Cavitation happens when liquid pressure drops low enough that the liquid turns into vapor, forming bubbles that violently collapse. It’s the same phase change as boiling, but driven by pressure drop instead of heat. This simple mechanism produces some of the most extreme conditions found outside of stars, and it shows up everywhere from ship propellers to cosmetic treatments to the claws of shrimp.
The Basic Physics of Bubble Formation
Every liquid has a vapor pressure, the threshold below which it transitions from liquid to gas. When you boil water on a stove, you’re raising the temperature until the vapor pressure exceeds atmospheric pressure. Cavitation takes the opposite path: instead of adding heat, you reduce the local pressure until it drops below the liquid’s vapor pressure at its current temperature. The phase change happens under essentially the same physics, just along a different axis of the pressure-temperature diagram.
This pressure drop occurs naturally wherever liquid accelerates. When water speeds up around a propeller blade, through a narrow valve, or along the leading edge of a hydrofoil, the faster-moving fluid creates zones of very low pressure. If those zones dip below the vapor pressure, tiny vapor-filled cavities appear. These bubbles don’t last long. As the fluid moves downstream into regions of higher pressure, the surrounding liquid crushes them inward. That collapse is where cavitation gets interesting and, often, destructive.
What Happens When a Bubble Collapses
A cavitation bubble’s life is short but violent. As surrounding pressure forces the bubble to shrink, the gas and vapor inside compress rapidly. The collapse is not gentle or symmetrical. It progresses through distinct stages: first, smaller isolated cavities collapse, then larger core cavities implode, and finally, rebound cavities form and collapse again in a series of diminishing pressure peaks. The large cavity collapse stage produces the most intense forces.
The temperatures and pressures generated inside a collapsing bubble are staggering. Computer simulations and experimental measurements converge on a core temperature of roughly 5,000 to 5,200 K, hot enough to rival the surface of the sun. Some single-bubble experiments have estimated temperatures as high as 20,000 K. Pressures inside the collapsing cavity can reach 250 megapascals or more. For context, that’s about 2,500 times atmospheric pressure, concentrated in a space smaller than a pinhead. A thin liquid shell immediately surrounding the bubble reaches around 1,900 K.
These extreme conditions produce two damaging mechanisms. When a bubble collapses near a solid surface, the collapse becomes lopsided: the side farther from the surface caves in faster, shooting a high-speed micro-jet of liquid directly at the material. When the collapse happens farther from the surface, the dominant force is a pressure shock wave radiating outward. Both mechanisms are powerful enough to pit and erode metal over time.
Inertial vs. Non-Inertial Cavitation
Not all cavitation bubbles behave the same way. The distinction comes down to what controls the bubble’s collapse. In non-inertial (sometimes called “stable”) cavitation, bubbles oscillate gently in response to pressure changes without fully imploding. The internal gas pressure resists collapse, and the bubble simply pulses back and forth. In inertial (or “transient”) cavitation, the bubble expands to more than twice its resting size, and the inrushing liquid overwhelms the internal gas pressure, driving a violent collapse.
The transition between these two behaviors is remarkably sharp. At frequencies below about 3 MHz, a pressure increase of just 5 to 10 percent can push a bubble from stable oscillation into inertial collapse, producing orders-of-magnitude jumps in the temperatures and pressures inside. One clear sign that inertial cavitation is occurring is sonoluminescence: the collapsing bubble gets so hot that it emits a brief flash of light visible under controlled laboratory conditions.
Cavitation Damage in Engineering
For engineers, cavitation is mostly a problem to prevent. The repeated micro-jet impacts and shock waves from collapsing bubbles chew through pump impellers, ship propellers, turbine blades, and pipe fittings. The damage looks like small pits in the metal surface, and over months or years, those pits grow into significant material loss. The erosion comes almost entirely from “detached” cavity collapses, bubbles that form and then collapse away from the attached vapor sheet, rather than from the vapor sheet itself.
The centrifugal pump industry manages this risk through a parameter called Net Positive Suction Head (NPSH), which essentially measures how much pressure margin exists above the liquid’s vapor pressure at the pump inlet. Industry guidelines from the Hydraulic Institute (most recently updated in 2024) specify recommended NPSH margins for different pump types and applications. When the recommended margin can’t be achieved, engineers follow mitigation strategies to reduce cavitation risk, such as lowering fluid temperature, raising inlet pressure, or reducing flow rate.
Cavitation in Nature
The pistol shrimp is the most famous biological example. When this small crustacean snaps its specialized claw shut, a plunger-like mechanism fires a water jet at roughly 28 to 31 meters per second. That jet creates a pressure drop of about 300,000 pascals, enough to vaporize water locally and form a cavitation bubble shaped like a vortex ring. The bubble travels outward at about half the jet’s speed, and when it collapses, it generates a pressure pulse exceeding 10 bar, with some estimates from simplified models suggesting collapse pressures as high as 2,000 bar. The shrimp uses this shock wave to stun or kill prey.
Mantis shrimp produce cavitation through a different strategy, striking prey with club-like appendages so fast that the water between the club and the target can’t get out of the way fast enough, dropping the local pressure below vapor pressure. Even some plants exploit cavitation: the rapid drying of xylem vessels in trees under drought stress can trigger cavitation events that block water transport, which is one reason prolonged drought kills trees.
How Ultrasonic Cavitation Works in Medicine
Cosmetic ultrasonic cavitation treatments use the same physics in a controlled way to destroy fat cells. A handheld device delivers focused ultrasound waves into the tissue beneath the skin, creating rapid cycles of compression and decompression. These pressure swings generate cavitation bubbles specifically at the boundary between fat droplets and the watery interior of fat cells.
As those bubbles form and collapse, they destabilize the fat cell’s outer membrane. The membrane develops clusters of tiny vesicles that weaken its structure, eventually causing small ruptures roughly 0.5 to 1.5 micrometers in diameter. These openings are large enough for stored fat (triglycerides) to leak out of the cell into the surrounding tissue, where the body’s lymphatic system gradually clears the released material. Many practitioners recommend lymphatic drainage massage after treatment to help move the freed triglycerides toward processing.
The FDA classifies focused ultrasound devices for aesthetic use as Class II medical devices with special controls. People with pacemakers, defibrillators, or other active implants should not have the treatment near those devices. The same applies to anyone with metallic implants in the treatment area, open wounds, severe acne, blood clotting disorders, or who is pregnant or breastfeeding.
Detecting Cavitation by Sound
Cavitation is loud, and its acoustic signature is one of the primary ways engineers detect and measure it. When bubbles collapse, they produce broadband noise across a wide frequency spectrum. Stable (non-inertial) cavitation tends to produce distinct peaks at specific frequencies, particularly subharmonics and ultraharmonics of the driving frequency. Inertial cavitation, by contrast, produces a wash of broadband noise between those peaks, the acoustic fingerprint of violent, chaotic bubble collapse.
The International Electrotechnical Commission has standardized how to measure cavitation noise in ultrasonic equipment, specifying measurements across a frequency range of 10 kHz to 5 MHz. By analyzing whether the acoustic energy sits in sharp peaks or spreads across the broadband spectrum, technicians can determine whether cavitation is occurring and whether it’s the gentle oscillating type or the destructive inertial type. In industrial pumps and valves, a sudden increase in broadband noise is often the first warning sign that cavitation has begun eating away at internal surfaces.