  Unrestricted particle transport through microfluidic channels is of paramount
importance to a wide range of applications, including lab-on-a-chip devices. In
this article, we study using video microscopy the electro-osmotic aggregation
of colloidal particles at the opening of a micrometer-sized silica channel in
presence of a salt gradient. Particle aggregation eventually leads to clogging
of the channel, which may be undone by a time-adjusted reversal of the applied
electric potential. We numerically model our system via the
Stokes-Poisson-Nernst-Planck equations in a geometry that approximates the real
sample. This allows us to identify the transport processes induced by the
electric field and salt gradient and to provide evidence that a balance thereof
leads to aggregation. We further demonstrate experimentally that a net flow of
colloids through the channel may be achieved by applying a square-waveform
electric potential with an appropriately tuned duty cycle. Our results serve to
guide the design of microfluidic and nanofluidic pumps that allow for
controlled particle transport and provide new insights for anti-fouling in
ultra-filtration.
