Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V37: Focus Session: Microfluidic Physics IV: Particles, Drops, and Mixing |
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Sponsoring Units: DFD Chair: Darren Link, Rain Dance Technologies Room: LACC 512 |
Thursday, March 24, 2005 11:15AM - 11:27AM |
V37.00001: Formation of Droplets of Different Compositions in Microfluidic Channels and Applications to Protein Crystallization Bo Zheng, Joshua Tice, Rustem Ismagilov This presentation reports characterization and applications of formation of nanoliter droplets of different compositions in microfluidic channels. In this method several different aqueous streams were injected into a flow of an immiscible oil to form droplets in the microfluidic channels. The conditions required to form the steady flow of the droplets in a microchannel were characterized as a function of the capillary number (Ca) and water fraction. Four flow regimes that were defined by Ca and water fraction were observed and characterized. The compositions of the adjacent droplets were found lineally related and allowed indexing compositions of droplets. We also demonstrated applications of these phenomena by conducting protein crystallization in droplets in the microchannels under the condition of microbatch and vapor diffusion, with characterization by on-chip x-ray diffraction. [Preview Abstract] |
Thursday, March 24, 2005 11:27AM - 11:39AM |
V37.00002: Enzyme-Inhibitor Assay Using Microdroplets Keunho Ahn, Darren Link, Andrew Griffiths, David Weitz We demonstrate a microdroplet enzyme-inhibitor assay based on a microfluidic device. Alternating microdrops of a buffer solution containing enzyme and inhibitor reagents and perfluorodecalin (PFD) are formed in a continuous flow of hexadecane. PFD microdrops act as spacers between the reagent microdrops and keep them from coalescence. A substrate solution is then introduced into the reagent microdrops after homogeneous mixing of the enzyme and the inhibitor. The fluorescence depends on the amount of product from the reaction between the substrate and the reagents. Reaction rates were measured with long time exposure using fluoresce microscopy. We also checked probability of inter-droplet contamination of reagents by measuring the fluorescence of each microdrops using an image intensified fast camera. [Preview Abstract] |
Thursday, March 24, 2005 11:39AM - 11:51AM |
V37.00003: Optical Detection and Magnetic Manipulation of Drops in Microfluidic Devices Charles Kerbage, Keunho Ahn, Tom Hunt, Robert Westervelt, David Weitz We demonstrate an integrated magneto-optic microfluidic device for drop detection and sorting. Optical detection of water drops formed in a continuous oil phase flow is performed using optical fibers which are integrated into the channels of the PDMS (Polydimethylsiloxane) based microfluidic device. The size and the velocity of the drops can be determined by measuring the transmission intensity as a function of time. We also show that such a device can be used to detect fluorescent materials introduced in the drop itself. Moreover, introducing nano-scale magnetic particles into the water drops allows for drop sorting by means of a magnetic field gradient. This magnetic field is generated through thin film permalloy integrated into the device itself and tuned by an external coil. We show that the sorting depends on the magnetic field gradient, material composite and volume fraction of the magnetic material in the drops. [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:03PM |
V37.00004: Nonlinear phenomena in two-phase flows in microfluidic systems Patrick Tabeling, Herve Willaime, Valessa Barbier, Laure Menetrier Experiments with drops produced in soft (PDMS) microfluidic systems show interesting behavior that underline the importance of nonlinear phenomena, which may be understood using dynamical systems theory and be exploited for practical purposes. First, water droplets are driven in oil along a main channel, across which a fraction of the flow is sucked off. Droplet sizes and velocites are measured by tracking interface velocities using PIV. In these experiments, there are two regimes: when the derivated flow-rate is small, the continuous phase can be sucked off without breaking up the droplets. When more flow is sucked off, the droplets break up to generate two droplets, one flowing in the main channel, the other in the derivation. Our theoretical explanation for the break-up condition fits remarkably well with hundreds of different experimental conditions. Second, we placed actuators close to a T junction where water droplets are produced in an oil stream. When the coupling between the flow and the actuation is large, the actuation accurately imposes the droplet sizes and emission frequency. More complex behaviour occurs when the coupling is weaker: frequency locking states and quasiperiodic regimes, organized into Arnold tongues and devil staircases. This behavior is captured by the circle map, a standard model for nonlinear coupling between an external forcing (the actuator) and an oscillating process (drop formation). [Preview Abstract] |
