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A water droplet immersed in an immiscible fluid under dc electric field can be charged by direct contact to the electrode. The droplet charged from the bottom electrode hops along the arc line of the electric field. It is recharged at the opposite bottom electrode immediately after arriving there. The droplet can be translated according to the intended direction if the signal is moved in the wanted direction at the moment when the drop is in contact with the opposite electrode. The droplet transport is relatively fast: the average velocity is about 10 mm·s−1. Furthermore, it causes less contamination of the chip substrate because a droplet is transported by hopping along the arc of the electric field. Accordingly, surface treatment of the chip substrate is much simpler.
Experimental condition: E = 1.5 kV/cm, V = 2 ml, n = 10 cSt from A novel actuation method of transporting droplets by using electrical charging of droplet in a dielectric fluid, Yong-Mi Jung (정용미) and In Seok Kang (강인석) , Biomicrofluidics 3, 022402 (2009). |
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Simulation of the coalescence of two sessile droplets: This video shows the equi-height contours of the droplets as they spread and coalesce on a partially wetting substrate. It illustrates the evolution of the footprint of the droplet system. Following the rapid growth of the neck bridging the two droplets, the resulting droplet slowly relaxes to a spherical cap configuration.
from Modeling the coalescence of sessile droplets, M. Sellier and E. Trelluyer, Biomicrofluidics 3, 022412 (2009). |
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Nanocolloids that assemble and disassemble within the nanochannel (width 100 µm and depth 200 nm) into a colloidal closed-packed structure.
from Understanding electrokinetics at the nanoscale: A perspective, Hsueh-Chia Chang and Gilad Yossifon, Biomicrofluidics 3, 012001 (2009). |
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Communication between neighboring nanochannels due to overlapping polarized layers at the entrance. The current for a seven-channel array is compared to seven times that for a single-channel array to determine the degree of communication. The single nanochannel dimensions are width w=0.1 mm, height h=200 nm, and length d=0.5 mm. The video shows an array of separate depletion regions emerging from the nanochannel array, just before their merging.
from Understanding electrokinetics at the nanoscale: A perspective, Hsueh-Chia Chang and Gilad Yossifon, Biomicrofluidics 3, 012001 (2009). |
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Rectification of a nanoslit with asymmetric entrances: the depletion-enrichment phenomenon when the anode is at the right reservoir. The left entrance is wider (w=2.3 mm) than the right (w =0.1 mm), as seen in the microscopic image (inset of b in the online article), and is obtained by intentional misalignment of the two Pyrex slides. The nanochannel height is h=200 nm and length is d=0.5 mm. The video shows the depletion-enrichment phenomenon when the anode is at the right reservoir. from Understanding electrokinetics at the nanoscale: A perspective, Hsueh-Chia Chang and Gilad Yossifon, Biomicrofluidics 3, 012001 (2009). |
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Rubinstein's vortex instability in front of a nanoslit develops beyond a critical voltage to select a thin diffusion layer much shorter than the distance to the electrode. The confocal images are taken at 40 V showing vortices at the depletion layer on the anodic side (left reservoir) for an ionic strength of ~0.1 mM. The diffusion layer is allowed to grow to a certain thickness with a low-frequency ac field and the vortices only appear when the frequency is below 0.1 Hz and when the voltage is beyond ~20 V.
from Understanding electrokinetics at the nanoscale: A perspective, Hsueh-Chia Chang and Gilad Yossifon, Biomicrofluidics 3, 012001 (2009). |
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Microflow induced by a pair of orthogonal electrodes at 140 µm separation Fluid conductivity is 20 mS/m. Microflow patterns are delineated by 0.5 µm fluorescent tracer particles. With an ac signal of 500 kHz at 15 VPP, the fluid moves along the pin electrode towards the gap. This microflow pattern is driven by ac electrothermal (ACET) effect. from Investigation of microflow reversal by ac electrokinetics in orthogonal electrodes for micropump design, Kai Yang and Jie Wu, Biomicrofluidics 2, 024101 (2008). |
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Microflow induced by an pair of orthogonal electrodes at 140 µm separation Fluid conductivity is 20 mS/m. Microflow patterns are delineated by 0.5 µm fluorescent tracer particles. With an ac signal of 200 Hz at 20 VPP, the fluid shoots from the pin electrode towards the gap. This is caused by Faradaic polarization at the pin electrode. At relatively high voltage, Faradaic polarization takes place and induces co-ions instead of counter-ions from capacitive polarization at lower voltages. Because the mobile ions are of opposite polarity, the flow direction is opposite to those at lower voltages. from Investigation of microflow reversal by ac electrokinetics in orthogonal electrodes for micropump design, Kai Yang and Jie Wu, Biomicrofluidics 2, 024101 (2008). |
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Microflow induced by a pair of orthogonal electrodes at 140 µm separation Fluid conductivity is 20 mS/m. Microflow patterns are delineated by 0.5 µm fluorescent tracer particles. Â With an ac signal of 1 kHz at 10 Vpp, fluid moves from the electrode gap up along the pin electrode and forms two vortices in the fluid bulk besides the pin electrode. This flow phenomenon is driven by ac electroosmosis (ACEO) effect, where counter-ions are attracted to the electrode surface and move under the influence of electric field to induce microflows. from Investigation of microflow reversal by ac electrokinetics in orthogonal electrodes for micropump design, Kai Yang and Jie Wu, Biomicrofluidics 2, 024101 (2008). |
