Experimental study of light emitted by spark-generated bubbles in water

Abstract

The emission of light from spark-generated bubbles freely oscillating in water far from boundaries is studied experimentally. The observations concentrate on light flashes radiated at final stages of the first bubble contraction and early stages of the following bubble expansion. It is shown that the shape of the emitted light pulses is not “Gaussian”, but asymmetric with a leading edge moderately growing and a trailing edge steeply decreasing. The maximum values and widths of these optical pulses are determined for bubbles having different sizes and oscillating with different intensities. The variation of the maximum values and pulse widths with bubble size and intensity of oscillation is discussed, as well as the observed weak correlation between these two quantities.

The Time Difference in Emission of Light and Pressure Pulses From Oscillating Bubbles

“…At present, there is no explanation for the existence of the time differences δ 1 . But their presence further confirms the earlier findings concerning the autonomous plasma behaviour in bubbles [25,26]. As shown in Figures 7-10, the time difference δ 1 may be of both positive and negative values and these values are only very weakly correlated with the bubble size R M1 , intensity of bubble oscillation p zp1 , and maximum values of the optical radiation u M1 .…”

“…It can be seen that the time difference δ 1 is very weakly correlated with the bubble oscillation intensity p zp1 and that is another proof of the autonomous behaviour of plasma in the bubble interior that has already been observed earlier in works [25][26][27]. As discussed in [25][26][27], this autonomous behaviour manifests itself in the relatively large independence of the light radiation from the plasma surface on the pressure at the bubble wall.…”

“…This phenomenon has also been observed by Golubnichiy et al [11], Huang et al [13] and Zhang et al [14]. Results discussed here have been presented in a brief form at conferences [22,23].…”

“…It can be seen in Figures 5 and 6 that the shapes of the pulses u 1 (t) and p 1 (t) differ, and that the times t u1 and t p1 are not identical. The difference in shapes of the pulses u 1 (t) and p 1 (t) is evidently connected with the autonomous behaviour of plasma in the bubble interior, already discussed in [25][26][27]. The difference in times t u1 and t p1 has not been discussed yet and the existence of this time difference is a surprising fact because a "reasonable" assumption is that the maxima in optical and acoustic radiations will occur at the same instant t c1 , when the bubble is contracted to the first minimum volume.…”

“…As said in Section 2, the spark-generated bubble is described by two parameters, by its size R M1 and oscillation intensity p zp1 . It is convenient to characterise the optical pulse u 1 (t) by two parameters as well [26]. First, it is the maximum value of the pulse u M1 .…”