Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session R9: Microscale Flows: Microfluidic Devices II |
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Chair: Matthew Hancock, Veryst Engineering Room: 109 |
Tuesday, November 24, 2015 12:50PM - 1:03PM |
R9.00001: Simulations of Micropumps Based on Tilted Flexible Fibers Matthew Hancock, Nagi Elabbasi, Melik Demirel Pumping liquids at low Reynolds numbers is challenging because of the principle of reversibility. We report here a class of microfluidic pump designs based on tilted flexible structures that combines the concepts of cilia (flexible elastic elements) and rectifiers (e.g., Tesla valves, check valves). We demonstrate proof-of-concept with 2D and 3D fluid-structure interaction (FSI) simulations in COMSOL Multiphysics\textregistered of micropumps consisting of a source for oscillatory fluidic motion, e.g. a piston, and a channel lined with tilted flexible rods or sheets to provide rectification. When flow is against the rod tilt direction, the rods bend backward, narrowing the channel and increasing flow resistance; when flow is in the direction of rod tilt, the rods bend forward, widening the channel and decreasing flow resistance. The 2D and 3D simulations involve moving meshes whose quality is maintained by prescribing the mesh displacement on guide surfaces positioned on either side of each flexible structure. The prescribed displacement depends on structure bending and maintains mesh quality even for large deformations. Simulations demonstrate effective pumping even at Reynolds numbers as low as 0.001. Because rod rigidity may be specified independently of Reynolds number, in principle, rod rigidity may be reduced to enable pumping at arbitrarily low Reynolds numbers. [Preview Abstract] |
Tuesday, November 24, 2015 1:03PM - 1:16PM |
R9.00002: Using micro-3D printing to build acoustically driven microswimmers. Nicolas Bertin, Olivier Stephan, Philippe Marmottant, Tamsin Spelman, Eric Lauga With no protection, a micron-sized free air bubble at room temperature in water has a life span shorter than a few tens of seconds. Using two-photon lithography, which is similar to 3D printing at the micron scale, we can build ``armors'' for these bubbles: micro-capsules with an opening to contain the bubble and extend its life to several hours in biological buffer solutions. When excited by an ultrasound transducer, a 20 $\mu$m bubble performs large amplitude oscillations in the capsule opening and generates a powerful acoustic streaming flow (velocity up to dozens of mm/s). A collaboration with the Dept. of Applied Mathematics and Theoretical Physics, University of Cambridge, is helping us predict the true resonance of these capsules and the full surrounding streaming flow. The present Bubbleboost project aims at creating red blood cell sized capsules ($\sim$ 10-20 $\mu$m) that can move on their own with a non-contact acoustic excitation for drug delivery applications. Another application of this research is in microfluidics: we are able to fabricate fields of capsules able to generate mixing effects in microchannels, or use the bubble-generated flow to guide passing objects at a junction. [Preview Abstract] |
Tuesday, November 24, 2015 1:16PM - 1:29PM |
R9.00003: Inertial microfluidic pump Pavel Kornilovitch, Alexander Govyadinov, David Markel, Erik Torniainen The inertial pump is powered by a microheater positioned near one end of a fluidic microchannel. As the microheater explosively boils the surrounding fluid, a vapor bubble expands and then collapses asymmetrically, resulting in net flow. Such devices become an effective means of transporting fluids at microscale. They have no moving parts and can be manufactured in large numbers using standard batch fabrication processes. In this presentation, physical principles behind pump operation are described, in particular the role of reservoirs in dissipating mechanical momentum and the expansion-collapse asymmetry. An effective one-dimensional dynamic model is formulated and solved. The model is compared with full three-dimensional CFD simulations and available experimental data. Potential applications of inertial micropumps are described. [Preview Abstract] |
Tuesday, November 24, 2015 1:29PM - 1:42PM |
R9.00004: Convective flow reversal in self-powered enzyme micropumps Henry Shum, Isamar Ortiz-Rivera, Arjun Agrawal, Ayusman Sen, Anna Balazs It was recently shown that a surface-bound patch of enzymes in a fluid filled chamber can drive large scale flow in the presence of the enzyme's substrate. Evidence suggested that the flow was buoyancy driven but the pumping speed, or even direction, was not always consistent with estimates based on heat released by the reaction. Hence, we develop and analyze a model for density variations due to changes in solution composition as the reaction proceeds. If the reaction causes an increase in solution density, then we intuitively expect the fluid to sink down and spread outward, away from the pump. If the reaction substrate and product have different diffusion coefficients, however, the pump can exhibit much more complex behavior, such as pushing fluid outwards at early times and pulling fluid inwards later on. Two parameters, the ratio of solutal expansion coefficients and the ratio of diffusion coefficients, determine the pump dynamics. The predicted reversal of pumping direction is experimentally verified with a urease pump. We further show that not only the speed but also the direction of pumping varies with the amount of enzyme present on the patch. A better understanding of these pumps will aid in the design of responsive, chemically powered microfluidic flow control. [Preview Abstract] |
