Sonoluminescence and sonochemiluminescence from a microreactor

Abstract

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.

Streaming flow by oscillating bubbles: quantitative diagnostics via particle tracking velocimetry

“…Such regimes greatly enhance sonochemical reactions when compared with conventional sonoreactors (Fernandez Rivas et al 2010, 2012a, 2013b. The pressure oscillations produced are also able to generate microcavitation in the surroundings of the bubble, which can be used to remove undesirable elements (Fernandez Rivas et al 2012b), but also cause erosion (Fernandez Rivas et al 2013a;van Wijngaarden 2016).…”

Enhancing acoustic cavitation using artificial crevice bubbles

“…The increase of cavitation activity would seem just a matter of increasing the acoustic amplitude, or input power. However, due to effects such as acoustic shielding and bubble-bubble interactions, this does not necessarily lead to an increase in efficiency [10][11][12]. There is also a frequency dependent lower acoustic power threshold for cavitation activity, usually referred to as the nucleation threshold [13].…”

Taming acoustic cavitation

“…radicals by reaction with terephthalate anions clearly proved that the presence of pits enhanced significantly the radical formation rate, which was proportional to the number of pits and power setting. Though, the slope of the reaction rate curve shows a descending trend as the number of pits and power applied increased, which could be attributed to bubble-bubble and bubble cluster-cluster interactions [95,96]. In contrast, non-pitted silicon substrates showed little evidence of ultrasonic effect.…”

“…By monitoring SL in water and aqueous propanol, evidence of transient cavitation bubbles could be inferred from the latter. It is also noteworthy that SL and SCL in aqueous solutions gave different light intensities trends, which suggest that bubble populations respon-sible for light emission are different from the number of chemically-active bubbles [95].…”

“…The main advantage of this method is that the location of cavitation occurrence is known a priori, which allows a detailed study of several relevant aspects of sonochemistry, such as SL, SCL and other effects such as erosion, cleaning action of bubbles, shock waves, jetting, streaming, etc. [38,95,96]; all of which have been difficult to address in the past, given the rather randomness of cavitating bubbles associated phenomena. Though the studies have been carried in the microscale, it has been proposed that this strategy of modifying the walls of a large scale sono-reactor, could lead to improvement in efficiency and versatility of already existent systems.…”