Xiao, Peng (2015-08). Investigation of Microplasma Generation in Dense Medium. Doctoral Dissertation.
This dissertation presents the experimental studies of low energy microplasma discharges in liquid and the model studies of microdischarge-generated microbubbles in liquid. The main focus is to determine the properties of the microbubble and relate them to the initiation and dynamics of the observed microplasma. Microplasma in liquid began to draw researchers' attention in recent decades because of its low energy input, micron scale size and nanosecond scale duration. The understanding of plasma discharges in gases has been well established; however, the mechanism of plasma initiation in liquid is still unclear. Several theories were proposed to provide different explanations of this mechanism, but none of them have been proven exclusively correct. Results here show that the generation of a microplasma needs higher energy than the generation of a microbubble from the same discharges. This supports the theory that during the microplasma initiation a lower density gaseous site is generated before the microplasma and the microplasma exists inside a microbubble. The typical form of a plasma and bubbles in liquid reported in literature are branch-like structures. Here we report spherical microplasma and microbubble afforded by relatively lower energy input, and it was found that for branched bubbles increasing the ambient pressure was able to reduce the microbubble size and eliminate its branches. The dynamics of the spherical bubbles could be modeled with a customized Rayleigh-Plesset model considering both condensable and incondensable gases in the bubble, the initial neutral temperature and neutral pressure were estimated to be as high as 550 K and 1.2 GPa.