Text: Cavitation is the formation and activity of bubbles (or cavities) in a liquid. Here the word 'formation' refers, in a general way, both to the creation of a new cavity or to the expansion of a preexisting one to a size where macroscopic effects can be observed. These bubbles may be suspended in the liquid or may be trapped in tiny cracks either in the liquid's boundary surface or in solid particles suspended in the liquid. The expansion of the minute bubbles may effected by reducing the ambient pressure by static or dynamic means. The bubbles then become large enough to be visible to the unaided eye. The bubbles may may contain gas or vapour or a mixture of both gas and vapour. If the bubbles contain gas, then the expansion may be by diffusion of dissolved gases from the liquid into the bubble, or by pressure reduction, or by temperature rise. If, however, the bubbles contain mainly vapour, reducing the ambient pressure sufficiently at essentially constant temperature causes an 'explosive' vaporization into the cavities which is the phenomenon that is called cavitation, whereas raising the temperature sufficiently causes the mainly vapour bubbles to grow continuously producing the effect known as boiling. This means that 'explosive' vaporization or boiling do not occur until a threshold is reached. There are thus four ways of inducing bubble growth: 1. For a gas-filled bubble, by pressure reduction or increase in temperature. This is called gaseous cavitation. 2. For a vapour-filled bubble, by pressure reduction. This is called vapourous cavitation. 3. For a gas-filled bubble, by diffusion. This is called degassing as gas comes out of the liquid. 4. For a vapour-filled bubble, by sufficient temperature rise. This is called boiling. The situation is complicated because the bubble usually contains a mixture of gas and vapour. Looking at it another way, we may distinguish between four different kinds of cavitation according to how it is produced: 1. Hydrodynamic cavitation is produced by pressure variations in a flowing liquid due to the geometry of the system. 2. Acoustic cavitation is produced by sound waves in a liquid due to pressure variations. 3. Optic cavitation is produced by photons of high intensity (laser) light rupturing in a liquid. 4. Particle cavitation is produced by any other type of elementary particles, e.g., a proton, rupturing a liquid, as in a bubble chamber. It has been pointed out that whereas hydrodynamic and acoustic cavitation are brought about by tension in the liquid, optic and particle cavitation are achieved by a local deposition of energy.

See Also: NEGATIVE ATTRACTION; IMPLOSION; VACUUM; VORTEX; ETHER; SCHAEFFER STEAM MACHINE; SUPERCAVITATION

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