Session AF: Microfluidics: Devices I

The hydrodynamic mobility of chiral colloidal aggregates

A recent advance in colloidal technology [Zerrouki \emph{et al.}, Nature \textbf{455}, 380 (2008)] uses magnetic aggregation to enable the formation of micron-scale particle clusters with helical symmetry from doublets composed of two micron-scale beads of different radii bonded together by a magnetic cement. Such self-assembled structures offer a means of controllable transport and separation in a low Reynolds number environment using externally applied magnetic or electric fields. We identify two necessary conditions that reveal further parameterized expressions that describe the positions of the beads in an aggregrate as a function of size ratio of the two beads composing the doublets. With the geometry of the structure known, we perform hydrodynamic calculations to ascertain entries of the mobility matrix for the aggregate and establish the relationship between the applied torque about the helical axis and translations parallel to this direction. For larger values of the particle radius ratio the coupling between rotations and translations changes sign as the number of doublets in the aggregate increases indicating that the clusters possess a more complex superhelical structure.   

Actuated cilial layers regulate deposition of microscopic solid particles

We use computational modeling to examine the three-dimensional interactions between oscillating, synthetic cilia and microscopic solid particles in a fluid-filled microchannel. The synthetic cilia are elastic filaments that are tethered to a substrate and are actuated by a sinusoidal force, which is applied to their free ends. The cilia are arranged in a square pattern and a neutrally buoyant particle is initially located between these filaments. Our computational studies reveal that depending on frequency of the beating cilia, the particle can be either driven downwards toward the substrate or driven upwards and expelled into the fluid above the cilial layer. This behavior mimics the performance of biological cilia used by certain marine animals to extract suspended food particles. The findings uncover a new route for controlling the deposition of microscopic particles in microfluidic devices.   

Size-based sorting of micro-particles using microbubble streaming

Oscillating microbubbles driven by ultrasound have shown great potential in microfluidic applications, such as transporting particles and promoting mixing [1-3]. The oscillations generate secondary steady streaming that can also trap particles. We use the streaming to develop a method of sorting particles of different sizes in an initially well-mixed solution. The solution is fed into a channel consisting of bubbles placed periodically along a side wall. When the bubbles are excited by an ultrasound piezo-electric transducer to produce steady streaming, the flow field is altered by the presence of the particles. This effect is dependent on particle size and results in size-based sorting of the particles. The effectiveness of the separation depends on the dimensions of the bubbles and particles as well as on the ultrasound frequency. Our experimental studies are aimed at a better understanding of the design and control of effective microfluidic separating devices. Ref: [1] P. Marmottant and S. Hilgenfeldt, Nature 423, 153 (2003). [2] P. Marmottant and S. Hilgenfeldt, Proc. Natl. Acad. Science USA, 101, 9523 (2004). [3] P. Marmottant, J.-P. Raven, H. Gardeniers, J. G. Bomer, and S. Hilgenfeldt, J. Fluid Mech., vol.568, 109 (2006).   

Whistling Bubbles: All-fluidic linear frequency sweep generators

Complex biochemical systems are often composed of networks of interacting elements with spatial and temporal dynamics. Unlike ever so prevalent electronic debugging tools, automated tools to chemically probe these systems are in their infancy. Here we demonstrate one such novel tool - an all-fluidic frequency sweep generator capable of linear and exponential sweeps from a few Hz to KHz response. This is accomplished by coupling a flow-focusing geometry driven by negative pressure (- FF), a non-linear resistor and retraction dynamics of fluid threads in micro-channels. The device exhibits various modes including accordions and bursts with negative ramps to generate the desired sweep. The sweep parameters including f$_{min}$, f$_{max}$ and T$_{off}$ can be programmed by knobs including the hydrostatic pressure at inlet geometries. Our work highlights the importance of exploiting dynamics of drops and bubbles in fluidic networks to engineer desired function.   

High Speed motion generated by an oscillating microfiber

We present detail regarding the flow field generated by the high-speed motion of a long, thin elastic fiber immersed in a viscous fluid. The fiber, made of glass, or carbon, is approximately 1 mm in length, has a diameter of 7 microns, and is immersed in water, seeded with sub-micron tracer particles. The fiber is oscillated back and forth at 32 kHz with a peak-to-peak tip amplitude of approximately 10 microns. The resultant flow field is measured using micro-PIV, imaged at high speed using an intensified high speed camera, capable of taking data up to 12kHz. Symmetric vortices around the tip are generated by the steady streaming effect, with fluid velocities approaching 1 m/s, and shear rates close to $10^5$ $s^{-1}$. The vortices have a strong three-dimensional structure due to the presence of the substrate below the moving tip, as well as the axial variation of the fiber amplitude. In addition to quantifying the fluid motion, the mixing of fluid and the dispersion, and accumulation of particles due to the fiber motion is measured and discussed.   

