## 2 Discussion
### Liquid Metal-Alloy-Nanoparticle-Enhanced Sonoluminescence  
To conclude this section, we review the results of a series of experimental and theoretical works, where it has been suggested that biocompatible microparticles and nanoparticles made of non-toxic room-temperature liquid metal alloys can be used to enhance the intensity of the UV component of sonoluminescence light by means of plasmonic effects [30; 87; 88; 89; 90]. Surface plasmons are optical waves that can be resonantly excited at the interface between a metal and a dielectric material [91]. Localised surface plasmon modes are typically observed at the surface of metal nanoparticles, where the plasmon resonance frequency depends on the nanoparticle size, its constituent material, and the optical properties of the surrounding environment, but the intensity of light confined to nanoparticle's surface can be higher than that of the incident light due to local optical field enhancement processes [8; 61; 92]. Nanoparticles suitable for applications in plasmonics are often made of gold and silver since these two metals exhibit the lowest possible optical absorption losses compared with other metals such as nickel and iron [93; 94]. However, metals other than gold and silver remain valuable in the field of plasmonics because their use may help extend the functionality of nanoparticles. Indeed, plasmon resonances of typical gold and silver nanoparticles lie in the visible and infrared spectral ranges, but their resonance frequencies cannot be readily changed because achieving this would require modifying the shape, size, and constituent material of the nanoparticle [61]. Yet, even though there exist techniques for tuning the resonance frequency of a nanoparticle using, for example, magnetic fields [61], the achievable tuning spectral range is insufficient to shift the resonance from the visible to UV spectral range. To resolve this problem, nanoparticles made of gallium and other metals have been investigated for their UV plasmonic properties (see [90] and the references therein). In particular, nanoparticles made of gallium alloys stand out for their intriguing properties, which include strong plasmon resonances in the UV spectral range, non-toxicity and biocompatibility as well as as the fact that certain gallium alloys can be liquid at room temperature [30; 87; 88; 89]. Subsequently, compared with solid metal nanoparticles that have a constant plasmon resonance frequency, the plasmon resonance frequency of liquid gallium alloy nanoparticles can be readily changed using external electric signals [88]. Yet, it has been suggested that liquid metal nanoparticles and larger structures such as macroscopic liquid metal drops could be deformed using acoustic pressure waves [95] and collapsing bubbles [8]. Indeed, let us recall that non-spherically collapsing bubbles (see the bubble in the middle of Figure 1b for an illustration) can produce a powerful water jet [18]. The direction of the water jet depends on the mechanical properties of the surface near which the bubble collapses. The bubble develops a jet directed towards a rigid surface, but away from a liquid-air interface. Yet, the jet splits near an elastic wall [96; 97]. In the work [39], it was suggested that the collapse of a microbubble near the surface of a macroscopic liquid metal drop can result in the deformation of the surface and the formation of a concave structure, which can serve as a focusing mirror for UV light. Moreover, it was hypothesised that the bubble collapse can lead to the emission of sonoluminescence light and that the interaction of the so-generated light with the liquid metal mirror can result in the formation of a UV light beam, where the intensity of the UV radiation is increased manifold due to the plasmonic properties of the metal when compared with the intensity of light produced as a result of a spherical bubble collapse.