Acoustic streaming in water induced by an asymmetric dielectric-barrier-discharge plasma actuator at the initiation stage

When acoustic waves with broadband frequency and high amplitude pass through a medium, it absorbs their momentum to induce a quasi-steady flow, which is commonly referred to as acoustic streaming (AS). The acoustic energy in AS is clean energy, and actuators that release acoustic energy by AS can control flow without contacting the controlled object and have considerable potential in microfluidic systems for enhancing transport and mixing. Recently, AS was observed to be induced in quiescent air by a dielectric-barrier-discharge plasma actuator. However, a normal AS flow and a tangential wall jet can be created by the plasma actuator in quiescent air. The AS flow suffers unavoidably from the induced wall jet. For example, the location of the production of the AS flow moves downstream of the upper electrode under the influence of the induced wall jet. In addition, whether the plasma actuator can generate AS in a liquid is the key to applying AS in biomedicine and remains unknown. Here, an asymmetric dielectric-barrier-discharge plasma actuator during the first sinusoidal high-voltage cycle when the induced flow field and the effect of the heating are not significant is suspended over the surface of distilled water but not in contact with the water. Importantly, AS in distilled water produced by a plasma actuator and causing depressions in the liquid surface is first observed by using the highly accurate phase-locked image-freezing schlieren technique. Based on the results, the formation process for AS in distilled water is proposed.