Tuesday, November 24, 2015 1:42PM - 1:55PM |
R9.00005: Chemically generated convective transport of micron sized particles Oleg Shklyaev, Sambeeta Das, Alicia Altemose, Henry Shum, Anna Balazs, Ayusman Sen A variety of chemical and biological applications require manipulation of micron sized objects like cells, viruses, and large molecules. Increasing the size of particles up to a micron reduces performance of techniques based on diffusive transport. Directional transport of cargo toward detecting elements reduces the delivery time and improves performance of sensing devices. We demonstrate how chemical reactions can be used to organize fluid flows carrying particles toward the assigned destinations. Convection is driven by density variations caused by a chemical reaction occurring at a catalyst or enzyme-covered target site. If the reaction causes a reduction in fluid density, as in the case of catalytic decomposition of hydrogen peroxide, then fluid and suspended cargo is drawn toward the target along the bottom surface. The intensity of the fluid flow and the time of cargo delivery are controlled by the amount of reagent in the system. After the reagent has been consumed, the fluid pump stops and particles are found aggregated on and around the enzyme-coated patch. The pumps are reusable, being reactivated upon injection of additional reagent. The developed technique can be implemented in lab-on-a-chip devices for transportation of micro-scale object immersed in solution. [Preview Abstract] |
Tuesday, November 24, 2015 1:55PM - 2:08PM |
R9.00006: Optimized open-flow mixing: insights from microbubble streaming Bhargav Rallabandi, Cheng Wang, Lin Guo, Sascha Hilgenfeldt Microbubble streaming has been developed into a robust and powerful flow actuation technique in microfluidics. Here, we study it as a paradigmatic system for microfluidic mixing under a continuous throughput of fluid (open-flow mixing), providing a systematic optimization of the device parameters in this practically important situation. Focusing on two-dimensional advective stirring (neglecting diffusion), we show through numerical simulation and analytical theory that mixing in steady streaming vortices becomes ineffective beyond a characteristic time scale, necessitating the introduction of unsteadiness. By duty cycling the streaming, such unsteadiness is introduced in a controlled fashion, leading to exponential refinement of the advection structures. The rate of refinement is then optimized for particular parameters of the time modulation, i.e. a particular combination of times for which the streaming is turned ``on'' and ``off''. The optimized protocol can be understood theoretically using the properties of the streaming vortices and the throughput Poiseuille flow. We can thus infer simple design principles for practical open flow micromixing applications, consistent with experiments. [Preview Abstract] |
Tuesday, November 24, 2015 2:08PM - 2:21PM |
R9.00007: Magnetically Actuated Cilia for Microfluidic Manipulation Srinivas Hanasoge, Drew Owen, Matt Ballard, Peter J Hesketh, Alexander Alexeev We demonstrate magnetic micro-cilia based microfluidic mixing and capture techniques. For this, we use a simple and easy to fabricate high aspect ratio cilia, which are actuated magnetically. These micro-features are fabricated by evaporating NiFe alloy at room temperature, on to patterned photoresist. The evaporated alloy curls upwards when the seed layer is removed to release the cilia, thus making a free standing `C' shaped magnetic microstructure. This is actuated using an external electromagnet or a rotating magnet. The artificial cilia can be actuated upto 20Hz. We demonstrate the active mixing these cilia can produce in the microchannel. Also, we demonstrate the capture of target species in a sample using these fast oscillating cilia. The surface of the cilia is functionalized by streptavidin which binds to biotin labelled fluorescent microspheres and mimic the capture of bacteria. We show very high capture efficiencies by using these methods. These simple to fabricate micro cilia can easily be incorporated into many microfluidic systems which require high mixing and capture efficiencies. [Preview Abstract] |
Tuesday, November 24, 2015 2:21PM - 2:34PM |
R9.00008: 3D flow focusing for microfluidic flow cytometry with ultrasonics Vaskar Gnyawali, Eric M. Strohm, Yasaman Daghighi, Mia Van de Vondervoort, Michael C. Kolios, Scott S.H. Tsai We are developing a flow cytometer that detects unique acoustic signature waves generated from single cells due to interactions between the cells and ultrasound waves. The generated acoustic waves depend on the size and biomechanical properties of the cells and are sufficient for identifying cells in the medium. A microfluidic system capable of focusing cells through a 10 x 10 $\mu$m ultrasound beam cross section was developed to facilitate acoustic measurements of single cells. The cells are streamlined in a hydro-dynamically 3D focused flow in a 300 x 300 $\mu$m channel made using PDMS. 3D focusing is realized by lateral sheath flows and an inlet needle (inner diameter 100 $\mu$m). The accuracy of the 3D flow focusing is measured using a dye and detecting its localization using confocal microscopy. Each flowing cell would be probed by an ultrasound pulse, which has a center frequency of 375 MHz and bandwidth of 250 MHz. The same probe would also be used for recording the scattered waves from the cells, which would be processed to distinguish the physical and biomechanical characteristics of the cells, eventually identifying them. This technique has potential applications in detecting circulating tumor cells, blood cells and blood-related diseases. [Preview Abstract] |
