High bubble concentrations produced by ultrasounds in binary mixtures

It was discovered that simultaneous insonification and air blowing of diffe rent aqueous binary solutions such as water/sodium-dodecyl-sulphate (SDS), water/methanol or water/potassium-sulphate yields a very concentrated bubbl e cloud invading the whole vessel in a few seconds. After the end of insoni fication, this cloudiness remained in the solution for about 1 min. The phe nomenon was investigated by computer-treatment of solution pictures recorde d every second after the end of insonification. Turbidity appeared to incre ase with ultrasound power, and also with SDS concentration. During the disa ppearance of the cloud, a turbidity front appeared rising and spreading upw ard. This front was studied in the characteristic plane and interpreted as a spatial segregation of different bubble sizes rising with different termi nal velocities. The bubble sizes involved were estimated to about 10 mum. A dsorption of surface active species are invoked to explain the cloud format ion and its abnormally slow disappearance, but the occurrence of the phenom enon for potassium-sulphate salt remains unexplained. (C) 2001 Elsevier Sci ence B.V. All rights reserved.