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Prime rat suggested that I explain cavitation. Please also refer to Roger's post which is informative. Cavitation in the present context is the formation of a gas phase (eg. a bubble) in liquid by transient supersaturation of the liquid due to a sudden reduction in the local ambient pressure. Remember that supersaturation exists if the gas and vapour pressure in a liquid exceeds ambient pressure. Such reductions in ambient pressure occur in a syringe when the plunger is withdrawn against a closed end as the volume is increased, another situation is when two closely opposed surfaces in a fluid are rapidly drawn apart, the flow of liquid into the resulting space is resisted by the viscosity of the liquid creating a transient localized reduction of ambient pressure. An example of the latter situation are the articular surfaces of a joint in the synovial capsule (eg. a knuckle). The resultant pressure reductions can be many hundreds of atmospheres. During such brief periods of negative pressure the solution is supersaturated and bubbles can form. The first event is the formation of a bubble of water vapour, vapourous cavitation. Since in the syringe and knuckle contain aqueous solutions there is plenty of water to form vapour from so a large gas phase can occur. The water is essentially boiling, a phenomena that is based on increasing water vapour pressure above ambient pressure (by raising the water temperature or lowering ambient pressure). This vapourous bubble will be extinguished as soon as the pressure increases, this is apparently the popping sound in a knuckle. Secondary to vapourous cavitation, gaseous cavitation may occur if gas dissolved in the liquid comes out of solution during the brief period of suppersaturation. Physics is stacked against the formation of small (there is less gas available than water vapour) bubbles because surface tension tends to collapse them, but the gas can easily evolve into the vapourous bubble. Under the appropriate conditions (another story), the gaseous phase may persist after the vapourous phase is extinguished. Some people call this form of cavitation tribonucleation. There are many other forms of cavitation, some already mentioned in other posts. I doubt these bubbles in the joints cause any DCI in situ, if you have ever seen an arthroscopy the synovial fluid is full of bubbles introduced during the procedure and these patients do not develop DCI. I suppose that theoretically the bubbles may track along the articular surfaces to somewhere that causes DCI but I can't think where. The point about cavitation is that it is evoked to explain the formation of gas nuclei from which bubbles can grow during decompression since the formation of gas nuclei by the `supersaturations' theoretically produced by diving is impossible. Cavitation may occur elsewhere in the body. Although the literature is divide, it seems that dead animals don't bubble as much as live ones. David Doolette ddoolett@me*.ad*.ed*.au*
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