Direct measurement of slip length in electrolyte solutions

Electrokinetic effects and electrostatic repulsion between tracer particles and glass surface have both been proposed as possible sources that would lead to false slip results obtained from velocimetry-based measurements. Using a three-dimensional total internal reflection velocimetry technique, we address such a concern by comparing the measured slip lengths between nonionic solutions and electrolyte solutions whose ionic concentrations have been predicted to reduce the electricity-induced slip effect to a submolecular level. It is observed that the presence of electrolytes has no effect on the measured slip lengths, suggesting that the observed slip velocities are most likely not due to electrostatic and electrokinetic effects, but are consequences of true boundary slip.