The behavior of small bubbles generated by electrical current impulses over a wide range of applied pressures

Very small bubbles are generated in chemically different insulating liquids in a repeatable and controlled way. A constant dc voltage is applied between a sharp point and a plane electrode. Above a threshold voltage, the current is ‘‘impulsional.’’ The origin of each impulse is an electron avalanche while the consequence is the formation of a small bubble at the point electrode. The relationship between the injected electrical energy and the maximum bubble size is examined. Depending on the liquid used, and therefore the level of energy injected, different behaviors were observed. In particular, results for very low energy injections were explained by mechanical rather than thermal processes. The nonequilibrium plasma formed following an electron avalanche, brings about the emission of a pressure transient, behind which there is an ‘‘afterflow’’ of matter. The liquid travelling outwards from this point source causes the liquid to break down mechanically, thus forming an almost empty cavity, which disappears on collapse. In the case of high energy injection, energy is lost to local heating of the liquid.