1,721,048 research outputs found
Effects of a DC offset on an electrothermal microparticle trap assembled with an AC electric field
No full textMicro and nano-scale colloidal particles can be rapidly assembled at electrode-electrolyte interfaces in highly organized structures. These particles aggregate through various forces which are, among others, chemical, electrical, and thermal in nature. Patterning biological cells in such structures has high impact applications but it remains a challenge due to their limited viability under said manipulation forces. Rapid electrokinetic patterning (REP) uses AC electrothermal micro-vortices to aggregate both synthetic particles and biological cells. In this work, we explore the effects of a DC offset on a REP trap performance in bio-relevant isotonic medium. REP traps were characterized by measuring the inter-particle distance under different DC offsets, using a Delaunay triangulation. The inter-particle distance was measured in a steady-state trap followed by the disassembly of the aggregate as the REP vortex was turned off. DC offset enhanced the trapping performance for micro-particles suspended in a sugar-based isotonic medium. It was observed that an increasingly negative DC offset increased the steady-state inter-particle distance and reduced the rate of disassembly. However, no such trend was found with positive DC offsets. Changing the offset in small steps (<500 mV) affected only the inter-particle distance whereas, larger steps significantly affected the trap stability and size
Characterization of electrothermal microfluidic tweezers (REP) in bio-relevant media
No full textRapid electrokinetic patterning (REP) has proven to be a powerful microfluidic tweezer that generates spatially and temporally specified microflow via electrothermal vortices. The ultra-small trapping forces on the scale of femtoNewtons exerted by REP, due to its viscous drag of the vortical flow on the trapped particles, attracts many potential bio-manipulation applications. We report, for the first time, use of isotonic sugar-based media to simulate bio-relevant environment for flow manipulation through REP. A DC field was introduced, in addition to the typical AC field, to enhance the vortical flow induced particle trapping performance. In this work, we study the effects of the magnitude and sign of the DC offset on the vortex characteristics. Results show that addition of the DC component in the electric field has a significant impact on the electrothermal micro-vortex and the electric double layer of the electrodes and the trapped particles. We also observe that an abrupt change in the DC offset destabilized the vortex and washed away some particles as the trap equilibrium re-established. However, the trap remained stable when the step change in the DC offset was smaller than ~500 mV
Optically modulated electrokinetic manipulation and concentration of colloidal particles near an electrode surface
Get PDFWe study a recently demonstrated AC electrokinetic technique for manipulation and concentration of colloidal particles on an electrode surface. The technique uses indium tin oxide (ITO)-based parallel-plate electrodes on which highly localized infrared (1064 nm) laser illumination is shone. We show that the highly localized laser illumination leads to a highly nonuniform heating of the electrode substrate, which in turn drives an electrothermal microvortex resulting in a rapid transport of particles toward the illuminated site. Hundreds of polystyrene particles, with diameters ranging from 2.0 to 0.1 ?m, suspended in a low conductivity solution (2.0 mS/m) could be aggregated at selected locations on the electrode by activating the laser illumination at suitable AC frequencies. Subsequent deactivation of the laser illumination causes the particles to scatter, and we explore this dynamical behavior for 1.0 ?m particles using Delaunay tessellations and high-speed videography. We establish that drag from the electrothermal microvortex acts against a repulsive force, which decreases with increasing AC frequency, to create stable particle clusters. Moreover, experimentally we show that this particle capturing technique can be characterized by a critical frequency: a frequency at which the captured colloidal particle cluster becomes unstable and particles are carried away into the bulk by the electrothermal microvortex. This critical frequency increases with decreasing particle diameter for similar particles. For 0.1 ?m particles, comparison of aggregation at different AC frequencies is achieved by the comparison of fluorescent intensity profiles of the aggregation
Optically induced electrokinetic concentration and sorting of colloids
No full textWe demonstrate an optically induced ac electrokinetic technique that rapidly and continuously accumulates colloids on a parallel-plate electrode surface resulting in a crystalline-like aggregation. Electrothermal hydrodynamics produce a microfluidic vortex that carries suspended particles toward its center where they are trapped by local ac electrokinetic hydrodynamic forces. We characterize the rate of particle aggregation as a function of the applied ac voltage, ac frequency and illumination intensity. Hundreds of polystyrene particles (1.0 mu m) suspended in a low conductivity solution (2.4 mS m(-1)) were captured at a range of voltages (5-20 V-pp) and frequencies (20-150 kHz) with an optical power of approximately 20 mW. This technique was not restricted to near infrared (1064 nm) illumination and was also demonstrated at 532 nm. The sorting capability of this technique was demonstrated with a solution containing 0.5 mu m, 1.0 mu m and 2.0 mu m polystyrene particles. This dynamic optically induced technique rapidly concentrates, sorts and translates colloidal aggregates with a simple parallel-plate electrode configuration and can be used for a variety of lab-on-a-chip applications
Optically induced electrothermal microfluidic tweezers in bio-relevant media
