Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session T12: Microfluidics II: Devices |
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Sponsoring Units: DFD Chair: Cindy Harnett, University of Louisville Room: B110-B111 |
Wednesday, March 17, 2010 2:30PM - 2:42PM |
T12.00001: Mechanical actuators at the nanoscale: molecular propellers, paddles and wheels Lela Vukovic, Boyang Wang, Petr Kral We model by molecular dynamics simulations nanosystems that could realize mechanical action in nanofluidics. First, we study molecular propellers formed by carbon nanotube rotors with attached aromatic blades that can pump liquids with efficiency dependent on the chemistry of the liquid-blade interface [1]. Next, we investigate nanorods with photoactive surfaces that can roll on water when driven by light [2]. Their rolling motion is realized when chromophores attached to their surfaces become anisotropically polarized by light and attracted to water. Finally, we examine nanoscale pumping induced by deformable nanoscale blades [3]. We show that the length, polarity, frequency and amplitude of oscillations of the nanoblades control their efficiency of water pumping.\\[4pt] [1] B. Wang and P. Kr\'al, . Rev. Lett. 98, 266102 (2007).\\[0pt] [2] L. Vukovic and P. Kr\'al, submitted.\\[0pt] [3] L. Vukovic, D. Astumian and P. Kr\'al, in preparation. [Preview Abstract] |
Wednesday, March 17, 2010 2:42PM - 2:54PM |
T12.00002: Smart Capsules: Engineering New Temperature and Pressure Sensitive Materials with Microfluidics Laura Adams, James Wilking, Anderson Ho Cheung Shum, Sebastian Seiffert, Shmuel Rubinstein, Yuanjin Zhao, David Weitz New smart materials that are responsive to external stimuli such as pressure and temperature can be carefully designed using microfluidics with double emulsions as building blocks. Here we introduce the synthesis of new smart core-shell structures with two different aqueous drops in the interior. By triggering the capsules with pressure and temperature, coalescence and mixing of the interior drops occurs and is studied with high speed video imaging techniques. [Preview Abstract] |
Wednesday, March 17, 2010 2:54PM - 3:06PM |
T12.00003: Directed Fluid Flow Produced by Arrays of Magnetically Actuated Core-Shell Biomimetic Cilia B. L. Fiser, A. R. Shields, B. A. Evans, R. Superfine We have developed a novel core-shell microstructure that we use to fabricate arrays of flexible, magnetically actuated biomimetic cilia. Our biomimetic cilia mimic the size and beat shape of biological cilia in order to replicate the transport of fluid driven by cilia in many biological systems including the determination of left-right asymmetry in the vertebrate embryonic nodal plate and mucociliary clearance in the lung. Our core-shell structures consist of a flexible poly(dimethylsiloxane) (PDMS) core surrounded by a shell of nickel approximately forty nanometers thick; by using a core-shell structure, we can tune the mechanical and magnetic properties independently. We present the fabrication process and the long-range transport that occurs above the beating biomimetic cilia tips and will report on progress toward biomimetic cilia induced flow in viscoelastic fluids similar to mucus in the human airway. These flows may have applications in photonics and microfluidics, and our structures may be further useful as sensors or actuators in microelectromechanical systems. [Preview Abstract] |
Wednesday, March 17, 2010 3:06PM - 3:18PM |
T12.00004: Microfluidic Mixing and Confined Flows with Biomimetic Cilia Arrays A.R. Shields, B.L. Fiser, B.A. Evans, R. Superfine We present results on fluidic mixing produced by the collective beating of arrays of biomimetic cilia. Our artificial cilia are arrays of free-standing microstructures, at the scale of biological cilia, which we actuate with permanent magnets to mimic their biological counterparts. The presence of mixing in biological ciliated systems has been a subject of recent speculation, with possible implications for a variety of biochemical processes. We have observed that biomimetic cilia actuation induces fluid mixing within the cilia layer that can be characterized as an enhanced diffusivity. Due to the similarity in size and hydrodynamic regime between our system and biology, our results provide the first experimental suggestion of mixing in ciliated systems. In addition, we have mapped three-dimensional flows in confined fluidic cells that recreate the flows observed in the embryonic node, where cilia-driven fluid transport determines vertebrate left/right body asymmetry. [Preview Abstract] |
