Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session P9: Microfluidic and Nanofluidic Devices |
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Sponsoring Units: DFD Chair: Patrick Tabeling, City of Paris Industrial Physics and Chemistry Higher Educational Institution Room: Morial Convention Center RO7 |
Wednesday, March 12, 2008 8:00AM - 8:12AM |
P9.00001: Microfluidic Fabrication of Bio-compatible Vesicles by Self-assembly in Double Emulsions Ho Cheung Shum, Jinwoong Kim, Daeyeon Lee, David Weitz Vesicles are compartments surrounded by bilayered membranes of amphiphilic molecules such as diblock copolymers and phospholipids. To minimize the exposure of their hydrophobic part to water, amphiphilic molecules self-assemble into aggregates of different structures. When the hydrophobic to hydrophilic ratio is close to unity, amphiphiles self assemble into bilayers, which tend to fold themselves into vesicles. These vesicles are useful for encapsulating and transporting actives such as drugs, flavor, and fragrance. To solve the problems of low encapsulation efficiency and large vesicle size distributions afforded by traditional techniques to create vesicles, we engineer a novel route to generate vesicles using monodisperse double emulsions prepared in microfluidics as templates. The double emulsion-to-vesicle transition exhibits different behaviors depending on the properties of the amphiphilic molecules such as the hydrophobic-to-hydrophilic ratio. Using this technique, we have fabricated both bio-compatible diblock copolymer vesicles, also known as polymersomes, and also lipid vesicles with high encapsulation efficiency. [Preview Abstract] |
Wednesday, March 12, 2008 8:12AM - 8:24AM |
P9.00002: Glass Coating for PDMS Microfluidic Channels by Sol-Gel Methods Thao Do, Adam Abate, David Weitz Soft lithography in polydimethylsiloxane (PDMS) allows one to fabricate complex microfluidic devices easily and at low cost. However, PDMS swells in the presence of many organic solvents, which can significantly degrade the performance of PDMS microfluidic devices. We present a method to coat PDMS channels with a glass-like layer using sol-gel chemistry. As a demonstration of chemical resistance, we flow toluene and aqueous Rhodamine B through coated PDMS channels. Toluene is an organic solvent that significantly swells PDMS in a matter of seconds. Rhodamine B is an organic fluorescent molecule that leaches into PDMS and can therefore be used as a fluorescent probe. Indeed, the coating suppresses swelling of the channels when exposed to toluene; it also prevents leaching of Rhodamine B into PDMS channels. In addition, the channels can be functionalized with silanes to precisely control surface properties. We exploit the high chemical resistance and precise surface functionalization of the coating to produce both direct toluene-in-water and inverted water-in-toluene emulsions in coated, functionalized, PDMS microfluidic channels. This combines the ease of fabrication afforded by soft-lithography with the precision control afforded by sol-gel glass. [Preview Abstract] |
Wednesday, March 12, 2008 8:24AM - 8:36AM |
P9.00003: Fluid Flow and Heat Transfer in a Dual-wet Micro Heat Pipe Jin Zhang, Stephen Watson, Harris Wong Micro heat pipes have been used to cool micro electronic devices, but their heat transfer coefficients are low compared with those of conventional heat pipes. In this talk, a dual-wet pipe is proposed as a model to study heat transfer in micro heat pipes. The dual-wet pipe has a long and narrow cavity. The bottom-half of the horizontal pipe is made of a wetting material and holds a wetting liquid, whereas the top-half is made of a non-wetting material and is filled with the vapor. As one end of the pipe is heated, the liquid evaporates and increases the vapor pressure. The higher pressure drives the vapor to the cold end where the vapor condenses and releases the latent heat. The condensate moves along the bottom half of the pipe back to the hot end to complete the cycle. Hence, the heat pipe is driven by the difference in equilibrium vapor pressure between the hot and cold ends, and not by the liquid-vapor interfacial curvature as is commonly believed. Our analysis provides an explanation for the comparatively low effective thermal conductivity in micro heat pipes [1]. \newline [1] Zhang, Watson {\&} Wong, J. Fluid Mech. \textbf{589}, 1 (2007) [Preview Abstract] |
Wednesday, March 12, 2008 8:36AM - 8:48AM |
P9.00004: Measuring velocity profiles and nanoparticle interactions between 20 and 300 nm from surfaces Patrick Tabeling, Cedric Bouzigues The observation of flows at a nanometric scale is crucial for understanding phenomena involving interactions between liquids and solid surfaces, such as slippage and electro-osmosis. Here we report a new method based on nanoparticle imaging by total internal reflection fluorescence, allowing the first observation of water flows between 20 and 300 nm from surfaces. We probed the energy landscape, leading to first local measurements of the Debye length and surface/nanoparticle interactions; and provide an unambiguous determination with 10~nm accuracy of the slip length for different surfaces - wetting, non-wetting, hard, soft. These results represent an improvement of one order of magnitude compared to the state of the art. In addition to investigating locally energetic and electrostatic properties of the wall/liquid system, this Letter lays down the foundations of a technique that can foster the development of nanofluidics: Imaging of Nanoparticles for Energy landscape and Speed flow measurements (INES). [Preview Abstract] |
