Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session L37: Microfluidic Physics I: Applications, Separations, Polymers |
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Sponsoring Units: DFD Chair: Todd Squires, Caltech Room: LACC 512 |
Tuesday, March 22, 2005 2:30PM - 3:06PM |
L37.00001: Biological Large Scale Integration Invited Speaker: The integrated circuit revolution changed our lives by automating computational tasks on a grand scale. My group has been asking whether a similar revolution could be enabled by automating biological tasks. To that end, we have developed a method of fabricating very small plumbing devices chips with small channels and valves that manipulate fluids containing biological molecules and cells, instead of the more familiar chips with wires and transistors that manipulate electrons. Using this technology, we have fabricated chips that have thousands of valves in an area of one square inch. We are using these chips in applications ranging from screening to structural genomics to ultrasensitive genetic analysis. However, there is also a substantial amount of basic physics to explore with these systems the properties of fluids change dramatically as the working volume is scaled from milliliters to nanoliters! [Preview Abstract] |
Tuesday, March 22, 2005 3:06PM - 3:18PM |
L37.00002: Multistream Laminar Flow: From a challenge in mixing to membraneless fuel cells and microreactors for cofactor regeneration. Paul Kenis Over the last decade a wide variety of research efforts in microchemical systems, in which fluid flow is laminar, has developed. The original challenge of mixing in the absence of turbulence in this laminar flow regime has been overcome through various technical approaches including zig-zag or serpentine-shaped channels (Branebjerg, Beebe \textit{et al.}), lamination (e.g. Manz, Jensen \textit{et al.}), hydrodynamic focusing (Austin \textit{et al}.), and integrated herringbone features (Stroock \textit{et al.}). Others have grasped the opportunity to utilize multistream laminar flow for example for a T-sensor for blood analysis (Weigl \textit{et al.}) and in microfabrication or cell studies (Whitesides \textit{et al.}). This presentation will highlight the development of (i) a membraneless fuel cell, and (ii) a microreactor for cofactor regeneration that utilize multistream laminar flow. Various performance-determining characteristics and engineering improvements will be discussed. [Preview Abstract] |
Tuesday, March 22, 2005 3:18PM - 3:30PM |
L37.00003: Continuous separation of human blood components through deterministic John Davis, David Inglis, James Sturm, Robert Austin Using a microfluidic device, the separation of red and white blood cells from their native blood plasma has been demonstrated. The device takes advantage of the asymmetric bifurcation of laminar flow around obstacles. This asymmetry creates a size dependent deterministic path through the device which depends on particle size.. All components of a given size follow equivalent migration paths, leading to high resolution. One-micron diameter fluorescent polystyrene beads were added to a mixture of blood to mark the flow patterns The blood WAS sorted into three distinct stream, consisting of 1 micron beads, red blood cells and white blood cells, respectively. [Preview Abstract] |
Tuesday, March 22, 2005 3:30PM - 3:42PM |
L37.00004: DNA Dynamics in a Microfluidic Device Patrick Doyle, Greg Randall We present a general analysis and experimental study of DNA deformation in complex electric field gradients created inside microfluidic devices. Double-stranded DNA, free of its native proteins, is a ``model polyelectrolyte'' because reliably monodisperse samples can be attained with a uniform negative charge. Furthermore, the molecules can be stained with fluorescent probes and their motion directly observed by single-molecule microscopy. Large DNA coils move at a size-independent velocity in uniform electric fields and can significantly deform in electric field gradients. Because the electrophoretic velocity field is a potential field, local deformation of DNA in any electric field gradient is pure elongation, quantified by a strain rate and axes of extension and compression. From this analysis we construct the electrophoretic Deborah number, the non-dimensionless parameter that governs deformation of a polyelectrolyte in an electric field gradient. Over a range of Deborah numbers, we experimentally study single-molecule DNA deformation dynamics in model geometries such as a sharp contraction and a non-conducting cylinder. Analogous to deformation in hydrodynamic velocity gradients, we report both critical coil-stretch behavior and a strong dependence on the zero-strain DNA configuration. Furthermore, we apply these results to DNA mapping and DNA sequencing innovations. [Preview Abstract] |
Tuesday, March 22, 2005 3:42PM - 3:54PM |
