The application of an alternating electric field to a suspension of colloidal particles above a planar electrode induces additional lateral and vertical forces on those particles, and can lead to net changes in vertical position, as well as aggregative or dis-aggregative lateral motion. In this work, the vertical motion of single colloidal particles in response to alternating electric fields ranging in frequency from 10 Hertz to 40 kHz was studied to determine the underlying physical mechanisms that induce the net forces. Multiple mechanisms were found to be dominant in driving net particle motion over different frequency ranges. At low frequencies, ∼10 Hz to 500 Hz, an electrode reaction driven mechanism was determined to be the primary cause of net particle motion, and from 500 Hz--10 kHz, the spatially nonuniform capacitive polarization of the electrode was determined to be the primary mechanism. The direction and magnitude of the induced forces were found to depend on the specific chosen electrolyte, the frequency and magnitude of the applied field, and the particle size.
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