Aspects of electrokinetic charging in liquid microjets

We have investigated the physicochemical basis of electrokinetic charge sep aration in methanol using micron-sized channel diameters under both turbule nt and laminar flow conditions. Turbulent flow studies were conducted using a 40 mu m diameter stainless steel aperture which had a channel length of 0.5 mm. Under these conditions, electrokinetic streaming currents arose fro m a charge stripping process in the region close to the aperture channel wa ll. The moving liquid removed the relatively weakly held charges from the o uter portion of the electrical double layer formed at the solid-liquid inte rface. Streaming currents were simultaneously measured at both the conducti ng 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 magnitu de of the streaming currents varied quadratically with mean liquid flow vel ocity. Studies under laminar flow conditions were conducted using a 3 cm le ngth of fused silica capillary which had an internal diameter of 25 mu m. U nder laminar flow conditions at higher flow velocities through the noncondu cting 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 i nfluences the extent of electrokinetic charging in the fused silica channel s.