5:06PM - 5:18PM |
T15.00014: Propagation modes of entropically trapped and extended DNA molecules Morten Mikkelsen, Walter Reisner, Henrik Flyvbjerg, Anders Kristensen Nanoconfinement is a powerful tool for controlling polymer conformation and dynamics in lab-on-a-chip type devices for the analysis of DNA and other biomolecules. We present a new device concept that combines confinement-based extension of DNA with the entropic trapping principle, leading to qualitatively new physics and applications. The device consists of a 50~nm slit channel with an array of transverse $100\times 100$~nm grooves, where the transport of DNA molecules perpendicular to the groove axis is investigated under pressure driven buffer flow. At low flow velocities the DNA remains trapped and extended in the nanogrooves while buffer circulates through the slit, enabling physical mapping of the DNA while performing real time buffer exchanges. For flow velocities above a molecular weight dependent escape threshold, we show that the molecule transport through the slit channel randomly alternates between two modes of propagation: A stepwise groove to groove hopping, called the 'sidewinder', and a continuous tumbling across the grooves, where the molecules feel the topology as an effective friction, called the 'tumbleweed'. The observed length dependence on the molecule velocity may lead to a novel separation methodology. [Preview Abstract] |
Wednesday, March 18, 2009 5:18PM - 5:30PM |
T15.00015: Multiplex selection and elution of aptamers using nanoporous sol-gel droplets and a microheater array Seung-min Park, Jiyoung Ahn, Minjoung Jo, Soyoun Kim, Dong-ki Lee, John Lis, Pangshun Zhu, Harold Craighead Aptamers are well-known protein capture reagents that bind to specific proteins and can be effective in inhibiting the protein's normal interactions. Here, we have described a process for selective binding and elution of aptimers from the nanoporous silicate sol-gel droplets within which target proteins are immobilized. These silicate sol-gel droplets are incorporated with polydimethylsiloxane (PDMS) microfluidic systems and individually addressable by electrical microheaters. These properties allow discrete protein -- nucleic acids interaction so that multiplexed selection is possible. It is shown that specific aptamers bind their respective protein targets and can be selectively eluted by micro-heating. Our microfluidic in vitro selection system improves selection efficiency, reducing the number of selection cycles needed to produce high affinity aptamers. We are also able to separate high-affinity nucleic acid species from a large random nucleic acid pool. The process is readily scalable to larger arrays of sol-gel-embedded proteins. [Preview Abstract] |
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