Influences of microplasma generated microbubble by moderate environmental pressure

Abstract

A microplasma and a spherical microbubble can be generated simultaneously in liquid. The microbubble dynamics and energy give insight into energy deposited by the plasma. A DC high voltage source in combination with spark gaps were used for pulsing power. The pulsing microsecond or nanosecond discharge was used to generate the microdischarges, and the rise time of the pulsing power was around 15 ns. The magnitude of pulsing voltage, hereafter refer to as the applied peak voltage (APV), and the duration of discharges were varied in this research to control the energy input. The APVs were tuned from −3 kV to −9 kV with 2 kV as interval. The initiation of microplasma and microbubbles have been addressed [1] in our previous research, and the hydrodynamics of microplasma generated microbubble at ambient pressure (0 psig) was studied by using Rayleigh-Plesset (RP) model. With the consideration of the coexistence of condensable and incondensable gases in the microbubble, the RP model matched well with our experimental results. In this paper, the hydrodynamics of microplasma generated microbubble in a pressure controlled environment is studied parametrically. The environmental pressure was adjusted from 0 psig to 80 psig. Two different surface tensions were prepared (71 mN/m and 35 mN/m at 25° C) as well, since the surface tension component in RP model contributes to the surface momentum balance. The generation and the oscillation of microbubbles were recorded by a CCD high speed camera (300 k fps). The radius change of microbubbles corresponding to time under different pressures was depicted. The RP model with different ambient water pressures and surface tensions were studied and compared with our experimental results. One of the advantages of RP model, which is built up on momentum balance, is to allow the estimation of the thermodynamic states and the energy of microplasmas and microbubbles. It was observed that the pressure of the microplasma and microbubble at the initiation stage was very high and decreased significantly in first hundreds of nanoseconds. It is concluded that the moderate environmental pressure won't affect this initiation significantly, but it does affect the hydrodynamics of microplasma generated microbubble after it expands in microsecond scale.