  Here, results are presented on the focusing of $1 \ \mu$m polystyrene
particle suspensions using a synchronous oscillatory pressure-driven flow and
oscillatory electric field in a microfluidic device. The effect of the phase
difference between the oscillatory fields on the focusing position and focusing
efficiency was investigated. The focusing position of negatively charged
polystyrene particles could be tuned anywhere between the channel centerline to
the channel walls. Similarly, the focusing efficiency could range from $20\%$
up to $90\%$, depending on the phase difference, for particle Reynolds numbers
of order $O(10^{-4})$. The migration velocity profile was measured and the peak
velocity was found to scale linearly with both the oscillatory pressure-driven
flow amplitude and oscillatory electric field amplitude. Furthermore, the
average migration velocity was observed to scale with the cosine of the phase
difference between the fields, indicating the coupled non-linear nature of the
phenomenon. Lastly, the peak migration velocity was measured for different
particle radii and found to have an inverse relation, where the velocity
increased with decreasing particle radius for identical conditions.
