A cavitation is a bubble that forms inside a liquid when the local pressure drops low enough for the liquid to vaporize. It’s the same basic physics as boiling water on a stove, but instead of heat driving the process, a sudden pressure drop does. These bubbles grow while the pressure stays low, then collapse violently when the surrounding pressure rises again. That rapid collapse is what makes cavitation powerful, destructive, and useful depending on where it happens.
How Cavitation Works
Every liquid has a vapor pressure, the threshold at which it transitions from liquid to gas. Heating water to 100°C at sea level reaches that threshold, producing steam. But you can also cross the threshold by lowering the pressure instead of raising the temperature. When fluid accelerates through a narrow space, its pressure drops (a relationship described by Bernoulli’s principle). If the pressure falls below the liquid’s vapor pressure, tiny vapor-filled cavities form spontaneously.
These bubbles don’t last long. As soon as they drift into a region of higher pressure, the vapor inside condenses and the bubble implodes. That collapse happens in microseconds and concentrates enormous energy into a tiny point. The implosion generates high-speed microjets of liquid and localized pressure spikes that can reach thousands of atmospheres. This is why cavitation can chew through solid metal or, in controlled settings, break apart kidney stones.
Inertial vs. Stable Cavitation
Not all cavitation is equally violent. Physicists distinguish two main types based on how the bubbles behave.
Stable cavitation occurs when bubbles oscillate in size without fully collapsing. They expand and contract rhythmically, often in response to an ultrasound wave. This gentler pulsing can still produce useful effects: the repeated motion creates tiny currents in the surrounding fluid that can push drugs across cell membranes or increase the permeability of blood vessels.
Inertial cavitation is the dramatic version. The bubble grows rapidly and then implodes with enough force to damage nearby surfaces or rupture cells. In focused ultrasound research, the threshold for inertial cavitation has been measured at around 0.45 megapascals of pressure. Below that level, bubbles oscillate without collapsing violently. Above it, they produce broadband acoustic emissions, essentially a burst of noise across many frequencies, that signals a full implosion has occurred.
Cavitation in Engineering
For anyone who designs pumps, propellers, or turbines, cavitation is a persistent enemy. When a propeller blade spins fast enough, it accelerates the water flowing over it until the pressure on the blade surface drops below vapor pressure. Bubbles form along the blade, then collapse as they move into higher-pressure zones downstream. The microjets from those collapses strike the metal surface repeatedly, creating pitting, erosion, and eventually structural failure.
In centrifugal pumps, the rotating flow field distorts cavitation bubbles into irregular ellipsoidal shapes rather than neat spheres, which makes the collapse pattern unpredictable. The result is uneven erosion that can weaken specific areas of a blade while leaving others untouched. Engineers combat this by designing blade geometries that minimize pressure drops, selecting harder alloys, or simply ensuring the system operates within pressure ranges that keep the liquid above its vapor pressure.
Cavitation in Medicine
The same destructive force that damages pump blades can be harnessed for surgical precision. High-intensity focused ultrasound (HIFU) uses controlled cavitation to destroy tissue without an incision. By focusing ultrasound waves on a small target area, clinicians can generate cavitation bubbles that implode and mechanically break apart the tissue. Applications include destroying tumors, breaking up blood clots, and delivering drugs across biological barriers that would normally block them.
Microbubbles injected into the bloodstream act as amplifiers for this process. When hit with ultrasound, they oscillate or collapse in ways that temporarily increase the permeability of nearby cell membranes and blood vessel walls. Research has shown that the blood-brain barrier can be opened at pressures as low as 0.3 megapascals using this approach, well below the inertial cavitation threshold, meaning the barrier can be crossed with stable bubble oscillation alone rather than violent implosion.
Safety in diagnostic ultrasound is governed by the Mechanical Index, a measure of how likely the ultrasound beam is to cause cavitation in tissue. The FDA caps this value at 1.9 for diagnostic imaging. Below 0.5, bubble growth essentially doesn’t occur. Standard heart imaging typically runs between 0.9 and 1.4, which provides good image quality while destroying some contrast agent bubbles in the process.
Cosmetic Fat Cavitation
Ultrasonic cavitation has become a popular noninvasive body-contouring treatment. A handheld device emits high-intensity sound waves, typically in the low megahertz range, that create microbubbles within the fat layer beneath the skin. When those bubbles collapse, the mechanical force ruptures the membranes of fat cells, releasing their contents (mostly triglycerides) into the surrounding fluid. The body then processes this released fat through the lymphatic and venous systems, eventually metabolizing it in the liver.
The process is selective because fat cells are more susceptible to this mechanical disruption than denser surrounding tissues like muscle or skin. Results vary, and multiple sessions are typical.
Cracking Your Knuckles
The satisfying pop when you crack a knuckle is cavitation in action, though the mechanism was debated for decades. The longstanding theory held that the sound came from a bubble collapsing inside the joint. Real-time MRI imaging has overturned that idea. The crack actually occurs at the moment a gas cavity forms, not when it collapses.
When you pull on a finger joint, the opposing cartilage surfaces resist separation until a critical point, then snap apart rapidly. This sudden separation drops the pressure inside the synovial fluid enough that dissolved gases come out of solution, creating a visible cavity. The sound coincides with that rapid cavity formation. The gas bubble then persists inside the joint for some time afterward, which is why you can’t immediately crack the same knuckle again. This process is called tribonucleation.
Cavitation in Plants
Trees and other plants face their own version of cavitation, and it can be fatal. Plants pull water from roots to leaves through narrow vessels called xylem, and that water column is under constant tension, essentially being sucked upward. During drought, the tension increases as the soil dries out. If it exceeds a critical threshold, air bubbles form inside the xylem vessels, blocking water flow. This is xylem cavitation.
A single air-filled vessel loses its ability to conduct water entirely. If enough vessels cavitate, the plant enters a dangerous feedback loop: reduced water flow means less ability to cool leaves and maintain basic functions, which increases stress, which causes more cavitation. Research on beech and poplar trees found that losing just 5 to 30 percent of water transport capacity can trigger runaway embolism, a cascading failure where vessels cavitate one after another until the plant loses hydraulic function completely. This hydraulic failure is a direct cause of tree death during severe drought. Interestingly, the water released by cavitating vessels can temporarily buffer the plant’s water status, buying time before conditions become irreversible.
Jawbone Cavitations
In dentistry, “cavitation” refers to something quite different: a hollow or poorly healed area within the jawbone, usually at the site of a previous tooth extraction. Instead of filling completely with new bone, the socket retains a pocket of soft, non-mineralized tissue beneath a thin bony cap. These are sometimes called NICO (neuralgia-inducing cavitational osteonecrosis) or fatty degenerative osteonecrosis of the jawbone.
Some patients with jawbone cavitations experience facial pain, numbness, or neuralgic symptoms, but many have no symptoms at all. Standard two-dimensional X-rays often miss these lesions. Cone beam CT scanning, which produces three-dimensional images, has significantly improved detection rates by revealing low-density areas within the bone that wouldn’t show on a flat X-ray. There is currently no evidence-based consensus on standardized diagnostic criteria, so diagnosis typically relies on a combination of imaging, clinical assessment, and sometimes surgical exploration with tissue analysis to rule out other conditions like cysts or tumors.