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Simulation of the generation of an asymmetric chemical gradient field Chemical was fed from an effusion hole located at the center of the chamber while stable lateral medium flow was pumped from left to right. The lateral medium flow provide homogeneous flow field and the central effusion generated a stable (yet variably controllable) chemical gradient. The distribution of the chemical gradient can be easily changed by controlling the relative flow rate between the lateral flow and the effusion flow. Experimental confirmation is described in the main text. from A transparent cell-culture microchamber with a variably controlled concentration gradient generator and flow field rectifier, Ji-Yen Cheng, Meng-Hua Yen, Ching-Te Kuo, and Tai-Horng Young, Biomicrofluidics 2, 024105 (2008). |
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Time-lapsed photography of dye replacement inside a modified Hele-Shaw microchamber The dimension of the cell is 14 mm x 26 mm x 105 µm. The flow rate was set at 10 µL/h by a syringe pump. Although parabolic flow front is observed, the flow field is homogeneous. The flow speed is measured by the shape of the flow front edge. The flow fronts were then digitized and the migration distance was measured to estimate flow speed. The result indicates that the difference between the speed at the centerline and that at the wall is about 7%, which is significantly smaller than a parabolic flow. Mammalian cancer cells have been stably cultured inside this chamber for more than 2 weeks. from A transparent cell-culture microchamber with a variably controlled concentration gradient generator and flow field rectifier, Ji-Yen Cheng, Meng-Hua Yen, Ching-Te Kuo, and Tai-Horng Young, Biomicrofluidics 2, 024105 (2008). |
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Simulation showing the culture medium replacement inside a modified Hele-Shaw microchamber The dimension of the cell is 14 mm x 26 mm x 105 µm. At a flow rate of 10 µL/h, the flow speed inside the chamber is 0.08 µm/s, resulting in insignificant shear stress on cells to be cultured inside the chamber. The observed parabolic front represents the flow front but not the flow speed. Transversally the flow speed is uniform within 90% of the chamber’s cross-section. The simulation shows that entire medium refreshment is achieved in about 10 hours. Slow and uniform flow field is obtained together with efficient medium replacement. from A transparent cell-culture microchamber with a variably controlled concentration gradient generator and flow field rectifier, Ji-Yen Cheng, Meng-Hua Yen, Ching-Te Kuo, and Tai-Horng Young, Biomicrofluidics 2, 024105 (2008). |
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A Dielectrophoretic-Meister for the age of red blood cells The video shows migration of blood cells near a quadrupole electrode as the frequency of the AC field at the electrodes increases. The blood cells start from the negative Dielectrophoretic trap at the middle, migrate to the high field region between the electrodes and migrate back to the center. Such migration pattern indicates that there are two cross-over frequencies for the blood cells. The two values can be easily estimated visually and their values are sensitively dependent on the age of the blood cells. We have also enhanced the sensitivity to age by cross-linking the membrane proteins with glutaldehyde. from Dielectrophoretic discrimination of bovine red blood cell starvation age by buffer selection and membrane cross-linking, J.E. Gordon, Z. Gagnon, and H.-C. Chang, Biomicrofluidics 1, 044102 (2007). |
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Trapping rate of fluorescent 5 µm latex particles in the crescent electrode The sorted latex particles are trapped at a trapping gate with electrodes on the top and bottom substrates working in the negative DEP region for the sorted particles. With the symmetric electrode design, a DEP trap exists at the tip of the trapping gate to trap the particles at a very high rate of 100 partilces per second. The number of trapped particles can be estimated from the impedance signal across the electrodes. There is no flow resistance with this trap as there is no electrical force on the liquid. from An integrated dielectrophoretic chip for continuous bioparticle filtering, focusing, sorting, trapping, and detecting, I.F. Cheng, H.C. Chang, D. Hou, and H.-C. Chang, Biomicrofluidics 1, 021503 (2007). |
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Sorting latex particles and Lactobacillus based on their negative DEP mobilities The sorter unit with 50-micron wide electrodes fabricated onto the top and bottom substrates and operated at a selected frequency to impart a high field between the two gating electrodes. The liquid goes through the gate unimpeded into a downstream channel and so do 1-micron latex particles suffering from positive DEP. Larger 5 micron latex particles cannot go through the gating electrode and slide along the electrode gate into a different channel. Sorting into different bins by size is hence achieved at about 100 particles per second. from An integrated dielectrophoretic chip for continuous bioparticle filtering, focusing, sorting, trapping, and detecting, I.F. Cheng, H.C. Chang, D. Hou, and H.-C. Chang, Biomicrofluidics 1, 021503 (2007). |
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Filtering and focusing stages on a dielectrophoretic (DEP) chip with C. albicans and latex The focusing units on the chip at high frequencies corresponding to negative DEP. The first set of periphery electrodes focus the particles to a 50 micron wide area in the middle of the channel and the second one, with a smaller orifice, line them up into a single queue within a 10 micron wide region. Such focusing allows each colloid to experience the same chemical and electrical environment in the chip and hence can be sorted according to their DEP mobility difference. from An integrated dielectrophoretic chip for continuous bioparticle filtering, focusing, sorting, trapping, and detecting, I.F. Cheng, H.C. Chang, D. Hou, and H.-C. Chang, Biomicrofluidics 1, 021503 (2007). |
US - NY - Hamilton
Assistant Professor of Biophysics
KLA-Tencor
US - CA - San Jose
Applications Development Engineer
Washington State Univ. / Institute for Shock Physics
US - WA - Pullman
Postdoc - Optical Spectroscopy
Washington State Univ. / Institute for Shock Physics
US - WA - Pullman
Postdoctoral Research – Computational Chemistry
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