We have investigated the physicochemical basis of electrokinetic charge separation in methanol using micron-sized channel diameters under both turbulent and laminar flow conditions. Turbulent flow studies were
conducted using a 40 μm diameter stainless steel aperture which had a channel length of 0.5 mm. Under
these conditions, electrokinetic streaming currents arose from a charge stripping process in the region close
to the aperture channel wall. The moving liquid removed the relatively weakly held charges from the outer
portion of the electrical double layer formed at the solid−liquid interface. Streaming currents were
simultaneously measured at both the conducting aperture and a downstream copper plate. The magnitudes of
the streaming currents were shown to be equal at the aperture and the plate; however, the sign of the current
at each measurement location was opposite. The magnitude of the streaming currents varied quadratically
with mean liquid flow velocity. Studies under laminar flow conditions were conducted using a 3 cm length
of fused silica capillary which had an internal diameter of 25 μm. Under laminar flow conditions at higher
flow velocities through the nonconducting fused silica channel, the extent of charge separation was ultimately
limited by the extent to which excess charge built up within the capillary could be neutralized. We develop
a simple model that shows how an interplay between fluid flow, ion mobility, and solid−liquid interfacial
chemistry influences the extent of electrokinetic charging in the fused silica channels.