## 3 Sonoluminescence in Microfluidics-Based Systems  
The results reviewed thus far have been achieved in laboratory conditions that are, in general, different from technologies and equipment employed in modern biomedical sensing and imaging devices based on microfluidic systems and adjacent approaches. Microfluidic technologies enable a quick and efficient processing of small quantities of fluids contained in microscopic channels [101]. The reduced turnaround time and increased productivity with a small device footprint facilitate the application of microfluidic devices in a variety of experimental situations, but their high portability makes them a technology of choice for point-of-care devices [22; 23]. The physical and chemical properties of oscillating and collapsing gas bubbles discussed in this article have also been essential for the development of microfluidic technologies [18; 22; 102; 103; 104]. Therefore, it is plausible that sonoluminescence can be achieved in microfluidics-based system. Indeed, in the work [86], micromachined pits on a solid substrate were employed to nucleate and stabilise microbubbles in a liquid irradiated with an ultrasound pressure wave, and ultrasound-driven collapse of such bubbles resulted in the emission of light. In turn, the generated light interacted with luminol, which was present in the liquid, thus producing a sonochemiluminescence effect. The intensities of sonoluminescence and sonochemiluminescence optical signals were recorded at several operating regimes related to the peak acoustic pressure amplitude at the ultrasonic frequency  
Figure 6: Spatial patterns of light emitted by (**a**) a spherical bubble in a bulk of water and (**b**) a non-spherical bubble near the liquid metal surface. The black curves outline the contours of the bubble and liquid metal surface. The equilibrium radius of the bubble is 1 μm. (**c**) UV radiation fluence as a function of the ultrasound wave frequency \(f\). The fluence of isotropic radiation is multiplied by 100. The fluence required for the inactivation of _Giardia_ and _Cryptosporidium_ pathogens is indicated. Reproduced from [39] under the terms of a Creative Commons license. of 200 kHz for a pure water environment and for aqueous luminol and propanol solutions. Several arrangements of pits on the substrate were investigated. At high acoustic pressures and in the presence of more than one pit on the substrate, the microbubbles combined into a chain or formed a triangular structure known as a streamer [18]. In all experiments, the intensity of sonochemiluminescence light was amplified as the number of pits was increased. This result was reproducible at both low and high acoustic pressure levels. In another relevant work [26], the conditions suitable for a realisation of sonoluminescence and sonochemistry were created using bubbles confined within a narrow channel of a microfluidic device (Figure 7a), where the bubbles assume a planar pancake-like shape. As a result of the collapse of such bubbles, the authors observed the formation of \(OH\) radicals and the emission of sonoluminescence light (Figure 7b). The observation of chemical reactions was closely linked to the gas-liquid interfaces (Figure 7c), and it was suggested that spatial control over them could be used to control sonochemical reactions. Furthermore, the decay time of the light emitted from the sonochemical reaction was several orders faster than that in the bulk of liquid, and the emission of light vanished immediately as the ultrasound was switched off (also see a relevant discussion of the Purcell effect below). Figure 7: (a) Sketch of the microfluidic device designed in [26] to observe sonochemistry and sonoluminescence effects. The piezoelectric transducer (PZT) used for the creation of the driving ultrasound wave is glued to a glass substrate to which a meandering microfluidic channel is bonded. The inlets consist of two ports for gas and water, respectively. The inset shows a typical distribution of gas and liquids along the microchannel.