Aspects of electrokinetic charging in liquid microjets

Abstract

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.