How Does a Shrimp Break Glass With a Snap? Water Tearing Itself Apart

A liquid boils when heated, and it can also be made to vaporise by lowering the pressure instead, since what matters is the pressure relative to the vapour pressure at that temperature rather than the temperature alone. Water flowing fast around an obstacle or through a constriction has its pressure reduced by the flow itself, and where that reduction is sharp enough the water vaporises locally, forming a cavity of vapour rather than of air. The cavity is carried along and reaches a region of higher pressure, where it cannot survive, and the surrounding liquid rushes in to fill it. That collapse is where all the interesting and destructive behaviour happens.

The inward rush concentrates energy into an extremely small region:

• •Pressures at the point of collapse reaching hundreds or thousands of atmospheres
• •Temperatures inside the collapsing cavity in the thousands of degrees
• •A visible flash of light in some conditions, which is still not fully explained
• •A microscopic jet of liquid punched through the cavity towards a nearby surface
• •A shock wave radiating outwards from the collapse point
• •Sound, which is why the whole process is audible as a hiss, rattle or crack

Engineers treat this as a serious and expensive problem rather than a curiosity. Ship propellers operating at speed generate these cavities on the blade surfaces, and the repeated collapses erode the metal, pitting the surface and eventually destroying the blade, which is why propeller design is constrained by it and why high speed operation is limited by more than power. Pump impellers suffer the same, along with a loss of performance and a characteristic rattling noise that operators recognise immediately. Valves, turbines and pipe fittings erode wherever flow separates sharply. The erosion attacks even hardened steel, because the mechanism is repeated impact at extreme local pressure rather than chemical attack, and no material is immune.

A collapsing cavity driven by sound in a carefully prepared liquid emits a brief flash of light, and the phenomenon has resisted full explanation for decades. The flashes last for picoseconds, which is extraordinarily short, and they repeat with remarkable regularity when a single cavity is trapped in a standing sound wave. The spectrum implies temperatures of tens of thousands of degrees inside the collapsing volume, far hotter than the surrounding liquid, and various proposals have invoked shock waves converging on the centre, plasma formation and other mechanisms. Extravagant claims that the conditions could sustain nuclear fusion were made in the early 2000s, generated a great deal of attention and were not reproduced, and the underlying light emission remains a genuine open question.

The same violence is exploited in several places. Ultrasonic cleaning baths generate the cavities throughout a liquid by sound waves, and the collapses scour surfaces including the insides of cracks and threads that no brush could reach, which is how surgical instruments, engine components and spectacles are cleaned. Medical lithotripsy uses focused shock waves to break kidney stones, with cavitation contributing substantially to the fragmentation. Industrial processing uses it to mix, emulsify and break cell walls. And several animals have evolved to produce it, including the pistol shrimp, which snaps a specialised claw shut fast enough to generate a collapsing cavity that stuns prey and emits a flash and a crack loud enough to interfere with sonar.

Water flowing fast enough has its pressure reduced until it vaporises locally, and the resulting cavity collapses when carried into higher pressure, concentrating energy into a tiny region at thousands of atmospheres and thousands of degrees. Repeated collapses erode propellers, pumps and valves regardless of how hard the metal is. The same effect cleans instruments ultrasonically, breaks kidney stones and lets a shrimp stun its prey.