(221h) A Method for Measuring Simultaneously the Fluid and Particle Mobilities under Strong DC and Low-Frequency Ac Fields

Recently, electrokinetic phenomena have gained considerable attention as a means of separating charged species and for the transport of suspensions in microdevices. The application of these phenomena in various ?lab-on-a-chip? devices requires the quantification of the motion of both the fluid and the suspended species. Moreover, for a fast and efficient separation and transport, strong field strengths are required, but their use has been limited because the standard microchannel surface treatment protocols employed to control the electroosmotic flow are unable to withstand the effects of high field strengths and are usually washed out. To overcome this difficulty, we have developed a method in which the fluid electroosmotic velocity and the particle ?apparent? velocity inside a capillary are measured simultaneously under strong DC fields. The particle electrophoretic velocity is then computed from their difference and is found to remain independent of the surface properties of the channel.

The experiments were conducted by employing strong DC fields (upto ~1kV/cm) along a capillary (50 micron inside diameter) initially filled with a dilute suspension (c~0.00025v/v) consisting of 4 micron diameter polystyrene particles uniformly dispersed in deionized water. The results of our DC field experiments showed that the fluid electroosmotic and particle electrophoretic velocities remained proportional to the applied field strength over the whole range from 100V/cm to 1kV/cm. We also obtained the apparent particle velocity under low-frequency AC field (frequency 5-50Hz) by measuring the amplitude of the particle oscillations which, as expected, was found to be proportional to the field strength (at fixed frequency), and inversely proportional to the frequency of the applied field (at fixed field strength). Close agreement was found between the values for the apparent mobilities as obtained from the DC and AC field experiments.