Wednesday, March 12, 2008 8:48AM - 9:00AM |
P9.00005: Poisson-Nernst-Planck model of ion current rectification through a nanofluidic diode Dragos Constantin, Zuzanna Siwy We have investigated ion current rectification properties of a recently prepared bipolar nanofluidic diode. This device is based on a single conically shaped nanopore in a polymer film whose pore walls contain a sharp boundary between positively and negatively charged regions. A semiquantitative model that employs Poisson and Nernst-Planck equations predicts current-voltage curves as well as ionic concentrations and electric potential distributions in this system. We show that under certain conditions the rectification degree, defined as a ratio of currents recorded at the same voltage but opposite polarities, can reach values of over 1000 at a voltage range (-2V, +2V). The role of thickness and position of the transition zone on the ion current rectification is discussed as well. We also show that the rectification degree scales with the applied voltage. [Preview Abstract] |
Wednesday, March 12, 2008 9:00AM - 9:12AM |
P9.00006: Non-reflecting boundary conditions for fluctuating hydrodynamics of compressible fluids Rafael Delgado-Buscalioni, Anne Dejoan Many important phenomena in microfluidics involve propagation of fast sound waves. Computational modeling of such problems requires a way to evacuate the reflected waves out of the computational box. However, a way to construct open boundary conditions for Fluctuating Hydrodynamics (FH) is lacking in the literature. This work presents open boundary conditions for fluctuating hydrodynamics solvers based on the Navier-Stokes Landau-Lifshitz equations. The objectives are i) ensure robust non-reflecting boundary conditions and ii) keep thermodynamic consistency for total mass fluctuation, i.e. agreement with the grand canonical ensemble. We show that by ensuring the fluctuation-dissipation balance for the total mass, one also gets the correct equilibrium power spectra of local mass and momentum at each point of the computational box. We consider real compressible fluids (argon and water) under isothermal condition and present results for the equilibrium and several out-of-equilibrium states involving generation of sound waves. [Preview Abstract] |
Wednesday, March 12, 2008 9:12AM - 9:24AM |
P9.00007: ABSTRACT WITHDRAWN |
Wednesday, March 12, 2008 9:24AM - 9:36AM |
P9.00008: Liquid precursor films spreading on chemically patterned substrates Antonio Checco We study the spreading of nonvolatile liquid squalane on chemically patterned nanostripes by using non-contact Atomic Force Microscopy (NC-AFM). The substrates are octadecylthrichlorosilane(OTS)-coated silicon wafers chemically patterned on multiple length-scales using a combination of UV and AFM oxidative lithographies. This process allows us to locally convert the terminal methyl groups of the OTS surface (non-wettable) into carboxylic acid groups (wettable) without affecting considerably the substrate roughness ($<$ 0.3nm rms). The patterned regions are shaped as a network of large (mm-sized) wettable lines connected to smaller and smaller (nm-sized) lines. Liquid squalane spreads across this ``microfluidic network'' starting from the large lines eventually reaching the nanolines (50 to 500 nm-wide). NC-AFM is used to image the morphology of the liquid as it spreads across the nanolines. We find that the liquid thickness on the nanolines grows with time (up to $\sim $10 nm) according to a power-law with exponent $\sim $1. These preliminary results suggest that the spreading dynamics of laterally-confined liquids slightly differs, as expected, from the one of laterally homogeneous precursor films. We compare our findings to recent theoretical predictions of confined liquid flow and also discuss its relevance to nanofluidics. [Preview Abstract] |
Wednesday, March 12, 2008 9:36AM - 9:48AM |
P9.00009: Velocity Dependent Selectivity of Deterministic Lateral Displacement Arrays Jason Puchalla, Keith Morton, Robert Austin Deterministic lateral displacement (DLD) has been demonstrated as a promising microfluidic method to circumvent diffusive dispersion while separating small particles based on size. At low average flow velocity, steric repulsion and diffusion seem sufficient to describe particle behavior and array separation characteristics. However, at higher but still laminar flow velocities, particle behavior changes drastically. We have investigated this regime using a silicon DLD array. We present how the local disruption of fluid flow about a moving particle and the effects inertial forces can alter DLD behavior and can be exploited for selective sorting. [Preview Abstract] |
Wednesday, March 12, 2008 9:48AM - 10:00AM |
P9.00010: Fluctuation effects and evolution in bacterial populations on a chip Jaan Mannik, Juan E. Keymer, Cees Dekker Fluctuation effects are ubiquitous in physics. Relatively little is known what role these effects play in systems involving biological organisms. How do random fluctuations originating from the environment and from the biological organisms itself affect the population dynamics and evolution? Here, we address this question using an experimental approach where we grow a large number of independent E. coli populations on a microfluidic silicon chip designed to evolve the body size distribution. We provide the same environmental conditions for different populations and follow their evolution in real time measuring number of bacteria in different colonies. We analyze fluctuations in these numbers and how the body size distribution of bacteria changes. [Preview Abstract] |
