Bubbles with tunable mobility of surfaces in ethanol-NaCl aqueous solutions

The mobility of bubble surfaces in aqueous solutions can be hindered by even a trace amount of contaminants at the air-liquid interface. Adding ethanol into an aqueous solution is expected to modify the activity of the contaminants and hence their level at the air-liquid interface, which can possibly tune the mobility of the bubble surface. The mobility of the bubble surface was characterized by measuring dynamic interactions between a millimeter-size air bubble and a flat mica surface in ethanol-NaCl aqueous solutions using the newly developed dynamic force apparatus and quantified by the Stokes-Reynolds-Young-Laplace model. For a given bubble approach velocity, the hydrodynamic boundary condition at the air-liquid interface deviated from a tangentially immobile boundary at a critical ethanol concentration and converged gradually to fully mobile boundary with further increasing the ethanol concentration. Increasing the bubble approach velocity was found to reduce the critical ethanol concentration that was needed to change the mobility of the bubble surface. By adding surfactants to the system of high ethanol concentration and fully mobile air-liquid interface, the boundary condition became immobile when the amount of surfactant reached a critical concentration. This study introduced a method to control the hydrodynamic boundary condition at the air-liquid interface in mixed liquids.