Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session MP: Microfluids: Mixing I |
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Chair: Jia Ou, University of Minnesota Room: Long Beach Convention Center 203A |
Tuesday, November 23, 2010 8:00AM - 8:13AM |
MP.00001: Breakup of particle clumps on liquid surfaces Sathish Gurupatham, M.M. Hossain, B. Dalal, I. Fischer, P. Singh, D.D. Joseph Although it is known that a clump of powder floating on a liquid surface breaks up to form a monolayer of particles on the surface, the mechanism by which this happens is not entirely understood. We show that when a floating clump comes in contact with the liquid surface particles on its outer periphery are pulled into the interface by the capillary force overcoming the cohesive forces that keep the clump together. Furthermore, the newly adsorbed particles move away from the clump which is a consequence of the fact that when a particle is adsorbed on to a liquid surface it causes a flow away from itself on the interface. This flow causes the newly adsorbed particles to not only move away from each other, but also away from the clump. Interestingly, when many particles are asymmetrically broken apart from the clump, the clump itself is moved away by the flow due to the newly adsorbed particles. [Preview Abstract] |
Tuesday, November 23, 2010 8:13AM - 8:26AM |
MP.00002: Electro-osmotic flows in rectangular cavities Viatcheslav Meleshko, Alexandre Trofimchuk, Alexandre Gourjii , Elina Bezym'yana The talk presents the results of investigation of the microfluidics mixing processes in a rectangular cavity flows induced by elctro-osmotic excitation. Enhanced mixing plays an important role in biological and chemical pharmaceutics analysis in microfluidics systems. Analytical solution is presented for the velocity field in the cavity under various electric potential distributions. The location of the periodic points in the flow are accurately established and the structure of stable and unstable manifolds is discussed. The optimal form of excitation is suggested in order to obtain most effective mixing regime in the cavity. The regular and chaotic regions are identified under various condition of excitation. Finally, we compare numerical and analytical solutions with the results of laboratory experiments for real microfluidic flows. [Preview Abstract] |
Tuesday, November 23, 2010 8:26AM - 8:39AM |
MP.00003: Targeting Complete Chaotic Mixing by Destabilizing Key Periodic Orbits in an Electro-osmotic Mixer Rodolphe Chabreyrie, Cristel Chandre, Pushpendra Singh, Nadine Aubry The ability to generate complete, or at least well spread, chaotic mixing is of great interest in numerous applications, especially microfluidics. For this purpose, we propose a strategy that allows us to quickly target the parameter values at which complete mixing occurs. The technique is applied to a time periodic, two-dimensional electro-osmotic flow with spatially and temporally varying Helmoltz-Smoluchowski slip conditions. The strategy consists of following the linear stability of some key periodic orbits in parameter space, particularly identifying bifurcation points at which such orbits become unstable. Poincar\'{e} maps, Lyapunov exponents and a box counting measure are all computed to validate the strategy. [Preview Abstract] |
Tuesday, November 23, 2010 8:39AM - 8:52AM |
MP.00004: Quantitative description of mixing in non-perturbative flows Radford Mitchell, Roman Grigoriev Studies of near-integrable fluid flows have identified two independent and complementary quantitative metrics which are needed to describe their mixing properties: (1) the mixing rate and (2) the size (and shape) of the mixed region. In the limit of weak perturbation away from integrability both metrics can be computed using multi-scale averaging theory. In this talk we show how these metrics can be computed in the non-perturbative regime using the formalism of Periodic Orbit Theory. Using a Taylor-Couette-like steady flow as an example, we show that the boundaries of the mixed region are formed by heteroclinic manifolds of periodic (or relative periodic) orbits while the mixing rate can be quantified using a weighted sum over a set of both periodic and relative periodic orbits. A similar description is expected to be valid for time-periodic flows and other geometries as well. [Preview Abstract] |
Tuesday, November 23, 2010 8:52AM - 9:05AM |
MP.00005: Characterization on performance of micromixer using DC-biased AC electroosmosis Bi-O Park, Simon Song An active micromixer using DC-biased AC-Electroosmosis (ACEO) is investigated to figure out the effects of design parameters on the mixing performance. The mixer consists of a straight microchannel, with a cross section of 60 x 100 $\mu $m, and gold electrode pairs fabricated in the microchannel. The design parameters include the number of electrode pairs, flow rate, DC-biased voltage, AC voltage and AC frequency. First, we found that a mixing index became 80{\%} 100 $\mu $m downstream of a single electrode pair with a length of 2 mm when applying a 25V$_{pp}$, 2.0 V$_{DC}$, 100 kHz sine signal to the electrodes. With decreasing AC frequency, the mixing index is affected little. But the mixing index significantly increases with increasing either DC-biased voltage or AC voltage. Also, we were able to increase the mixing index up to 90{\%} by introducing alternating vortices with multiple electrode pairs. Finally, we discovered that the mixing index decreases as the flow rate increases in the microchannel, and there is an optimal number of electrode pairs with respect to a flow rate. Detailed quantitative measurement results will be presented at the meeting. [Preview Abstract] |
Tuesday, November 23, 2010 9:05AM - 9:18AM |