Multi - directional Electrophoretic Positioning of Charged Drops

We demonstrate a multi-directional electrophoretic technique to provide precise control over the position, trajectory and velocity of a charged object. Two or more pairs of electrodes, oriented along different directions, are periodically activated and deactivated to move the charged object along a desired trajectory. Two perpendicular sets of electrodes, for example, can induce the charged object to take a series of sudden $90^\circ$ turns. We derive scaling expressions for the rates of acceleration and deceleration as the electric field direction is switched, and we corroborate the expressions experimentally for water droplets in oil. Surprisingly, we observe that the electrode geometry significantly alters the trajectory of charged drops, even in the absence of an applied electric field. The results have broad implications for any system that requires dynamic three-dimensional positioning, including lab-on-a-chip devices and electrophoretic displays.   

A theoretical model of electrophoretic biomolecule separation in periodic nanofilter arrays

We present a theoretical model describing Ogston sieving---pore size comparable to or larger than the characteristic molecular dimension---of rigid isotropic and anisotropic biomolecules in nanofluidic filters, comprising of a periodic array of alternating deep and shallow regions. We obtain one-dimensional analytical results and two-dimensional numerical solutions of a drift-diffusion description, which captures the interplay between the driving electric force, entropic barrier and molecular diffusion. The analytical solution yields explicit results for the effective mobility and trapping time, and shows that the configurational entropy, which arises from the reduction of available configurations in the confined space of the nanochannel, dominates the resulting separation behavior. Our results are in line with experimental observations, and elucidate the effects of field strength, device geometry and entropic barrier height, providing a robust tool for the design and optimization of nanofilter/nanopore systems.   

Colon Cancer Cell Separation by Dielectrophoresis

Separation of cancer cells from the other biological cells can be useful for clinical cancer diagnosis and cancer treatment. In this presentation, conventional dielectrophoresis (c-DEP) is used in a microfluidic chip to manipulate and collect colorectal cancer HCT116 cell, which is doped with Human Embryonic Kidney 293 cells (HEK 293). It is noticed that, the HCT116 cell are deflected to a side channel from a main channel clearly by apply electric field at particular AC frequency band. This motion caused by negative DEP can be used to separate the cancer cell from others. In this manuscript, chip design, flow condition, the DEP spectrum of the cancer cell are reported respectively, and the separation and collection efficiency are investigated as well. The sorter is microfabricated using plastic laminate technology. -/abstract- This work has been financially supported by the NSF RII funding (EP   

Simulation of Taylor bubble flow in T-shaped micro-channel by MPS method

Moving Particle Semi-implicit (MPS) method uses particles and their interactions to simulate incompressible flow and it is a promising meshless method for multiphase flow simulation. In order to capture the interface in micro-scale channel, Taylor bubble flow in a T-shaped micro-channel is simulated in this paper. Firstly available surface tension and wettability model are improved and validated by simulating the droplet vibration and static shapes of a droplet attached on the solid surface, respectively. Afterwards, by discretizing the liquid and gas phases into moving particles with different density, bubble slug generation in T-shaped micro-channel is reproduced by MPS method with above models. The good agreement between numerical simulation with visualization experiment confirmed the capacity of MPS for the micro-scale two-phase flow. Finally, bubble generation mechanism is revealed by the velocity field of typical squeezing and shearing regime. The influences of viscosity, surface tension and contact angle on the bubble slug length are also discussed in detail.   

A Microscale Multi-Inlet Vortex Nanoprecipitation Reactor: Turbulence Measurement and Simulation

Microreactors capable of generating turbulent flow are used in Flash NanoPrecipitation, a novel approach to produce functional nanoparticles. Microreactor design and optimization can be greatly enhanced by developing reliable computational models. Scalar mixing in a multi-inlet micro-vortex reactor has been previously studied and a RANS simulation with a scalar mixing model had been successfully validated for fully turbulent flow. To further validate the flow field, the reactor, operated in flow regimes from laminar to turbulent, was investigated using microscopic particle image velocimetry and computational fluid dynamics. In the experiments, instantaneous velocity fields were recorded on three different planes in the reactor and flow statistics such as the mean velocity and turbulent kinetic energy were computed for comparison with flow simulations. Laminar simulations were performed for the low-Reynolds-number cases, and large-eddy simulations (LES) were performed for higher-Reynolds-number cases. The simulation results were in good agreement with experiments for all cases, demonstrating the accuracy of the simulation models in the laminar-turbulent transition range.