Wednesday, March 17, 2010 3:18PM - 3:30PM |
T12.00005: Devices for the production and sorting of microfluidic droplets Donald Aubrecht, John Heyman, Jeremy Agresti, Sarah K\"oster, David Weitz Droplets produced in microfluidic devices are a great set of tools for studying large cell populations and permutations of reactions. Sample populations of $10^6$ - $10^7$ can be studied with relative ease, as encapsulation and screening rates in the kHz range are accessible. Previous droplet work has shown encapsulation of cells in droplets allows individual cells and their products to be studied. Advantages include correlation between detected products and initial drop contents, as well as minimized sample cross-contamination. Most microfluidic-based biological assays rely on fluorescent labeling of cells or use of cellular products to initiate a fluorescence-producing reaction. Detection of the fluorescence provides a trigger for sorting those cells or cell-containing droplets away from the general population. Though this allows some cellular processes to be studied, detection and quantification of all products, not just those expressed to the cell surface or those that catalyze reactions, would impact development of better therapeutics. We are currently working to adapt benchtop biological assays that label and detect cellular products for use in a droplet-based system. The work presented here details the chain of modular microfluidic devices we use to encapsulate, incubate, interrogate, and sort a population of droplets containing a model system. [Preview Abstract] |
Wednesday, March 17, 2010 3:30PM - 3:42PM |
T12.00006: Surface-functionalized Microelectrofluidic Biosensors Michael A. Stanton, Guilherme N.M. Ferreira, Jean-Luc Fraikin, A. N. Cleland We are developing all-electronic, label-free biosensors for the rapid, selective and label-free detection of viruses and viral proteins. We are using a thiol-based self assembled monolayer (SAM) on gold as the functional element, sensed using radiofrequency reflectometry, with the functionalized sensor embedded in a microfluidic channel. We are exploring using mixed-length SAMs to bind single-chain fragments and single domain antibodies for the recognition of HIV1 and other virus targets. RF reflectometry is used to measure impedance changes in the sensors, which occur upon binding of viral or antibody targets. With an active sensing volume of a few hundred attoliters and sensitivity to impedance changes of order 1 part in 10$^{5}$ we expect to be able to detect the binding of small numbers of viral antibodies or viral particles. [Preview Abstract] |
Wednesday, March 17, 2010 3:42PM - 3:54PM |
T12.00007: Electrode-based detection technique for microfluidic devices Evgeniya Moiseeva, Adrian Fletcher, Cindy Harnett We report on a droplet-producing microfluidic device with electrode-based detection techniques. The microfluidic devices are made of polydimethylsiloxane (PDMS) and glass. Immiscible fluids containing the hydrophobic and hydrophilic phases are injected into the microfluidic device using syringe pumps. When a particle passes between a pair of electrodes in a medium having different electrical conductivity, the resulting impedance change signals the presence of the particle for closed-loop feedback during processing. The circuit produces a digital pulse for input into a computer control system. The detected signal can be used for evaluating droplet size, droplet shape, and droplet formation frequency. The detector also allows estimation of a droplet's arrival time at the microfluidic chip outlet for dispensing applications. Electronic feedback provides the ability to count, sort, and direct microfluidic droplets. Microelectrode-based techniques should find several applications in digital microfluidics and in three-dimensional printing technology for rapid prototyping and biotechnology. [Preview Abstract] |
Wednesday, March 17, 2010 3:54PM - 4:06PM |
T12.00008: Microfluidic high-throughput nanoparticle counter Jean-Luc Fraikin, Andrew Cleland We have developed a high-throughput sensor for the all-electronic sizing of synthetic and biological nanoparticles suspended in a fluid. We have demonstrated detection of unlabeled particles with diameters ranging from 50 nanometers to 1 micron, and the rapid response time of the sensor permits detection of particles at rates greater than 200000 particles per second. Our current efforts focus on developing the sensor to detect single virus particles suspended in complex cytoplasmic fractions. [Preview Abstract] |
