Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session A9: Micro-fluidics |
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Sponsoring Units: DFD Chair: Alex Alexeev, Georgia Institute of Technology Room: D220 |
Monday, March 21, 2011 8:00AM - 8:12AM |
A9.00001: Hydrodynamic resistance of confined cells in rectangular microchannels Zeina S. Khan, Siva A. Vanapalli Several microfluidic approaches have been developed to screen suspended cells mechanically in microchannels by exploiting characteristics that are linked to their individual mechanical properties. Typically changes in cell shape due to shear-induced deformation and transit times are reported; while these measurements are qualitative compared to more precise techniques such as atomic force microscopy and micropipette aspiration their advantage lies in throughput, with the ability to screen hundreds to thousands of cells in a minute. We study the potential of a microfluidic cell squeezer to characterize the hydrodynamic resistance of LNCaP prostate cancer cells by measuring dynamical pressure-drop variations along a micrometer-sized channel. The hydrodynamic resistance of the cell introduces an excess pressure drop in the narrow channel which depends on the mechanical stiffness of the cell. We additionally visualize the cell size and assess the influence of cell size on the hydrodynamic resistance of each cell, demonstrating the capability of the microfluidic cell squeezer to yield the hydrodynamic resistance as a mechanical fingerprint of cells. [Preview Abstract] |
Monday, March 21, 2011 8:12AM - 8:24AM |
A9.00002: Bio-inspired artificial iriodphores based on capillary origami Supone Manakasettharn, J. Ashley Taylor, Tom Krupenkin Many marine organisms have evolved complex optical mechanisms of dynamic skin color control that allow them to drastically change their visual appearance. In particular, cephalopods have developed especially effective dynamic color control mechanism based on the mechanical actuation of the micro-scale optical structures, which produce either variable degrees of area coverage by a given color (chromatophores) or variations in spatial orientation of the reflective and diffractive surfaces (iridophores). In this work we describe bio-inspired artificial iridophores based on electrowetting-controlled capillary origami. We describe the developed microfabrication approach, characterize mechanical and optical properties of the obtained microstructures and discuss their electrowetting-based actuation. The obtained experimental results are in good agreement with a simple theoretical model based on electrocapillarity and elasticity theory. The results of the work can enable a broad range of novel optical devices. [Preview Abstract] |
Monday, March 21, 2011 8:24AM - 8:36AM |
A9.00003: Micropipette as Coulter counter for submicron particles Yauheni Rudzevich, Tony Ordonez, Grant Evans, Lee Chow Coulter counter based on micropipette has been around for several decades. Typical commercial Coulter counter has a pore size of 20 $\mu $m, and is designed to detect micron-size blood cells. In recent years, there are a lot of interests in using nanometer pore size Coulter counter to detect single molecule and to sequence DNA. Here we describe a simple nanoparticle counter based on pulled micropipettes with a diameter of 50 -- 500 nm. Borosilicate micropipettes with an initial outer diameter of 1.00 mm and inner diameter of 0.5 mm are used. After pulling, the micropipettes are fire polished and ultrasound cleaned. Chlorinated Ag/AgCl electrodes and 0.1 M of KCl solution are used. The ionic currents are measured using an Axopatch 200B amplifier in the voltage-clamp mode. Several types and sizes of nanoparticles are measured, including plain silica and polystyrene nanospheres. The results will be discussed in terms of pH values of the solution and concentrations of the nanoparticles. [Preview Abstract] |
Monday, March 21, 2011 8:36AM - 8:48AM |
A9.00004: ABSTRACT WITHDRAWN |
Monday, March 21, 2011 8:48AM - 9:00AM |
A9.00005: On demand fusion and triggering of confined chemical reactions in femtoliter volume aqueous droplets controlled by interfacial tension Pat Collier, Seung-Yong Jung, Scott Retterer Droplet-based microfluidic platforms offer many opportunities to confine chemical and biochemical reactants in discrete ultrasmall reaction volumes, and investigate the effects of increased confinement on reaction dynamics. Current state-of-the-art microfluidic sampling strategies for creating ultrasmall reaction volumes are predominately steady-state approaches, which result in difficulty in trapping reacting species with a well-defined time-zero for initiation of biochemical reactions in the confined space. This talk describes stepwise, on-demand generation and fusion of femtoliter aqueous droplets based on interfacial tension. Sub-millisecond reaction times from droplet fusion were demonstrated, as well as a reversible chemical toggle switch based on alternating fusion of droplets containing acidic or basic solution, monitored with the pH-dependent emission of fluorescein. [Preview Abstract] |
