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 (<1.5 vol%). A combination of the saturation gases argon (79 vol%), oxygen (20 vol%), and carbon dioxide (1 vol%) gives the highest radical production. Argon has a low heat capacity which results in high hot spot temperatures. Oxygen participates in new chemical reactions, which gives a higher probability of conversion of initial radicals towards the end product. The mechanism through which carbon dioxide enhances the radical production is speculative. It may be possible that the shape of the cavitation bubbles and micro bubble ejection is altered by enhanced mass transport of carbon dioxide into the bubble. Sometimes a high reactivity by radicals is undesired in an ultrasound process. An example of such a process is the study of scission of polymer-metal complexes by strain induced by a collapsing bubble. By mechanical scission of the complex the metal inside it becomes exposed and becomes available for catalytic reactions. This opens up interesting applications such as self-regenerating materials or materials that change colour when they are exposed to strain. Thermal activation and activation by radicals must be low compared to mechanical scission to be able to study the mechanical activation precisely. This has been done by changing the saturation gas from argon to methane or nitrogen. The radical production is 2 times higher under argon than under a nitrogen atmosphere, and 20 times higher under argon than under a methane atmosphere. The hot spot temperature correlates with the radical production. The scission percentages are roughly the same under these gases, indicating that mechanical scission is the most important mechanism of complex breakage. This is supported by model calculations of a collapsing bubble in a liquid saturated with these gases. An alternative method to create cavitation and its effects is by creating a flow through a constriction. The pressure inside the constriction drops to values sufficient to induce bubble growth. Subsequently the bubble collapses. Miniaturisation of this process is desired because it allows easy study in precisely manufactured geometries, and because the apparent frequency of the process increases. Experimental observations have been compared to computational fluid dynamics modeling results. This shows that it is important that the constriction exits in an unconfined area. If the constriction exit is confined, an extended low pressure region can occur, which induces excessive bubble growth and a non-violent implosion of the bubbles. Values of about 160 kHz in 0.75 mm capillaries are reached in 0.2 mm constrictions operated with a flow of 90 mL min-1, equal to a liquid velocity in the constriction of 59 m s-1. A radical production in the same range as those reported in previous work on hydrodynamic cavitation is measured with these settings. Chlorohydrocarbons have been added to the liquid to increase radical production. Simulations of the temperature of a collapsing bubble show a qualitative correlation with the radical production. A physical representation of the propagation of sound waves in a liquid-gas slug flow has been developed to be able to safely operate a gas-liquid micro separator. An undesired pressure increase by e.g. temporary blocking of an upstream microchannel can result in leaking of the liquid to the gas side. A safety pressure release valve and a safety capillary are incorporated into the process. The safety valve releases any undesired over pressure to the atmosphere. The safety capillary and the pressure pulse propagation speed are needed to be able to have enough time to open the valve. A second advantage of the developed equation is that, together with an equation for pressure drop, the gas bubble length and gas fraction inside the capillary can be calculated. This offers opportunities for online characterisation of slug flow in micro reactors.