Thursday, March 24, 2005 12:03PM - 12:15PM |
V37.00005: Flow and clogging of colloids in microfluidic devices Hans Wyss, Daniel Blair, David Weitz We study the flow and clogging of colloidal particles in porous materials by using microfluidid devices. Our experiments aim at getting a better understanding of the mechanisms that lead to flow-driven jamming of colloids in porous materials. The dynamics of the clogging process is investigated in terms of the dependence on particle size, colloid volume fraction, pressure gradient, as well as particle-particle interactions. We present results that show the influence of these parameters on the dynamics and nature of the clogging process. [Preview Abstract] |
Thursday, March 24, 2005 12:15PM - 12:27PM |
V37.00006: Statistical reconstruction of velocity profiles for nano-PIV Peter Mucha, Christel Hohenegger Velocities and Brownian effects at nano-scales near microchannel walls have been measured by evanescent-wave illumination techniques [R. Sadr et al., J. Fluid Mech. 506, 357-367 (2004)]. Assuming moblility of spherical particles is dominated by hydrodynamic interaction between the particle and wall, and that fluid velocity is directed in one in-plane direction, the out-of-plane dependence of mobility and velocity are clearly coupled. We investigate such systems computationally, using a Milstein algorithm that is both weak- and strong-order 1. We demonstrate that a maximum likelihood algorithm can reconstruct the out-of-plane velocity profile given known mobility dependence and ideal particle identification. We further test this reconstruction for measurements obtained by cross-correlation techniques applied to windowed simulation data. Application to physical data is proposed via analytical results about the influence of Brownian motion in this setting on the correlation peak, combined with simulation results to help identify nearly-optimal parameters. [Preview Abstract] |
Thursday, March 24, 2005 12:27PM - 12:39PM |
V37.00007: Flow-induced currents in nanotubes: a Brownian dynamics approach Sriram Ramaswamy, Moumita Das, Ajay Sood, Garani Ananthakrishna Motivated by recent experiments reporting that carbon nanotubes immersed in a flowing fluid displayed an electric current and voltage, we numerically study the behaviour of a collection of Brownian particles in a channel, in the presence of a flow field applied on similar but slower particles in a wide chamber in contact with the channel. For a suitable range of shear rates, we find that the flow field induces a unidirectional drift in the confined particles, and is stronger for narrower channels. The average drift velocity initially rises with increasing shear rate, then shows saturation for a while, thereafter starts decreasing, in qualitative agreement with recent theoretical studies (cond-mat/0407803) based on Brownian drag and ``loss of grip''. Interestingly, if the sign of the interspecies interaction is reversed, the direction of the induced drift remains the same, but the flow-rate at which loss of grip occurs is lower, and the level of fluctuations is higher. [Preview Abstract] |
Thursday, March 24, 2005 12:39PM - 12:51PM |
V37.00008: Electro-thermal-fluidic behavior near dielectrophoretic micro-constrictions Chia-Fu Chou, Venkatraman Ramamurthy, Frederic Zenhausern We present a detailed study of the electro-thermal-fluidic behavior in the close proximity of single dielectric micro-constrictions using multiphysics computational fluidics modules in the device level. These micro-constrictions may be used to perform dielectrophoresis for molecular trapping and bioparticle separation. When an external electric potential is applied across such constrictions defined in microfluidic channels filled with ionic buffers, the electric field and current will be focused at these dielectric constrictions. The focused nonuniform local field gives rise to the dielectrophoretic effect to dielectric particles near the constrictions. On the other hand, the focused local current may generate Joule heating which, in turn, induces temperature gradient that causes convective flow to potentially disturb or even frustrate the dielectrophoretic effect. However, the raised local temperature and the induced local flow field may provide another degree of freedom for stringency control, say in a hybridization assay, and the mixing effect, respectively. We will discuss the various aspects of these effects and their potential applications. [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:03PM |