Monday, March 21, 2011 9:00AM - 9:12AM |
A9.00006: Acoustic actuation and sorting of droplets and cells at ultrahigh rates in microfluidics Thomas Franke, Lothar Schmid, Susanne Braunmueller, Achim Wixforth, David A. Weitz We direct the motion of droplets in microfluidic channels using a surface acoustic wave device. This method allows individual drops to be directed along separate microchannel paths at high volume flow rates, which is useful for droplet sorting. The same principle can be applied for biological cell sorting which operates in continuous flow at high sorting rates. The device is based on a surface acoustic wave cell-sorting scheme and combines many advantages of fluorescence activated cell sorting (FACS) and fluorescence activated droplet sorting (FADS) in microfluidic channels. It is fully integrated on a PDMS device, and allows fast electronic control of cell diversion. We direct cells (HaCaT, MV3 melanoma, fibroblasts) by acoustic streaming excited by a surface acoustic wave. The device underlying principle works without additional enhancement of the sorting by prior labeling of the cells with responsive markers such as magnetic or polarizable beads. We have combined the acoustic device successfully with a laser based fluorescence detection system and demonstrate sorting of fluorescent labeled drops at rates of several kHz without any false sorting. [Preview Abstract] |
Monday, March 21, 2011 9:12AM - 9:24AM |
A9.00007: Microfluidic mixing using an array of superparamagnetic beads Wenbin Mao, Zhengchun Peng, Peter J. Hesketh, Alexander Alexeev We present a combined numerical and experimental study on the dynamics of superparamagnetic beads in a microfluidic channel, wall of which is decorated with an array of stationary magnetic disks. When exposed to a rotating magnetic field, the beads circulate around the magnetic disks. We conduct experiments with micrometer-sized supeparamagnetic beads and use a numerical method that is based on the lattice Boltzmann model to examine the dynamics of this microfluidic system. We isolate the conditions in which beads exhibit stable periodical motion around magnetic disks and probe the effect of microchannel flow on the bead dynamics. We demonstrate that the fluid circulations created by rotating beads can be exploited for microfluidic mixing, thereby offering a new approach for designing highly-efficient active microfluidic mixers. [Preview Abstract] |
Monday, March 21, 2011 9:24AM - 9:36AM |
A9.00008: Separating Magnetically Labeled and Unlabeled Biological Cells within Microfluidic Channels Tom Byvank, Greg Vieira, Brandon Miller, Bo Yu, Jeffrey Chalmers, L. James Lee, R. Sooryakumar The transport of microscopic objects that rely on magnetic forces have numerous advantages including flexibility of controlling many design parameters and the long range magnetic interactions generally do not adversely affect biological or chemical interactions. We present results on the use of magnetic micro-arrays imprinted within polydimethylsiloxane (PDMS) microfluidic channels that benefit from these features and the ability to rapidly reprogram the magnetic energy landscape for cell manipulation and sorting applications. A central enabling feature is the very large, tunable, magnetic field gradients ($>$ 10$^{4}$ T/m) that can be designed within the microfluidic architecture. Through use of antibody-conjugated magnetic microspheres to label biological cells, results on the transport and sorting of heterogeneous cell populations are presented. The effects of micro-array and fluid channel design parameters, competition between magnetic forces and hydrodynamic drag forces, and cell-labeling efficiency on cell separation are discussed. [Preview Abstract] |
Monday, March 21, 2011 9:36AM - 9:48AM |
