Materials synthesis in a bubble
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Introduction The generation and oscillation of transient bubbles in liquids is termed “cavitation.” At maximum expansion, the bubble pressure is low and the bubble will collapse due to liquid pressure and surface tension. Important types of cavitation include hydrodynamic, acoustic (e.g., ultrasonic), and laser-induced cavitation.1–3 Cavitation phenomena involve a high degree of spatiotemporal energy concentration upon bubble collapse that can result in modifications of adjacent structures (e.g., cavitation erosion). The high pressure and temperature produced upon bubble collapse can also be used for materials processing and synthesis of new materials. In addition, the gas phase of the bubble could contain solid particles and liquid droplets produced by local heating or laser ablation of a solid target immersed in liquid, setting nanoparticles free after collapse. Finally, the shock waves generated by the rebounding of a collapsed bubble4,5 can have substantial effects on solid particulates suspended nearby in the liquid,6,7 including fragmentation of friable, brittle materials and agglomeration of malleable ones.8 Cavitation erosion is an important feature of hydrodynamic cavitation (e.g., in pumps and propellers) and is associated with the interaction of individual bubbles and bubble clouds with adjacent boundaries.2,9–11 It has been the historical starting
point for intense research on bubble dynamics. Bubble interactions with solid or elastic boundaries generate jetting phenomena arising from the focusing of the liquid flow rushing into the collapsing bubble under aspherical boundary conditions.1,9,12 Jets concentrate bubble energy at locations away from the bubble center and may thus contribute to material erosion or intended modifications. However, they also transform part of the bubble energy into rotational flow energy, which reduces the collapse pressure and temperature.9 The highest pressures and temperatures are observed during spherical bubble dynamics, when the entire bubble energy contributes to the compression of the bubble interior and the surrounding liquid. Spherical bubble oscillations can be induced optically or acoustically. In optical cavitation, tightly focused short laser pulses induce plasma formation at the laser focus (“optical breakdown”13) that drives rapid bubble expansion followed by several spherical oscillations.1,3 In acoustic cavitation, preexisting nuclei are excited by an external sound field.1 When the frequency of the sound field matches the eigenfrequency of the bubble, resonant excitation produces large bubbles that vigorously collapse, concentrating the ambient acoustic energy by 12 orders of magnitude,14–16 which produces pressures >103 MPa and temperatures >104 K. Thus, spherical collapse may be associated with luminescence and even plasma emission from an optically opaque core—a
Stephan Barcikowski, Technical Chemistry I and Center for Nanointegration Duisburg-Essen, University of Duisburg-Essen, Germany; [email protected] Anton Plech, Institute for Photon
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