L37.00005: DNA transport in magnetic arrays Nicolas Minc, Jean-Louis Viovy, Kevin Dorfman We present a experimental/theoretical study of the microfluidic electrophoresis of long DNA in an innovative matrix consisting of a hexagonal array of magnetic bead columns. Using videomicroscopy, we examined the motion of long T4 DNA (169 kbp) under a wide range of array densities and electric fields. By tracking the motion of many individual DNA molecules, we computed (i) the distribution of collision times; (ii) the collision probability; (iii) and the mean passage time through the viewing area. Based on our single molecule results, we will present a model focusing upon the non-Markovian characteristic of the transport in the array. DNA transport is represented by a Scher-Lax walk, where each molecule undergoes cycles of collisions. The model qualitatively and quantitatively captures the main features on separation in microarrays. As this work represents the first systematic theoretical study of dispersion in these devices, it is a significant step towards a detailed understanding of realistic miniaturized separation systems. [Preview Abstract] |
Tuesday, March 22, 2005 3:54PM - 4:06PM |
L37.00006: Shear Flow Induced Chain Migration in Nanochannels Rajesh Khare, Juan de Pablo, Michael Graham Flow induced migration of polymer chains in nanochannels has potential applications for DNA separation and sequencing operations. In this work, we use molecular dynamics (MD) simulation to investigate the effect of shear flow on chain migration effects. In particular, flow behavior of a dilute polymer solution is studied using a bead-spring model of the polymer chain and a coarse grained model of the solvent. A purely repulsive Lennard-Jones potential is used to represent all of the intermolecular interactions in the system. The diffusion and hydrodynamic effects are determined by the intermolecular interactions in such a model system. Our results are used to identify the flow conditions that cause chain migration both towards and away from the channel walls. The MD simulation results are compared with the experimental data and the predictions from recent Brownian dynamics simulation and kinetic theory. Our results are used to assess the relative importance of the wall hydrodynamics and chain diffusion on the chain migration effects. [Preview Abstract] |
Tuesday, March 22, 2005 4:06PM - 4:18PM |
L37.00007: The interplay of fluid inertia and fluid viscoelasticity in complex microfluidic flows Lucy Rodd, David Boger, Justin Cooper-White, Gareth McKinley The interplay of inertia and elasticity is explored by investigating the extensional flow behavior of dilute solutions of high molecular weight polyethylene oxide (PEO) flowing through micro-fabricated contractions. The small length scales and high deformation rates inherent to microfluidic devices make it possible to induce strong non-Newtonian effects even in dilute polymer solutions. By preparing solutions with varying solvent viscosities, it is possible to tune the ratio of elasticity to inertia at the same polymer concentration. The kinematics upstream of the contraction are characterized in terms of the strong viscoelastic enhancement of the corner vortex, which is accompanied by an increase in the pressure drop across the contraction plane that reaches a value in excess of 6 times the equivalent Newtonian pressure drop. The shape and maximum of the pressure drop/flowrate curve is found to be strongly dependent on the elasticity number, which is only a function of fluid properties and the size of the channel. This work provides valuable insight into the behavior of macromolecular fluids that are common in lab-on-a-chip processes, such as those containing proteins or dilute solutions of DNA. [Preview Abstract] |
Tuesday, March 22, 2005 4:18PM - 4:30PM |
L37.00008: Dynamics of DNA Molecules in Slit Microchannels Yeng-Long Chen, Hongbo Ma, Michael Graham, Juan de Pablo Microfluidic devices used for high accuracy/throughput bio- chemical analysis could potentially revolutionize genome analysis. As an example, DNA adsorption on microfluidic channel walls has been fruitfully exploited for single DNA restriction mapping. Brownian dynamics simulations accounting for full hydrodynamic interactions (including perturbations from the wall) are employed to investigate the dynamics of a single DNA molecule undergoing pressure-driven shear flow in rectangular channels of height comparable to and less than the radius of gyration of the molecule. Good agreement is found between theory predictions, simulation results, and experimental measurements. Further investigations explore how shear flow may be used to manipulate the chain conformation and transport properties. We also examine how predictions of the chain dynamics and conformation in confined environments are affected by the degree of coarse-graining in the simulated DNA molecule, and propose a novel approach to dynamically optimize the resolution of our models. [Preview Abstract] |
Tuesday, March 22, 2005 4:30PM - 4:42PM |
