ICCD light emission image collection and processing

The aforementioned ICCD was also used to image the light emission from the cavitation singularity using a macro lens (AF-S VR Micro NIKKOR 105mm f/2.8G IF-ED) at a 1:1 magnification ratio and f/2.8 aperture. The ICCD image exposure time was 100 or 200 μs with a gain setting of 1000 and was artificially delayed from the clutch release time sensed by the hydrophone for around 880 μs (chosen on the basis of analysis of the high–frame rate videos). This timing correlated to be around the first singularity and was long enough to account for most jitter in the clutch release and short enough to avoid excessive background signal. Multiple background images were taken without triggering the bioinspired device in a dark room. These background images were averaged and subtracted from images acquired during device operation for presentation in this article. The average intensity of background images taken for background subtraction is around 60 counts (out of 4095 counts for a 12-bit system) for distilled water and 73 counts for saline water with an average SD (σbackground) close to 34 ± 3 counts. For the ICCD results and images presented in this article the units are ICCD counts with average background value subtracted. For colorized ICCD images the counts values was scales to range from 3σbackground to maximum intensity with a jet color map to highlight light emission of interest in the image.

For analysis of the light emission ICCD images, a threshold was chosen for distinguishing background and pixels that captured the light emission. A threshold of 9σbackground was chosen; thus, the bright pixels above threshold were statistically significant and corresponded to light emission signals. Because of the stochastic influences in the cavitation formation and collapse, not all singularities had the same brightness. One of the 20 trials did not have light emission exceeding the 9σbackground threshold (see Fig. 4B). In addition, cosmic rays and similar induced excitations could randomly induce ICCD camera signal over limited pixel regions. However, such observed signals did not correlate with the expected location of the singularity or the timing from the clutch release, as was the case for light emission during the cavitation collapse, which is shown in Fig. 4C.