V37.00009: Electrothermal Stirring for Heterogeneous Assays Carl Meinhart, Marin Sigurdson AC Electrokinetic effects are exploited to develop tools for improving response of lab-on-a-chip biosensors through augmenting transport in the sensor. This is applicable to immunoassays as well as DNA hybridization, and to a variety of formats, from microfluidic to microarray. AC electric fields in a microchannel or microcavity can generate forces on both the fluid itself, through AC electroosmosis and electrothermal forces, and on suspended particles through dielectrophoresis (DEP). The appropriate combination of these forces can concentrate or sort particles, or mix solutions. Here we describe the use of electrothermally generated flow in a microcavity to circulate suspended analyte past immobilized ligand to increase binding opportunities. A finite element model predicts the electrothermally generated force on the fluid, the subsequent fluid circulation, and the resulting increase in binding rate - up to a factor of eight for a heterogeneous immunoassay. The fluid velocity solution has been qualitatively corroborated by micro-PIV experiments using fluorescent tracing particles. The increase in binding rate has been tested through the reaction of fluorescently tagged streptavidin with biotinylated glass. Experimental results for increasing binding rate with electrothermal stirring are encouraging. [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:15PM |
V37.00010: Optimal mixing by chaotic advection in two-dimensional droplet microfluidics Reza Miraghaie, Ali Nadim The problem of mixing within microfluidic droplets bounded by parallel plates is studied numerically. Flow in the droplet can be induced by azimuthally varying the surface tension on the non-solid boundary of the droplet, or by applying an external tangential stress (e.g., "wind stress") at that boundary. Periodic switching of such boundary conditions results in stretching and folding of the streamlines within the droplet. Due to periodic changes of the flow direction and crossing of the streamlines, chaotic advection can be generated for certain periods of switching. Lyapunov exponents as well as Poincare$\acute{}$ maps for several periodic switching scenarios are presented. Optimal mixing protocols in terms of a combination of periods and total number of switching events are also discussed. As an alternative to classical measures, the first and second moments of a swarm of passive particles, initially concentrated in a small region in the drop, are tracked to quantify the mixing quality inside the droplet. [Preview Abstract] |
Thursday, March 24, 2005 1:15PM - 1:27PM |
V37.00011: Formation of nanoscale structures through driven flow of nano-particles in microchannels Rolf Verberg, Julia Yeomans, Anna Balazs Using a computational model, we consider an isothermal binary fluid that contains tracer particles, which are advected by the fluid within a microchannel. The flow is modeled directly using a Lattice Boltzmann algorithm for a binary fluid. We then introduce nanoparticles that travel along trajectories that obey a stochastic differential equation, such that the concentration of nanoparticles in the bulk obeys a standard convection-diffusion equation. To simulate the affinity of the nanoparticles to one of the components, we modified the stochastic diffusion equation in order to include a drift term that contains the gradient of the order parameter. Reactions with the solid boundaries will be incorporated by introducing a reaction probability when the particle's trajectory crosses the solid-fluid surface. The results provide guidelines for creating microfluidic devices with surfaces that contain well-controlled spatial patterns on the nanometer to micron range. [Preview Abstract] |
Thursday, March 24, 2005 1:27PM - 1:39PM |
V37.00012: Mixing of Fluids in Nanochannels Eric Oliver, Gary W. Slater We report on numerical simulations of fluid mixing in nanochannels with widths on the order of (or less than) 30 nanometers. Previous work in this field has concentrated on modelling the system with the macroscopic equations of fluid dynamics; however, onsuch length scales considered here a continuum approximation may no longer be valid. We address this issue by resorting to a Molecular Dynamics model where we explicitly include two solvents. The scale of our channels and the forces involved implies a very low Reynolds number and hence, the flow is laminar and we must rely on diffusion to mix the fluids. By patterning the walls of the channel such that, mimicking a chemical pattern, they repel or attract different species of the unmixed fluids or by physically modifying the geometry of the channel we can overcome the limitations of very slow diffusive mixing. These modifications disrupt the laminar flow profiles, thus introducing a mechanism that can accelerate mixing in much the same way that turbulence does in non-laminar flow. The simulations performed have allowed us to follow the mixing molecule by molecule, thus providing us with a complete picture of the mixing process. [Preview Abstract] |