Wednesday, March 17, 2010 4:06PM - 4:18PM |
T12.00009: Analysis and fabrication of micro scale self-terminated electrochemical growth by a pressure-driven method Fatemeh Soltani, Alex Wlasenko, Geoff Steeves A self-terminated electrochemical method was used to fabricate microscopic-scale contacts between two Au electrodes in a microfluidic channel. The conductance of contacts varies in a stepwise fashion with a tendency to quantize near the integer multiples of the conductance quantum ($G_0 )$. The mechanism works by a pressure-driven flow parallel with a pair of Au electrodes with a gap in order of micron in an electrolyte of HCl. When applying a bias voltage between electrodes, metal atoms are etched off the anode and deposited onto the cathode. Consequently, the gap decreases to the atomic scale and then completely closed as the two electrodes form a contact. The electrochemical fabrication approach introduces large variance in the formation and location of individual junctions. Controlling this process will enable the precise positioning of reproducible geometries into nano-electronic devices. To investigate the high speed behavior of a QPC, it can be integrated with a transmission line structure patterned on a photoconductive GaAs substrate. The nonlinear conductance of the QPC (due to the finite density of states of the conductors) can be examined and compared with recent theoretical studies. Samples are fabricated in situ using an electrochemical procedure to produce QPCs along the transmission line structure. This method may provide insight into Terahertz Optoelectronic devices and ultrafast communication systems. [Preview Abstract] |
Wednesday, March 17, 2010 4:18PM - 4:30PM |
T12.00010: Improved Performance of Deterministic Lateral Displacement Arrays with Triangular Posts Kevin Loutherback, Kevin Chou, Jason Puchalla, Robert Austin, James Sturm Deterministic lateral displacement arrays have shown great promise for sized-based particle analysis and purification in medicine and biology.~~Here we demonstrate that use of an array of triangular rather than circular posts significantly enhances the performance of these devices by reducing clogging, lowering hydrostatic pressure requirements and increasing the range of displacement characteristics.~~Experimental data and theoretical models are presented to create a compelling argument that future designs of deterministic lateral displacement arrays should employ triangular posts [Preview Abstract] |
Wednesday, March 17, 2010 4:30PM - 4:42PM |
T12.00011: Hydrodynamic Trap for Single Cells and Particles Melikhan Tanyeri, Charles Schroeder The ability to trap individual particles, cells and macromolecules has revolutionized many fields of science during the last two decades. Several methods of particle trapping and micromanipulation have been developed based on optical, magnetic and electric fields. In this work, we describe an alternative trapping method, the hydrodynamic trap, based on the sole action of hydrodynamic forces in a microfluidic device. A microfluidic cross slot device is fabricated consisting of two perpendicular microchannels where opposing laminar flow streams converge. In this device, a purely extensional flow field is created at the microchannel junction, thereby resulting in a semi-stable potential well at the stagnation point which enables particle trapping. We implement an automated feedback-control mechanism to adjust the location of the stagnation point which facilitates active particle trapping. Using the hydrodynamic trap, we successfully demonstrate trapping and manipulation of single particles and cells for arbitrarily long observation times. This technique offers a new venue for observation of biological materials without surface immobilization, eliminates potentially perturbative optical, magnetic and electric fields, and provides the capability to change the surrounding medium conditions of the trapped object. [Preview Abstract] |
Wednesday, March 17, 2010 4:42PM - 4:54PM |
