Abstract
In the present review, complexity in multibubble sonoluminescence (MBSL) is discussed. At relatively low ultrasonic frequency, a cavitation bubble is filled mostly with water vapor at relatively high acoustic amplitude which results in OH-line emission by chemiluminescence as well as emissions from weakly ionized plasma formed inside a bubble at the end of the violent bubble collapse. At relatively high ultrasonic frequency or at relatively low acoustic amplitude at relatively low ultrasonic frequency, a cavitation bubble is mostly filled with noncondensable gases such as air or argon at the end of the bubble collapse, which results in relatively high bubble temperature and light emissions from plasma formed inside a bubble. Ionization potential lowering for atoms and molecules occurs due to the extremely high density inside a bubble at the end of the violent bubble collapse, which is one of the main reasons for the plasma formation inside a bubble in addition to the high bubble temperature due to quasi-adiabatic compression of a bubble, where “quasi” means that appreciable thermal conduction takes place between the heated interior of a bubble and the surrounding liquid. Due to bubble–bubble interaction, liquid droplets enter bubbles at the bubble collapse, which results in sodium-line emission.
Highlights
Multibubble sonoluminescence (MBSL) is the light emission phenomenon from cavitation bubbles in liquid irradiated by strong ultrasound (Figure 1) [1,2,3]
MBSL has been used to visualize an acoustic field through the spatial distribution of active bubbles in SL especially in studies on ultrasonic cleaning, sonochemical reactors, and medical applications such as cancer treatment using high-intensity focused ultrasound (HIFU) [59,60,61,62,92,93]
MBSL from vaporous bubbles is by chemiluminescence of OH as well as emissions from weakly ionized plasma such as electron-atom bremsstrahlung
Summary
Multibubble sonoluminescence (MBSL) is the light emission phenomenon from cavitation bubbles in liquid irradiated by strong ultrasound (Figure 1) [1,2,3]. Due to the inertia of the ingoing liquid, the bubble collapse becomes very violent. At the end of the violent bubble collapse, temperature and pressure inside a bubble significantly increase to more than 4000 K and 300 bar (1 bar = 105 Pa = 0.987 atm), respectively [7,8], due to quasi-adiabatic compression of a bubble, where “quasi-“ means the appreciable amount of thermal conduction takes place between the heated interior of a bubble and the surrounding liquid. 2. ThAenoorethtiecralsiMgnoidfiecal nt finding in SL research is the evidence of (thermal) plasma formation inFisrisdtelya, bmuobdbelel oinf bsuulbfubrleicdaycnidaminicospdtiecavlesloppeectdrainofstSuBdSiLesboyfFSlaBnSnLigisanreavniedwSeuds.liSckomine 2b0u0b5b[l1e7s]i.n MBSL are nearly isolated and spherical, which are similar to SBSL bubbles [18].
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