Wednesday, March 12, 2008 10:00AM - 10:12AM |
P9.00011: Modelling colloidal dynamics in complex systems Christopher Smith, Colin Denniston We present a lattice Boltzmann method for dealing with solid moving boundaries in a fluid. A novel method is introduced to distribute a solid surface onto the fluid mesh. We show that for a single particle in a chute with Stokes flow, the quantitatively correct Stokes drag is obtained. Comparing two scenarios at the same Reynolds number, where the walls induce the flow or where the particle is moving, we show there is little discernible difference in the force measured. Next, we have a system with two particles and show we get quantitative agreement for the interaction between the two particles measured by our algorithm and the interaction expected according to the Rotne-Prager (RP) tensor or the Oseen tensor, in the regimes in which they are expected to be accurate. Moving away from irrotational flow, for a cylinder in a two dimensional chute the Reynolds number of the flow is increased further into the laminar region and we show the formation of eddies shedding off the solid surface. We incorporate this new algorithm into liquid crystals simulations to look at novel colloidal interactions through topological defects. [Preview Abstract] |
Wednesday, March 12, 2008 10:12AM - 10:24AM |
P9.00012: Experimental and Theoretical Studies of Electroosmotic Membrane Micropumps Zuli Xu, Jianying Miao, Ning Wang, Ping Sheng Electroosmotic (EO) effect means fluid flow (through a porous medium) induced by an applied electric field E. EO pumps have the advantages of no moving parts and easily-controlled accurate flow rate at low applied voltages. We have fabricated nano-channel EO membrane pumps using anodic aluminum oxide (AAO) as the template [1]. The diameter of the uniform-sized nanochannels can range from 60-300nm, with a membrane thickness of 30-100 microns. The EO effect is enhanced by coating the nano-channels with silica. By using de-ionized water, the nanopump performance is shown to agree reasonably well with the theoretical model, with factors such as the ratio of the double layer thickness to channel diameter, channel geometry, and treatment of the AAO membranes playing important roles. With silica coating to the nanochannels, the nanopump can produce a maximum pressure of 1 atm and a maximum flow rate of 86,000$\mu $L/min$\cdot $cm2 under an applied field of 0.94 V/$\mu $m. Besides DI water, the micropumps have also been tested to work well with salt, acid or base solution. [1] J.Y. Miao, Z.L. Xu, X.Y. Zhang, N. Wang, Z.Y. Yang, P. Sheng, submitted to Advanced Materials (Appeared online: 10.1002/adma.200700767). [Preview Abstract] |
Wednesday, March 12, 2008 10:24AM - 10:36AM |
P9.00013: Investigation and Characterisation of Resizeable Nanopores in an Elastomeric Membrane Geoff Willmott Experimental and theoretical work relating to the development of resizeable synthetic nanopores will be presented. The nanopores, which are roughly conical, are formed by puncturing a relatively thick ($\sim $250 $\mu $m) elastomeric membrane with an STM tip. The aperture can be closed and the size can be dynamically controlled by stretching the elastomer [1]. Use of this technology presents a collection of interesting physical problems, covering topics that include the failure and mechanical properties of the elastomer, flow of ionic current through the aperture and particle sensing using the resistive pulse technique. Synthetic nanopores have potential applications in many fields, but especially relating to nanoscale sensing and diagnostic devices, and replication of ion channels in living cells. [1] S. J. Sowerby, M. F. Broom, G. B. Petersen, Dynamically Resizable Nanometre-Scale Apertures for Molecular Sensing, Sensors and Actuators B: Chemical 123 (1), pp. 325-330 (2007) [Preview Abstract] |
Wednesday, March 12, 2008 10:36AM - 10:48AM |
P9.00014: The physics of densely-packed emulsions Donald M. Aubrecht, David F. Marran, Darren R. Link, David A. Weitz One strategy for microfluidic lab-on-a-chip applications is to use water droplets as tiny reaction vessels in a carrier stream of oil. As biochemical and cell-based experiments often require control over events that take place over a wide range of time scales, strategies need to be developed to ensure adequate timing without limiting droplet throughput. In general, longer time scales can be achieved by using longer channels or more densely packed droplets. Long channels become increasingly impractical at high throughputs for times exceeding tens of minutes, thus motivating work with densely packed droplets. Dense packing of droplets can be achieved by generating droplets on-chip, collecting them off-chip to allow the oil to drain, and re-injecting them back on-chip as a packed emulsion. This strategy is limited in that it only provides access to time scales in excess of hours. Moderate time scales can be accessed by removing carrier oil from the flow without removing the droplets. Here we present some of the physical principles governing how this can be implemented and discuss the flow of the resulting dense collections of droplets through microchannels. [Preview Abstract] |
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