Bubble, bubble, toil and trouble… a closer look at cavitation – Chew Hong En

The next time you boil water in a beaker over a Bunsen flame, consider how the water actually boils: Even before reaching boiling point, small bubbles begin to form near the base and sides of the beaker, with the bubbling becoming more and more violent as the temperature rises. Here, heating the liquid raises the vapour pressure of liquid to match atmospheric pressure so that bubbles can form within the bulk liquid.

There is, however, a much more interesting way in which bubbles can form in bulk liquid. Instead of raising the liquid vapour pressure (by increasing temperature) to equal external atmospheric pressure, why not lower the local liquid pressure to a point below that of the liquid’s vapour pressure? This phenomenon, known as cavitation, is an important research area in the study of fluid dynamics, and can be used for a mindboggling range of application ranging from industrial mixing machines to water purification to the removal of kidney stones and even to catching fish!

Meet the pistol shrimp. This little creature competes with much larger animals like the Sperm Whale and Beluga Whale for the title of ‘loudest animal in the sea’. The animal snaps a specialized claw shut to create a cavitation bubble that generates acoustic pressures of up to 80 kPa at a distance of 4 cm from the claw. As it extends out from the claw, the bubble reaches speeds of 97 km/h and releases a sound reaching 218 decibels (just for comparison, a plane taking off around 100m away is only 120 decibels loud). Although the duration of the click is less than 1 millisecond, the spike in pressure is strong enough to stun and even kill small fish.

More interestingly, the snap can also produce sonoluminescence from the collapsing cavitation bubble. As it collapses, the cavitation bubble reaches temperatures of over 5000 K (the surface temperature of the sun is estimated to be around 5800 K!). And if all this is not enough, the pistol shrimp also has a bigger cousin – the mantis shrimp – whose club-like forelimbs can strike so quickly and with such force (these creatures have been known to break aquarium glass) as to induce sonoluminescent cavitation bubbles upon impact! So how does motion (e.g. of those claws you see in the picture) actually result in cavitation? The simple answer is that if the motion of a body within a fluid is fast enough such that the region behind the object is “vacated” by the object faster than water can rush in to fill its place, a region of localized low pressure develops and cavitation bubbles can form.

Cavitation was first studied by Lord Rayleigh in the late 19th century, when he considered the collapse of a spherical void within a liquid. As briefly discussed earlier, cavitation inception occurs when the local pressure falls sufficiently far below the saturated
vapour pressure, a value given by the tensile strength of the liquid. This may occur behind the blade of a rapidly rotating propeller or on any surface vibrating in the liquid with sufficient amplitude and acceleration. In order for cavitation to occur, the bubbles generally need a surface on which they can nucleate (e.g. the sides of a container, impurities in the liquid, or even small undissolved microbubbles within the liquid). It is generally accepted that hydrophobic surfaces stabilize small bubbles. These pre-existing bubbles start to grow unbounded when they are exposed to a pressure below the threshold pressure, termed Blake’s threshold.

However, physical motion of bodies in liquid is not the only means by which cavitation can occur. Acoustic cavitation occurs whenever a liquid is subjected to sufficiently intense sound or ultrasound (that is, sound with frequencies of roughly 20 kHz to 10 MHz). When sound passes through a liquid, it produces compressions and rarefactions (low pressure regions!). Hence, if the sound intensity high enough, it can cause the formation, growth, and rapid recompression of vapour bubbles in the liquid. Other ways of generating cavitation voids involve the local deposition of energy, such as an intense focused laser pulse (optic cavitation) or with an electrical discharge through a spark.

To correct our previous hand-waving explanation on the region being “vacated” faster than water can rush in, the cavitation bubble is not actually a vacuum. Vapour gases evaporate into the cavity from the surrounding medium; thus, the cavity is not a perfect vacuum, but has a relatively low gas pressure. Such a low-pressure cavitation bubble in a liquid begins to collapse due to the higher pressure of the surrounding medium. As the bubble collapses, the pressure and temperature of the vapour within it increases. The bubble eventually collapses to a minute fraction of its original size, at which point the gas within dissipates into the surrounding liquid via a rather violent mechanism, which releases a significant amount of energy in the form of an acoustic shock wave and as visible light. At the point of total collapse, the temperature of the vapour within the bubble may be several thousand Kelvin, and the pressure several hundred atmospheres.

In engineering, cavitation is undesirable in many propulsion and hydraulic systems because it produces extensive erosion of rotating blades, additional noise from the resultant knocking and vibrations, and a significant reduction of efficiency because it distorts the flow pattern. However, cavitation is also utilized in many interesting applications, such as high-power ultrasonics which utilize the inertial cavitation of microscopic vacuum bubbles for the cleaning of surfaces or homogenizing colloids such as paint mixtures or milk. Water purification devices have also been designed, in which the extreme conditions of cavitation can break down pollutants and organic molecules. Cavitation also plays an important role for the destruction of kidney stones in shock wave lithotripsy, and nitrogen cavitation is a method used in research to lyse cell membranes while leaving organelles intact. So don’t ever look at bubbles in a beaker in the same way again!

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