A9.00009: Coating microchannels to improve Field-Flow Fractionation Tyler N. Shendruk, Gary W. Slater We propose a selective-steric-mode Field-Flow Fractionation (ssFFF) technique for size separation of particles. Grafting a dense polymer brush onto the accumulation wall of a microchannel adds two novel effects to FFF: the particles must pay an entropic cost to enter the brush and the brush has a hydrodynamic thickness that shifts the no-slip condition. For small particles, the brush acts as a low-velocity region, leading to chromatographic-like retention. We present an analytical retention theory for small but finite-sized particles in a microchannel with a dense Alexander brush coating that possesses a well-defined hydrodynamic thickness. This theory is compared to a numerical solution for the retention ratio given by a flow approximated by the Brinkman equation and particle-brush interaction that is both osmotic and compressional. Large performance improvements are predicted in several regimes. Multi-Particle Collision simulations of the system assess the impact of factors neglected by the theory such as the dynamics of particle impingement on the brush subject to a flow. [Preview Abstract] |
Monday, March 21, 2011 9:48AM - 10:00AM |
A9.00010: Digital Flow Control of Electroosmotic Pump: Onsager Coefficients and Interfacial Parameters Determination Zuli Xu, Jianying Miao, Ning Wang, Ping Sheng Electroosmosis (EO) and streaming potential (SP) are two complementary electrokinetic processes related by the Onsager relation. In particular, electroosmotic pump (EOP) is potentially useful for a variety of engineering and bio-related applications. By fabricating samples consisting of dry-etched cylindrical pores (50 $\mu $m in length and 3.5 $\mu $m in diameter) on silicon wafers, we demonstrate that the use of digital control via voltage pulses can resolve the flow regulation and stability issues associated with the EOP, so that the intrinsic characteristics of the porous sample medium may be revealed. Through the consistency of the measured electroosmosis and the streaming potential coefficients as required by the Onsager relation, we deduce the zeta potential and the surface conductivity, both physical parameters pertaining to the liquid-solid interface. [Preview Abstract] |
Monday, March 21, 2011 10:00AM - 10:12AM |
A9.00011: AC electrophoretic effect in inhomogeneous electrical field: potentials for single molecule trapping Weihua Guan, Jae Hyun Park, Predrag Krstic, Mark Reed In micro-fabricated fluidic devices, we have experimentally observed trapping of objects in the supposed unallowed positive dielectrophoresis (pDEP) region. This `anomalous' trapping behavior motivates us to investigate the missing contributions in the trapping dynamics. We present here a study on overlooked aspects of alternating current (AC) electrokinetics-AC electrophoretic (ACEP) phenomena. The dynamics of a particle with both polarizability and net charges in an \textit{inhomogeneous} AC electric trapping field are investigated. It is found that either electrophoretic (EP) or dielectrophoretic (DEP) effects can dominate the trapping dynamics, depending on experimental conditions. A dimensionless parameter is developed to predict the relative strength of EP and DEP effect. Contrary to conventional thought, an ACEP trap is feasible for charged particles in `salt-free' or low salt concentration solutions. In contrast to DEP traps, an ACEP trap favors the down scaling of particle size. We anticipate that this feature will allow the confinement of single nanometer-sized objects or macromolecules. [Preview Abstract] |
Monday, March 21, 2011 10:12AM - 10:24AM |
A9.00012: Making robust electrowetting processes: dielectric breakdown and satellite droplets Greg Randall, Brent Blue For over ten years, charge-related wetting phenomena such as electrowetting or dielectrophoresis have been used to manipulate individual liquid droplets on grids of patterned electrodes. Many proof-of-principle droplet actuations have been shown, however some physics-based problems are complicating this technology's move to industry. These problems include: breakdown of a device's dielectric coating at field strengths lower than anticipated and generation of satellite droplets from the primary droplet's surface. We use atomic layer deposition (ALD) to fabricate high-quality dielectric layers required for robust droplet electrowetting and generate operating plots for several dielectric materials. Using scanning electron microscopy and X-ray spectroscopy, we study damage and ionic penetration into the device's dielectric layer. Using video and current measurements, we examine the physics of satellite droplet generation. We apply these findings to engineer a microfluidic process to mass produce inertial fusion energy targets. [Preview Abstract] |
Monday, March 21, 2011 10:24AM - 10:36AM |
