Physical mechanisms behind plasma-induced liquid flow

Plasma-induced liquid flow is an emerging topic receiving accelerated interest in recent years within the larger inter-disciplinary field of plasma-liquid interactions. The liquid phase transport of reactive oxygen and nitrogen species, plasma produced molecules that have found use in a wide variety of applications, is governed by convective processes. Therefore, controlling the mechanisms that drive plasma-induced liquid flow would lead to control over reactive species transport in liquids, with the potential to thereby improve the efficiency of all plasma-liquid applications. Despite this potential, very little is currently known about the underlying physical processes of plasma-induced liquid flow. This review covers the present limited understanding of the mechanisms behind plasma-induced liquid flow, namely a combination of neutral gas flow, multi-phase electro-hydrodynamic forces, and Marangoni stresses driven by gradients in surface tension. A description is given on the underlying theory behind each of these mechanisms from a general fluid mechanical perspective, along with describing how these mechanisms would arise in plasma-liquid systems. Gaps in the understanding of the fluid mechanics of plasma-liquid interactions are identified and discussed, along with how present and future methods could be used to address these outstanding questions.