Cavitation in gas-saturated liquids

Oscillating gas bubbles can be created in a liquid by exposing it to ultrasound. These gas bubbles implode if the sound pressure is high enough. This process is called cavitation. Interesting phenomena take place during the collapse. The gas and vapour inside the bubble are compressed and reach temperatures of several thousand Kelvin and pressures of several hundred bars, a dense plasma is formed inside the bubble, light is emitted, reactive radical molecules are formed, and there is a liquid flow around the bubble which can be utilised for mixing or even scission of polymers by strain rates. An eroding jet is formed if the bubble collapses near a wall. The pressure and temperature are at ambient conditions around the bubble during this process. There is a plethora of applications which can be operated or intensified by cavitation, e.g. micro mixing, catalyst surface renewal and material synthesis, treatment of kidney stones, waste water treatment and other radical-induced chemistry such as polymerisation, and polymer weight distribution control. In this thesis, several forms of cavitation have been investigated. Special attention is paid to the influence of gas and vapour content in the cavitation bubble. The gas and vapour content of the bubble play a crucial role in the extent of the effects of cavitation. First and foremost the thermal properties adiabatic index and heat conductivity determine the maximum temperature during the collapse. Gases with a high adiabatic index, such as noble gases, and liquids with a low vapour pressure and a high adiabatic index, such as water or sulfuric acid, yield the highest hot spot temperatures and therefore the most intense effects. Some gases such as oxygen also participate in chemical reactions. Another difference between cavitation effects of several gases may be caused by changing gas solubility in the liquid. This changes the concentration gradient around the bubble, and the mass transport to and from the bubble. The effects of the gas on the cavitation process depend on the ultrasound frequency. At frequencies above 20 kHz air has a higher efficiency of radical formation than argon as a saturation gas. This has been measured by following the oxidation of potassium iodide to iodine spectroscopically. The carbon dioxide in the air contributes to this increase at low ultrasound input power. This is surprising since carbon dioxide addition in the cavitation bubble gas phase likely suppresses the hot spot temperature. The enhancement of radical production by carbon dioxide only occurs when it is present in low quantities (