Tuesday, November 24, 2015 2:34PM - 2:47PM |
R9.00009: Viscoelastic focusing and separation of bioparticles in straight microchannels Guoqing Hu, Chao Liu Viscoelasticity-induced particle migration has recently received increasing attention due to its ability to obtain high-quality focusing over a wide range of flow rates. However, its application is limited to low throughput regime since the particles can defocus as flow rate increases. Using an engineered carrier medium with constant and low viscosity and strong elasticity, the sample flow rates are improved to be one order of magnitude higher than those in existing studies. Utilizing differential focusing of particles of different sizes, here we present sheathless particle/cell separation in simple straight microchannels that possess excellent parallelizability for further throughput enhancement. The present method can be implemented over a wide range of particle/cell sizes and flow rates. We successfully separate small particles from larger particles, MCF-7 cells from red blood cells (RBCs), and \textit{Escherichia coli} (\textit{E. coli}) bacteria from RBCs in different straight microchannels. We recommend further study on engineering rheological properties of carrier media for improving the separation performance of viscoelasticity-based microfluidic devices. [Preview Abstract] |
Tuesday, November 24, 2015 2:47PM - 3:00PM |
R9.00010: Paper-based flow fractionation system for preconcentration and field-flow fractionation. Seokbin Hong, Rhokyun Kwak, Wonjung Kim We present a novel paper-based flow fractionation system for preconcentration and field-flow fractionation. The paper fluidic system consisting of a straight channel connected with expansion regions can generate a fluid flow with a constant flow rate for 10 min without any external pumping devices. The flow bifurcates with a fraction ratio of up to 30 depending on the control parameters of the channel geometry. Utilizing this simple paper-based bifurcation system, we developed a continuous-flow preconcentrator and a field-flow fractionator on a paper platform. Our experimental results show that the continuous-flow preconcentrator can produce a 33-fold enrichment of the ion concentration and that the flow fractionation system successfully separates the charged dyes. Our study suggests simple, cheap ways to construct preconcentration and field-flow fractionation systems for paper-based microfluidic diagnostic devices. [Preview Abstract] |
Tuesday, November 24, 2015 3:00PM - 3:13PM |
R9.00011: Fabrication of thermo-responsive microfluidic membrane using photopolymerization patterning Hyejeong Kim, Sang Joon Lee The programmed manipulation of responsive functional hydrogels is receiving large attention because of its unique functions and wide range of engineering applications. In this study, we developed an innovative stomata-inspired membrane (SIM) by fabricating a temperature-responsive hydrogel with a simple, cost effective, and high-throughput photopolymerization patterning process. Polymerization-induced diffusion on the macro-scale surface gives rise to form a multi-parted polymer membrane with fine pores by simple UV irradiation. After heating the SIM, the less deformable thick frame supports the whole structure, and the highly deformable thin base regulates the size of pores. The morphological configuration of the SIM can be easily changed by varying the solution composition or selecting a suitable photomask with different pattern. The developed SIM has the special sensing-to-actuation functions of stimuli-responsive hydrogels. This membrane with temperature-responsive pores would be potentially utilized in numerous practical applications, such as filter membranes with self-adjustable pores, membrane-based sensors, membrane-based actuators, and multi-functional membranes etc. [Preview Abstract] |
Tuesday, November 24, 2015 3:13PM - 3:26PM |
R9.00012: Roll-to-Roll Nanoimprint Lithography Simulations for Flexible Substrates Andrew Spann, Akhilesh Jain, Roger Bonnecaze UV roll-to-roll nanoimprint lithography enables the patterning of features onto a flexible substrate for bendable electronics in a continuous process. One of the most important design goals in this process is to make the residual layer thickness of the photoresist in unpatterned regions as thin and uniform as possible. Another important goal is to minimize the imprint time to maximize throughput. We develop a multi-scale model to simulate the spreading of photoresist drops as the template is pressed against the substrate. We include the effect of capillary pressure on the bending of the substrate and show how this distorts uniformity in the residual thickness layer. Our simulation code is parallelized and can simulate the flow and merging of thousands of drops. We investigate the effect of substrate tension and the initial arrangement of drops on the residual layer thickness and imprint time. We find that for a given volume of photoresist, distributing that volume to more drops initially decreases the imprint time. We conclude with recommendations for scale-up and optimal operations of roll-to-roll nanoimprint lithography systems. [Preview Abstract] |
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