reactions are not seen in the ultrasonic irradiation of solids or solid–gas systems. For example, in chemical kinetics, it has been observed that ultrasound can greatly enhance chemical reactivity in a number of systems by as much as a million-fold; effectively acting to activate heterogeneous catalysts. In addition, in reactions at liquid-solid interfaces, ultrasound breaks up the solid pieces and exposes active clean surfaces through microjet pitting from cavitation near the surfaces and from fragmentation of solids by cavitation collapse nearby. This gives the solid reactant a larger surface area of active surfaces for the reaction to proceed over, increasing the observed rate of reaction., While the application of ultrasound often generates mixtures of products, a paper published in 2007 in the journal Nature described the use of ultrasound to selectively affect a certain cyclobutane ring-opening reaction. Atul Kumar has reported multicomponent reaction Hantzsch ester synthesis in Aqueous Micelles using ultrasound. Some water pollutants, especially chlorinated organic compounds, can be destroyed sonochemically. Sonochemistry can be performed by using a bath (usually used for ultrasonic cleaning) or with a high power probe, called an ultrasonic horn, which funnels and couples a piezoelectric element's energy into the water, concentrated at one (typically small) point. Sonochemistry can also be used to weld metals which are not normally feasible to join, or form novel alloys on a metal surface. This is distantly related to the method of calibrating ultrasonic cleaners using a sheet of aluminium foil and counting the holes. The holes formed are a result of microjet pitting resulting from cavitation near the surface, as mentioned previously. Due to the aluminium foil's thinness and weakness, the cavitation quickly results in fragmentation and destruction of the foil. See also Ultrasound Sonication Ultrasonics ultrasonic homogenizer homogenizer Homogenization (chemistry) Sonoelectrochemistry Kenneth S. Suslick References External links The Chemical and Physical Effects of Ultrasound by Prof. K. S. Suslick Sonochemistry – Short Review and Recent Literature Sonochemistry: New Opportunities for Green Chemistry by Gregory Chatel (Université Savoie Mont Blanc, France) Physical phenomena Chemistry Quantum chemistry Fluid dynamics Ultrasound Acoustics