T12.00012: Control of Molecular Transport and Chemical Reaction Dynamics in Confined Volumes Pat Collier Understanding how confinement, crowding and reduced dimensionality modulate biochemical reactivity and reaction dynamics will aid in the discovery of functionality unique to nanoscale systems. Biochemical reaction kinetics in biomimetic reaction vessels with confined volumes present in microfabricated structures have been determined, including monitoring single-enzyme reaction kinetics in femtoliter-volume (10$^{-15}$ L) chambers with millisecond mixing times, and the discovery and characterization of shear-induced redistribution of surfactant at the oil-water interface in femtoliter-volume droplets split off from larger aqueous plugs at a microfabricated T-junction, which resulted in increased interfacial adsorption of enzymes. The fabrication capabilities at the Center for Nanophase Materials Sciences at ORNL are extending these studies to nanoscale structures, interfaces and architectures, with the ability to control chemical reaction kinetics with precise spatiotemporal control of molecular/mass transport via integration with microfluidics. [Preview Abstract] |
Wednesday, March 17, 2010 4:54PM - 5:06PM |
T12.00013: Cost effective fabrication method for microscaled interdigitated electrodes for fast 3D ACEO pumps Yehya Senousy, Cindy Harnett Electro-osmotic pumping is a promising battery-powered replacement for traditional pumping systems at the micro scale when dilute electrolytes are used. To avoid the drawbacks of pumping using DC, ``ac electro-osmotic'' (ACEO) pumps have been recently introduced. The advantages over DC electro-osmotic pumps include lower operating voltages at integrated electrodes, and absence of gas generation from electrolysis. The microchannels of these ACEO pumps consisted first of asymmetric, planar electrodes. A non-planar ACEO pump geometry was then introduced with electroplated three dimensional (3D) stepped electrodes. This design had a faster flow rate than the planar ACEO pump by an order of magnitude, but the fabrication process was complex. In this paper, we demonstrate a new fabrication method for these 3D interdigitated microelectrode arrays. The method eliminates the need for electroplating thick 3D electrodes; instead 3D interdigitated electrodes are created by shadow evaporation of thin films on 3D structures that could be injection molded. The pumps were characterized for flow speed versus applied voltage amplitude and frequency. [Preview Abstract] |
Wednesday, March 17, 2010 5:06PM - 5:18PM |
T12.00014: Interaction of thin-film microcoils with the air/water interface and applications in microfluidics Cindy K. Harnett, Thomas M. Lucas, Julia W. Aebersold Capillary forces at the air-water interface are widely regarded as a nuisance in fabricating micro- and nanoelectromechanical (MEMS/NEMS) devices, since the forces can pull suspended cantilevers permanently to the substrate. However, this phenomenon leads to interesting and potentially useful behavior with highly strained three-dimensional metal/oxide microcoils that can balance the capillary forces. These out-of-plane coiled filaments have typical curvature radii of 100 microns, but the fabrication technique scales from sub-10 nm to more than 1mm. Hydrophobic and hydrophilic self-assembled monolayers (SAMs), applied selectively to metal and oxide surfaces, enable fine control over the wettability of these structures. The resulting microcages and microtubes can contain nonpolar fluids (air, oils) in a polar fluid, or vice versa. Applications include controlled bubble capture for dry sample storage, quantification of the reaction rate of electrolytic reactions by detection of gas bubbles exceeding a threshold size, and confinement of liquids in porous microcontainers that enable diffusion of gaseous reactants. [Preview Abstract] |
Wednesday, March 17, 2010 5:18PM - 5:30PM |
T12.00015: Development of an AFM-based hanging fiber rheometer for interfacial microrheology Shuo Guo, Xiaomin Xiong, Zuli Xu, Ping Sheng, Penger Tong A new interfacial microrheology technique using atomic force microscope (AFM) as a force sensor is developed. The probe used for microrheology contains a long vertical glass fiber with one end glued onto a rectangular shaped cantilever beam and the other end immersed through a water-air interface. The motion of the modified cantilever can be accurately described by the Langevin equation for a damped harmonic oscillator, from which we obtain the friction coefficient $\xi$ of the glass fiber in contact with the water. It is found that $\xi$ contains two contributions. One is generated by the bulk fluid, which increases with the immersion length of the glass fiber. The other contribution comes from the contact line between the water-air interface and the glass fiber, which is obtained by a linear extrapolation of the measured $\xi$ at the limit of zero immersion length. The experiment thus demonstrates an application of AFM in the studies of interfacial microrheology and contact line dynamics. [Preview Abstract] |
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