Get PDFNon-contact micro-manipulation tools have enabled invasion-free studies of fragile synthetic particles and biological cells. Rapid electrokinetic patterning (REP) traps target particles/cells, suspended in an electrolyte, on an electrode surface. This entrapment is electrokinetic in nature and thus depends strongly on the suspension medium’s properties. REP has been well characterized for manipulating synthetic particles suspended in low concentration salt solutions (~ 2 mS/m). However, it is not studied as extensively for manipulating biological cells, which introduces an additional level of complexity due to their limited viability in hypotonic media. In this work, we discuss challenges posed by isotonic electrolytes and suggest solutions to enable REP manipulation in bio-relevant media. Various formulations of isotonic media (salt and sugar-based) are tested for their compatibility with REP. REP manipulation is observed in low concentration salt-based media such as 0.1× phosphate buffered saline (PBS) when the device electrodes are passivated with a dielectric layer. We also show manipulation of murine pancreatic cancer cells suspended in a sugar-based (8.5% w/v sucrose and 0.3% w/v dextrose) isotonic medium. The ability to trap mammalian cells and deposit them in custom patterns enables high-impact applications such as determining their biomechanical properties and 3D bioprinting for tissue scaffolding
Hybrid opto-electric manipulation in microfluidics-opportunities and challenges
Get PDFHybrid opto-electric manipulation in microfluidics/nanofluidics refers to a set of methodologies employing optical modulation of electrokinetic schemes to achieve particle or fluid manipulation at the micro-and nano-scale. Over the last decade, a set of methodologies, which differ in their modulation strategy and/or the length scale of operation, have emerged. These techniques offer new opportunities with their dynamic nature, and their ability for parallel operation has created novel applications and devices. Hybrid opto-electric techniques have been utilized to manipulate objects ranging in diversity from millimetre-sized droplets to nano-particles. This review article discusses the underlying principles, applications and future perspectives of various hybrid opto-electric techniques that have emerged over the last decade under a unified umbrella
Confined flow cavitation in hydraulic oil: Test rig design, pressure measurement, and PIV
No full textNumerical advances in cavitation modeling require experimental measurements to validate them. For this purpose an oil cavitation test rig was designed to generate experimental data of a submerged cavitating orifice in oil. An important feature of the test section was the ability to have optical access while preserving structural integrity. Experiments showed that visible inception occurred coincidently with both noise detection and the point where flow starts to undershoot predicted values. With cavitation at constant flow it was observed that upstream pressure was strongly independent of downstream pressure compared to single-phase flow where the two are linked. A serendipitous aside of related measurements was the notion that the traditional definition of cavitation number may not be valid for confined flows because of its tie to flow rate. Using experimental data and two different approaches, a new definition was proven to be unaffected by the existence of cavitation and shown to closely match theoretical values. Dynamic pressure data showed that higher back pressures led to higher frequency noise and the suppression of lower frequency noise; data was collected that shows the spectral shift at 1 bar decrements. Particle Image Velocimetry was used to capture instantaneous and average velocity fields. Entrainment and recirculation were evident in all the flow fields. Under certain flow conditions, the entire cavitating jet velocity could be resolved using bubbles as tracer particles. With high speed imagery, the jet length was observed to shorten with increased pressures while the averaged flow fields remained broadly uninfluenced by the pressure
In Situ Morphological and Structural Study of High Capacity Anode Materials for Lithium-Ion Batteries
No full textLithium-ion batteries (LIBs) have dominated the energy storage market in the past two decades. The high specific energy, low self-discharge, relatively high power and low maintenance of LIBs enabled the revolution of electronic devices and electric vehicle industry, changed the communication and transportation styles of the modern world. Although the specific energy of LIBs has increased significantly since first commercialized in 1991, it has reached a bottleneck with current electrode materials. To meet the increasing market demand, it is necessary to develop high capacity electrode materials.Current commercial anode material for LIB is graphite which has a specific capacity of 372 mAh g-1 . Other group IV elements (silicon (Si), germanium (Ge), tin (Sn)) have much higher capacities. However, group IV elements have large volume change during lithiation/delithiation, leading to pulverization of active materials and disconnection between electrode particles and current collector, resulting in fast capacity fading. To address this issue, it is essential to understand the microstructural evolution of Si, Ge and Sn during cycling.This dissertation is mainly focused on the morphological and structural evolution of Sn and Ge based materials. In this dissertation, an in situ focused ion beam-scanning electron microscopy (FIB-SEM) method is developed to investigate the microstructural evolution of a single electrode particle and correlate with its electrochemical performance. This method is applied to all projects. The first project is to investigate the microstructural evolution of a Sn particle during cycling. Surface structures of Sn particles are monitored and correlated with different states of charge. The second project is to investigate the morphological evolution of Ge particles at different conditions. Different structures (nanopores, cracks, intact surface) appear at different cycling rates. The third project is to study selenium doped Ge (GeSe) anodes. GeSe and Ge particles are tested at the same condition. Se doping forms Li-Ge-Se network, provides fast Li transport and buffers volume change. The fourth project is to study the reaction front of Ge particle during lithiation. Micronsized Ge particles have two reaction fronts and a wedge shape reaction interface, which is different from the well-known core-shell mode. The fifth project is to investigate antimony (Sb)-coated porous Ge particles. The Sb coating suppresses electrolyte decomposition and porous structure alleviates volume change. The results in this dissertation reveal fundamental information about the reaction mechanism of Sn and Ge anode. The results also show the effects of doping, porous structuring and surface coating of anode materials
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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