Mapping alternating current electroosmotic flow at the dielectrophoresis crossover frequency of a colloidal probe |
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| Authors: | Jingyu Wang Ming‐Tzo Wei Joel A Cohen H Daniel Ou‐Yang |
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| Institution: | 1. Department of Physics, Lehigh University, , Bethlehem, PA, USA;2. Bioengineering Program, Lehigh University, , Bethlehem, PA, USA;3. Department of Biomedical Sciences, University of the Pacific, , San Francisco, CA, USA;4. Emulsion Polymers Institute, Lehigh University, , Bethlehem, PA, USA |
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| Abstract: | AC electroosmotic (ACEO) flow above the gap between coplanar electrodes is mapped by the measurement of Stokes forces on an optically trapped polystyrene colloidal particle. E2‐dependent forces on the probe particle are selected by amplitude modulation (AM) of the ACEO electric field (E) and lock‐in detection at twice the AM frequency. E2‐dependent DEP of the probe is eliminated by driving the ACEO at the probe's DEP crossover frequency. The location‐independent DEP crossover frequency is determined, in a separate experiment, as the limiting frequency of zero horizontal force as the probe is moved toward the midpoint between the electrodes. The ACEO velocity field, uncoupled from probe DEP effects, was mapped in the region 1–9 μm above a 28 μm gap between the electrodes. By use of variously sized probes, each at its DEP crossover frequency, the frequency dependence of the ACEO flow was determined at a point 3 μm above the electrode gap and 4 μm from an electrode tip. At this location the ACEO flow was maximal at ～117 kHz for a low salt solution. This optical trapping method, by eliminating DEP forces on the probe, provides unambiguous mapping of the ACEO velocity field. |
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| Keywords: | Alternating current electroosmotic flow Colloids Dielectrophoresis Optical tweezers |
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