L37.00009: Brownian Dynamics Simulations of Polyelectrolyte Adsorption in Shear Flow Ajay Panwar, Satish Kumar The adsorption of polyelectrolytes onto charged surfaces often occurs in microfludic devices and can influence their operation. We employ Brownian dynamics simulations to investigate the effect of a simple shear flow on the adsorption of an isolated polyelectrolyte molecule onto an oppositely charged surface. The polyelectrolyte is modeled as a freely-jointed bead-rod chain where the total charge is distributed uniformly among all the beads, and the beads are allowed to interact with one another and the charged surface through screened Coulombic interactions. The simulations are performed by placing the chain some distance above the surface, and the adsorption behavior is studied as a function of the screening length. Specifically, we look at the components of the radius of gyration, normal and parallel to the adsorbing surface, as functions of the screening length, both in the absence and presence of the flow. We find that in the absence of flow, the chain lies flat and stretched on the adsorbing surface in the limit of weak screening, but attains free solution behavior in the limit of strong screening. In the presence of a shear flow, the chain orientation in the direction of the flow increases with increasing Weissenberg number over the entire range of screening lengths studied. We also find that increasing the strength of the shear flow leads to an increased contact of the chain with the surface compared to the case when no flow is present. [Preview Abstract] |
Tuesday, March 22, 2005 4:42PM - 4:54PM |
L37.00010: Flow-modulated ssDNA reaction in microchannels Thomas John, Igor Mezic We model the recombination of very-short-strand DNA in microchannels with the assumption that two complementary strands will combine only if they are close together in position as well as alignment. We describe this as a reaction-advection-diffusion system in position-orientation space with avenues for control in the form of velocity fields and external potentials. We prove that, without reaction, chaotic (uniformly and non-uniformly hyperbolic) flows can lead to simultaneous mixing of the strands and their alignment. We develop numerical simulation of the process showing that reaction is enhanced by chaotic mixing. We analyze the dynamics in a flow caused by an active (shear superposition) micro-mixer. [Preview Abstract] |
Tuesday, March 22, 2005 4:54PM - 5:06PM |
L37.00011: Electrophoresis of a Bead-rod Chain through a Narrow Slit: A Brownian Dynamics Study Satish Kumar, Ajay Panwar, Seung Ha Kim, Kyung Hyun Ahn, Seung Jong Lee We use two-dimensional Brownian dynamics simulations to study the electrophoresis of a bead-rod chain through a narrow slit. A constant electric field is assumed to act inside and outside the slit, and the total charge on the chain is distributed equally among all beads. We study the dependence of the polymer transit velocity on the chain length, slit dimensions (width-to-length ratio), and electric field strength. We find that for sufficiently narrow slits, the transit velocity increases non-linearly with the applied field for low field strengths, whereas it increases linearly for high field strengths. In the low-field-strength region and for sufficiently narrow slits, the transit velocity decreases rapidly for small chain lengths and then decreases slowly beyond a critical chain length. With increasing width of the slit, the decrease in velocity is observed to be more continuous, and becomes independent of chain length for large slits. These results show the sensitivity of the transit velocity vs. chain length relationship to the slit dimensions and electric field strength, and could be useful for microfluidic separations. [Preview Abstract] |
Tuesday, March 22, 2005 5:06PM - 5:18PM |
L37.00012: Electrokinetic Concentration of Charged Macromolecules In Nanoporous Media Brian J. Kirby, Daniel J. Throckmorton, Anup K. Singh We present a technique for concentrating proteins and other charged macromolecules in microfabricated systems through the manipulation of molecular electromigration at the interface between bulk flow in microchannels and flow through micro/nanoporous material with a bimodal pore distribution. The presence of a bimodal pore structure allows for the creation of metastable electrokinetic regions in areas of double layer overlap without the attendant electrokinetic pressure generation required for continuity in transitions between open microchannels and unimodal nanoporous structures. Directed experiments on the dependence of the observed phenomena on pH, macromolecule charge state, chemi- and electrosorption are supportive of a relatively simple model that defines the criteria that must be satisfied to observe macromolecule concentration. This technique has been applied to concentrate proteins by 2-3 orders of magnitude before pressure elution for analysis, and has been implemented with both polymeric and silicate materials in capillaries and in microfabricated glass microfluidic devices. [Preview Abstract] |
Tuesday, March 22, 2005 5:18PM - 5:30PM |
L37.00013: Continuous microfluidic immunomagnetic cell separation David Inglis, Robert Riehn, Robert Austin, James Sturm We present a continuous-flow microfluidic device that enables cell by cell separation of cells selectively tagged with magnetic nanoparticles. The cells flow over an array of microfabricated magnetic stripes, which create a series of high magnetic field gradients that trap the magnetically labeled cells and alter their flow direction. The process was observed in real-time using a low power microscope. The device has been demonstrated by the continuous flow separation of leukocytes from whole human blood. The dependence of the process on the flow rates and direction of flow with respect to the magnetic stripes will be described. [Preview Abstract] |
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