Thresholds for microbubble and microplasma generation in liquid

Thresholds for low density regions (microbubbles) and microplasma light emissions generation in liquid are studied. With nanosecond microdischarges at different applied peak voltages with the different sizes of microelectrode tip, the low density regions (microbubbles) are captured by a high speed ICCD camera and the microplasmas light emissions are detected by both the ICCD camera and a Photomultiplier Tube (PMT). Deionized water with NaCl was employed as the experimental liquid. Different concentrations of NaCl present different conductivities. With the high conductivity (17600μS/cm), the mciroplasmas light emissions are always accompanied with microbubbles generation at various applied peak voltages (-3kV to -12kV). For a low conductivity (1990μS/cm), there were thresholds of applied peak voltage for different phenomenon of the microplasmas and the microbubbles. With the applied peak voltage in the range of 0kV to -4kV, neither the microbubble formation nor the detectable microplasma slight emission was observed by ICCD and PMT. With the applied peak voltage in the range of -4kV to -6kV, the visible microbubbles was able to be observed, but no microplasma light emission could be detected by both ICCD and PMT. With the applied voltage larger than -6kV in magnitude, both of the visible microbubbles and microplasma light emission can be observed. The thresholds of applied peak voltages are determined from observations. With different applied voltage and same conductivity, the energy inputs are proportional to the applied voltages. Experimental results presented by ICCD and PMT suggest that there exist a correlation of the generation of microplasmas and microbubbles to the applied peak voltages and energy inputs. Different scenarios for the appearances of both of microbubbles and microplasmas, only microbubbles without microplasmas or neither of microbubbles and microplasmas assist our investigation of the mechanism of microplasmas in liquid.