Electrolytic flow in partially saturated charged micro-channels: Electrocapillarity vs electro-osmosis

Wettability is the main factor controlling the fluid flow in an electrically neutral partially saturated micro-channel. If the micro-channel body carries electric charges and is fully saturated by a conductive fluid, electro-osmosis is considered the driving force for fluid movement. The flow of electrolytes in an electrically charged partially saturated micro-channel, however, needs further attention where the electrocapillary and electro-osmosis can simultaneously exist. We, thus, investigated the movement of KCl electrolytes with different concentrations (0.1, 0.5, 1, and 3 M) in a partially saturated (air-filled) and electrically charged micro-channel fabricated in a conductive substrate (aluminum) using micro-fluidics. We additionally studied the contact angle-based wettability alteration of an electrolyte/air/aluminum substrate system under an electric field. This allowed us to link the change in capillary forces due to the electricity-induced wettability alteration to micro-fluidic flow observations, i.e., a link between electro-osmosis and capillary forces. Our theoretical analysis revealed that at low concentration, the role of electro-osmosis and electrocapillarity on fluid flow in partially saturated charged micro-channel is relatively comparable. At 0.1 M KCl concentration, the change in wettability due to the applied electric field contributed to over 42% of the induced flow of the solution in the micro-channel. As the ionic concentration increases, the role of capillary pressure fades and electro-osmosis becomes the dominant process controlling the flow. At 3.0 M KCl concentration, electrocapillarity contributed only 23% to the induced flow under the applied electric field in the micro-channel. The results reveal the importance of electro-osmosis along with electrocapillary flow in partially saturated electrically charged micro-channels.