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Mapping alternating current electroosmotic flow at the dielectrophoresis crossover frequency of a colloidal probe
Author(s) -
Wang Jingyu,
Wei MingTzo,
Cohen Joel A.,
OuYang H. Daniel
Publication year - 2013
Publication title -
electrophoresis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.666
H-Index - 158
eISSN - 1522-2683
pISSN - 0173-0835
DOI - 10.1002/elps.201200614
Subject(s) - dielectrophoresis , electrode , electric field , microelectrode , low frequency , amplitude , crossover , analytical chemistry (journal) , chemistry , optics , materials science , molecular physics , physics , quantum mechanics , astronomy , artificial intelligence , chromatography , computer science
AC electroosmotic ( ACEO ) flow above the gap between coplanar electrodes is mapped by the measurement of S tokes forces on an optically trapped polystyrene colloidal particle. E 2 ‐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. E 2 ‐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.