A9.00013: Introducing the Hybrid Free Surface Microfluidics for Gas Sensing Meysam Barmi, Carl Meinhart Free-Surface MicroFluidics (FSMF) have recently received much attention for their applications especially their ability for airborne chemical detection [Piorek, PNAS 2007]. Due to their sensitivity to the ambient condition and possibility of contamination, hybrid configuration is introduced to perform the measurement more accurately. The hybrid free surface microfluidics are combination of free surface and closed surface microfluidics. The gas is absorbed by the working fluid through a small opening on the microchannel and transported to the closed surface reaction chamber to carry out the measurements. The working fluid is transported by surface tension and regulated by temperature-controlled evaporator at the outlet. The microchannels are fabricated on Silicon substrates with built-in Ti/Pt electrodes to measure the conductivity of the working fluid before and after the gas absorption to find the concentration of the absorbed gas. It proves that the hybrid free surface microfluidics are appropriate for gas sensing and the minimum exposing time and required opening size are calculated. Numerical simulations are carried out by COMSOL multiphysics. Navier-Stokes equations along with the mass transport with reaction are solved simultaneously to find the correlation between vapor pressure of the surrounding gas and concentration of the absorbed gas. [Preview Abstract] |
Monday, March 21, 2011 10:36AM - 10:48AM |
A9.00014: Motion-Reversal Transitions in Self-Assembled Colloidal Walkers Stephanie Moran, Charles Sing, Alfredo Alexander-Katz Nature has created a variety of designs in order to move fluids and transport objects within living organisms. At microscopic scales (in the region of micrometers) two motifs are common: flagella and cilia. Within the cell, however, molecular motors with nanometer dimensions transport small sized vesicles. Here, we describe a novel approach that combines properties from two systems: cilia and molecular motors, to create self-assembled colloidal walkers. These walkers are assembled by superparamagnetic beads in the presence of a rotating homogeneous magnetic field, and are able move in a given direction due to the presence of surfaces which provide an effective friction. The motion is somewhat reminiscent of a person doing cartwheels on ice, where the friction is not high enough to avoid slip, but overall one can attain directed motion in one direction. Interestingly, the motion of the center of mass of these walkers is a non-monotonic function along one cycle of revolution. By exploiting this non-monotonicity, we show that motion reversal is possible in this systems if one carefully controls the friction properties of the surface as well as the confining ``gravitational'' field that maintains the beads near the surface. Our results our important in understanding the motion of micron scale organisms and may be useful in the development of virtual microfluidic platforms. [Preview Abstract] |
Monday, March 21, 2011 10:48AM - 11:00AM |
A9.00015: Optimizing Nanopore Surface Properties for High-Efficiency Water Desalination David Cohen-Tanugi, Jeffrey Grossman As water resources worldwide become rapidly scarcer, it is becoming increasingly important to devise new techniques to obtain clean water from seawater. At present, water purification technologies are limited by costly energy requirements relative to the theoretical thermodynamic limit and by insufficient understanding of the physical processes underlying ion filtration and fluid transport at the molecular scale. New advances in computational materials science offer a promising way to deepen our understanding of these physical phenomena. In this presentation, we describe a new approach for high-efficiency water desalination based on surface-engineered porous materials. This approach is especially relevant for promising technologies such as nanofiltration and membrane distillation, which offers promising advantages over traditional desalination technologies using mesoporous membranes that are only permeable to pure water vapor. More accurate molecular modeling of mesoporous and nanoporous materials represents a key step towards efficient large-scale treatment of seawater. Results regarding the effect of pore properties (surface texture, morphology, density, tortuosity) on desired performance characteristics such as ion selectivity, maximal water flux and energy requirements will be presented. [Preview Abstract] |
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