Surface Bubbles in Micro- and Nanofluidics

In this Proceeding we review and summarize recent findings of the Twente Physics of Fluids group on controlled cavitation of surface microbubbles [1-4] and surface nanobubbles [5] and in addition give some overview on the Twente findings on surface nanobubbles [6-9]. - Gas accumulation at liquid-solid interfaces can occur in the form of bubbles, for instance at a surface defect. The resulting micro- and nanoscale air pockets can act as nucleation sites in shockwave-induced cavitation experiments, leading to the random nature of cavitation, both in space and time. However, by controlling the size and the position of the defects, Bremond et al. [2; 3] succeeded to make cavitation perfectly reproducible both in time and space. This technique made it possible to study the dynamics of individual bubbles, bubble pairs, and bubble clusters emerging from the defects in an extremely reproducible fashion. In addition, by further minaturizing the pits down to diameters of 100nm, Borkent et al. [4] could measure the pressure at which these bubbles start to nucleate, giving results in perfect agreement with the theory by Atchley and Prosperetti [10]. In contrast, so-called surface nanobubbles, though comparible in size and volume to the bubbles in the nanopits, do not act as nucleation sites in cavitation experiments [5]. Finally, we report on some typical properties of surface nanobubbles [6] and report that they can be created in a reproducible and controllable way by electrolysis [8]. That suggests that these surface nanobubbles are in a dynamic equilibrium, i.e., gas in- and outflux are balanced. Such a dynamic stabilization has also been suggested for standard surface nanobubbles [11].