MP.00006: Coherent structures in 3D viscous time-periodic flow J.G. Znaien, M.F.M. Speetjens, R.R. Trieling, H.J.H. Clercx Periodically driven laminar flows occur in many industrial processes from food-mixing devices to micro-mixer in lab-on-a-chip systems. The present study is motivated by better understanding fundamental transport phenomena in three-dimensional viscous time-periodic flows. Both numerical simulation and three-dimensional Particle Tracking Velocimetry measurements are performed to investigate the 3D advection of a passive scalar in a lid-driven cylindrical cavity flow. The flow is forced by a time-periodic in-plane motion of one endwall via a given forcing protocol. We concentrate on the formation and interaction of coherent structures due to fluid inertia, which play an important role in 3D mixing by geometrically determining the tracer transport. The disintegration of these structures by fluid inertia reflects an essentially 3D route to chaos. Data from tracking experiments of small particles will be compared with predictions from numerical simulations on transport of passive tracers. [Preview Abstract] |
Tuesday, November 23, 2010 9:18AM - 9:31AM |
MP.00007: High-throughput continuous millisecond solution exchange for particle suspensions Daniel Gossett, Henry Tse, Jaideep Dudani, Dino Di Carlo Mixing and solution exchange are routine tasks in macroscale systems. However, they can be challenging and slow in microfluidic systems which lack turbulence, and mixing is often achieved by manipulation of passive diffusion. In previous work we characterized geometry-dependent inertial lift forces which act on objects entrained in high velocity flows in confined microchannels. Using inertial focusing we can precisely focus spherical particles or cells to equilibrium positions with throughputs of thousands per second. Inertial forces have been employed in microfluidic systems for membrane-free filtration, size-based sorting, and cytometry. Here, we present a novel microfluidic system which manipulates these lift forces to rapidly transfer particles and cells from one solution to another by geometrically defining a new equilibrium position in a coflow. Due to the high Peclet number convection is dominant and diffusion is negligible for the length of the channel. We intend to use this technique for on-chip, in-line sample preparation or for studying reaction kinetics and molecular binding events where rapid and complete solution exchange or mixing is required. [Preview Abstract] |
Tuesday, November 23, 2010 9:31AM - 9:44AM |
MP.00008: Spinning Convection: Particle-induced Secondary Flow in Straight Microchannels Hamed Amini, Elodie Sollier, Dino Di Carlo In microfluidic systems, flow is generally approximated with Stokes flow and inertial forces are assumed negligible. However, at finite Reynolds number (small, yet non-zero), inertial forces have been shown to be useful, for instance by causing randomly distributed particles to order on specific lateral equilibrium positions in a confined flow. To further study these inertial effects at the microfluidic scale, we investigated the local disturbances induced by spinning particles in straight microchannels. We observe, numerically and experimentally, unexpected net cross-stream disturbances that generate a unique secondary flow pattern, which resembles the well-known Dean flow in curved channels. This behavior requires fluid inertia and micro-particles offset from the channel center, spinning due to the local shear rate across the particle. This phenomenon provides a novel technique for fluid transfer and solution switching in microsystems. Compared to other microfluidic mixing approaches the technique requires a simple channel geometry, with no external forces for operation, enabling biological applications in which the cells present within the flow themselves can induce the fluid transfer required for labeling, lysis, and other high-throughput sample preparations. [Preview Abstract] |
Tuesday, November 23, 2010 9:44AM - 9:57AM |
MP.00009: Inertial Droplet Mixing in a Confined Microchannel Gas Flow Brian Carroll, Brian Robinson, Carlos Hidrovo Efficient mixing at the microscale remains a formidable engineering challenge. Recent advancement and proliferation of Lab on a Chip and Micro Total Analysis Systems has demanded accelerated development and demonstration of novel micromixers as successful mixing is critical to device performance. In here we present a new droplet-based mixing technique currently being developed which aims to improve micromixing rates by increasing droplets Reynolds numbers in a microchannel prior to collision interaction. High speed gaseous flows are used to detach and transport discrete droplets to a collision zone where droplet interaction and subsequent mixing is achieved under highly inertial conditions. The design utilizes variants of the standard T-junction arrangement for both the detachment and collision process. Two fluorescing droplets with different fluorophore concentrations are brought into contact in a collision zone and allowed to interact. Mixing rates are quantified using an optical based measurement technique that examines temporal changes in droplet intensity as mixing progresses. [Preview Abstract] |
Tuesday, November 23, 2010 9:57AM - 10:10AM |
MP.00010: Mixing Enhancement in a Very Low Reynolds Number Liquid Shear Flow Rohit Nehe, Hui Hu, Manoochehr Koochesfahani We consider the shear flow created by two parallel streams where the Reynolds number is so low that the natural flow instabilities are completely damped and mixing occurs only at the diffusion interface between the two streams. Our interest is to increase the number of diffusion interfaces, in order to enhance the amount of molecular mixing, by imposing an external perturbation on the flow streams. LIF flow visualization is used to investigate the general features of the flow, while chemically reacting LIF is employed to quantify the extent of molecular mixing. Results will be presented over a range of perturbation frequencies and amplitudes. [Preview Abstract] |
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