  Micromachined pits on a substrate can be used to nucleate and stabilize
microbubbles in a liquid exposed to an ultrasonic field. Under suitable
conditions, the collapse of these bubbles can result in light emission
(sonoluminescence, SL). Hydroxyl radicals (OH*) generated during bubble
collapse can react with luminol to produce light (sonochemiluminescence, SCL).
SL and SCL intensities were recorded for several regimes related to the
pressure amplitude (low and high acoustic power levels) at a given ultrasonic
frequency (200 kHz) for pure water, and aqueous luminol and propanol solutions.
Various arrangements of pits were studied, with the number of pits ranging from
no pits (comparable to a classic ultrasound reactor), to three-pits. Where
there was more than one pit present, in the high pressure regime the ejected
microbubbles combined into linear (two-pits) or triangular (three-pits) bubble
clouds (streamers). In all situations where a pit was present on the substrate,
the SL was intensified and increased with the number of pits at both low and
high power levels. For imaging SL emitting regions, Argon (Ar) saturated water
was used under similar conditions. SL emission from aqueous propanol solution
(50 mM) provided evidence of transient bubble cavitation. Solutions containing
0.1 mM luminol were also used to demonstrate the radical production by
attaining the SCL emission regions.