Thursday, March 24, 2005 1:39PM - 1:51PM |
V37.00013: The Shear Superposition Micromixer : An Efficient and Fast Micromixer Frederic Bottausci, Caroline Cardonne, Igor Mezic, Carl Meinhart In the current paper we present new experimental and numerical results showing high efficiency mixing procedures for an active shear superposition micromixer (SSM). This micromixer consists of a main mixing channel where unmixed fluids are perturbed by jet flows emanating from a series of transverse channels. Mixing of two fluids is achieved using the kinetic of the side jet flows. mixing process is studied numerically and experimentally using flow visualizations techniques. The numerical simulations are performed for the 3-D flow using Fluent. The parameters (flow rate, frequency, and amplitude of oscillations) are accurately controlled using Labview. We quantify, numerically and experimentally, the degree of mixing achieved using the Mixing Variance Coefficient (MVC). We present some flow properties, optimization of the mixing for the parameters mentioned above and some biological applications. [Preview Abstract] |
Thursday, March 24, 2005 1:51PM - 2:03PM |
V37.00014: Coherent Particle Scattering Eric Cummings The concept and modeling of coherent particle scattering (CPS) are presented. CPS is transport that arises from interactions of molecules with periodic, spatially non-uniform fields. CPS supports a variety of novel micro- and nanofluidic technologies including coherent nonlinear chromatography (CNC). CNC is a novel separation technique that promises ultra-rapid sorting of particles and molecules. Employing interactions with field nonuniformities in the bulk suspending fluid, CNC avoids chromatography's reliance on repeated diffusion of particles to and from surfaces and can theoretically separate protein-scale molecules greater than 100,000X faster than conventional chromatography. CPS transport is a particle-specific secondary flow produced when particles interact with periodic field nonuniformities. These nonuniformities can be created by macromolecular self-assembly or lithographic patterning. The spatial arrangement of non-uniformities and type of applied field controls the nature of CPS. The term ``coherent'' in CPS and CNC refers to spatially coherent patterning that can rapidly drive and linearly ``amplify'' transport effects. Specific examples of CPS based on dielectrophoresis, electrokinesis, and entropic effects are detailed. [Preview Abstract] |
Thursday, March 24, 2005 2:03PM - 2:15PM |
V37.00015: Mixing it up for Protein Crystallization Carl Hansen, Morten Sommer, James Berger, Stephen Quake In the post-genomic era, X-ray crystallography has emerged as the workhorse of large-scale structural biology initiatives that seek to understand protein function and interaction at the atomic scale. Despite impressive technological advances in X-ray sources, phasing techniques, and computing power, the determination of protein structure continues to be severely hampered by the difficulties in obtaining high-quality protein crystals. Emergent technologies utilizing microfluidics now have the potential to solve these problems on several levels. We will present two microfluidic devices that have been shown to dramatically improve protein crystallization. The first is a formulation device which allows for the rapid combinatorial mixing of reagents to systematically explore protein solubility behavior. \textit{A priori} solubility mapping allows for the rational design of optimal crystallization screens that are tailored to a specific target. A second screening device allows for massively parallel sample processing while exploiting the properties of mass transport manifest at the micron scale to ensure slow and efficient mixing kinetics that are difficult to achieve in macroscopic reactors. [Preview Abstract] |
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V37.00016: Dielectrophoretic processes in a linear electrode array Frederic Bottausci, Yanting Zhang, Igor Mezic We present experiments on dielectrophoretic (DEP) separation and trapping performed in a titanium-based microchannel linear electrode array. The device is designed to allow for effects driven by nonhomogeneities in electric-field magnitude driven (p-DEP and n-DEP) and nonhomogeneities in electric-field phase-driven (traveling wave) DEP. It is also capable of inducing multi-frequency DEP, in contrast with most of the previous, single-frequency, designs. We show that fluid flow effects are substantial and can affect the particle motion in a positive (enhanced trapping) and negative (trapping when separation is desired) way. We discuss the advantages of multi-frequency dielectrophoretic handling of bioparticles. [Preview Abstract] |
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