Cavitation is the formation and collapse of tiny gas or vapor bubbles in a liquid, triggered by rapid changes in pressure. When pressure in a liquid drops low enough, small bubbles form almost instantly. When pressure rises again, those bubbles collapse violently, releasing concentrated bursts of energy. The forces involved are surprisingly powerful: the stress from a single collapsing cavitation bubble can reach 1 gigapascal, roughly 10,000 times atmospheric pressure. This makes cavitation both a destructive force that damages industrial equipment and a useful tool in medicine, dentistry, and body contouring.
How Cavitation Works
Every liquid has a vapor pressure, the point at which it starts turning into gas. When local pressure in a fluid drops below that threshold, pockets of vapor form as microscopic bubbles. This can happen when liquid flows rapidly through a narrow space (like a pump or valve), when an ultrasound wave passes through tissue, or when a shockwave hits a surface. The bubbles don’t last long. As soon as the surrounding pressure rises again, often within microseconds, they collapse inward with extreme force.
That collapse is where cavitation gets interesting. As a bubble implodes, it generates tiny high-velocity jets of fluid, shock waves, and in some cases localized temperatures hot enough to emit light (a phenomenon called sonoluminescence). These micro-scale events can erode metal, shatter kidney stones, or strip bacteria from a tooth surface, depending on the context.
Stable vs. Inertial Cavitation
Not all cavitation is the same. Physicists divide it into two broad categories based on what happens to the bubbles.
In stable cavitation, bubbles oscillate in size without fully collapsing. They expand and contract rhythmically in response to pressure waves, producing predictable acoustic signals at harmonic and subharmonic frequencies. At low acoustic pressures, this gentle pulsation is enough to increase the permeability of nearby cell membranes without destroying them. This is the type of cavitation used in some drug delivery research, where the goal is to open cells temporarily rather than obliterate them.
Inertial cavitation (sometimes called transient cavitation) is the violent version. Bubbles grow rapidly and then collapse completely, producing broadband acoustic emissions, high temperatures, extreme pressures, and high-velocity micro-jets. This is the type responsible for both industrial damage and many therapeutic applications. The collapse energy is high enough to damage surrounding structures, which is exactly what you want when breaking apart a kidney stone but exactly what you don’t want inside a pump.
Cavitation Damage in Pumps and Valves
In engineering, cavitation is mostly a problem. It occurs in pumps, propellers, turbines, and valves wherever liquid accelerates through a constriction and pressure drops sharply. The repeated collapse of bubbles against metal surfaces acts like millions of tiny hammer blows. Over time, this causes a specific type of material failure called cavitation erosion.
The process unfolds in stages. During the initial incubation period, the surface develops plastic deformation: microscopic slip strips and shallow craters appear as the metal bends under repeated impacts. In the acceleration period that follows, micro-cracks form and propagate along grain boundaries. The surface layer becomes refined into nanoscale grains, and cracks initiate between them. Eventually, chunks of material break free, leaving the pitted, cratered surfaces that engineers recognize as classic cavitation damage.
The dominant mechanisms behind this damage are micro-jets (tiny streams of liquid fired at the surface when a bubble collapses asymmetrically near it) and shock waves from the collapse itself. Some specialized alloys resist this well. Stellite 6, a cobalt-chromium alloy used in nuclear power plant components, actually hardens in response to cavitation impacts. The repeated stress transforms its crystal structure and increases its internal resistance, which is why it’s a common choice for valves and pump components in high-risk environments.
Breaking Kidney Stones With Shockwaves
Cavitation plays a central role in extracorporeal shockwave lithotripsy, the most common non-surgical treatment for kidney stones. During the procedure, focused shockwaves pass through the body and converge on the stone. When each shockwave hits the stone’s surface, the intense negative pressure in the surrounding fluid spawns a cluster of small gas bubbles. As each bubble rapidly collapses, it fires a jet of high-energy fluid directly at the stone’s surface.
This is just one of several fragmentation mechanisms at work (others include shear stress, spall fracturing, and fatigue), but cavitation is a key contributor, particularly for eroding the stone’s outer surface. Over hundreds or thousands of shockwave pulses, the combined effect breaks the stone into fragments small enough to pass naturally through the urinary tract.
Ultrasonic Fat Cavitation
Cosmetic body contouring has adopted cavitation as a non-invasive alternative to liposuction. Devices called ultracavitation units emit high-intensity sound waves, typically at frequencies around 4 MHz, into subcutaneous fat. These waves create rapidly alternating zones of high and low pressure in the tissue, generating microbubbles within and around fat cells.
The microbubbles grow unstable and implode. When they do, the mechanical force ruptures nearby fat cell membranes, spilling their contents (triglycerides and lipids) into the surrounding extracellular space. The body’s lymphatic system then processes and eliminates these released fats over the following days. Studies using ultracavitation at 4 MHz and 5 watts per square centimeter have demonstrated significant reductions in fat cell size and measurable loss of subcutaneous fat thickness.
Treatments are typically scheduled once per week, with a minimum gap of 72 hours between sessions. The spacing matters because your lymphatic system needs time to clear the released fat. Most protocols call for multiple sessions to achieve visible contouring results, and staying well hydrated between treatments helps the body process the freed lipids efficiently.
Cavitation in Dental Cleaning
Ultrasonic scalers, the vibrating instruments dentists use to remove plaque and tartar, rely partly on cavitation. The rapidly vibrating tip creates pressure fluctuations in the surrounding water, spawning tiny bubbles that collapse against tooth and implant surfaces. Because these microbubbles are far smaller than the acoustic wavelength driving them, the pressure gradients and shear forces they generate are concentrated into extremely small areas. This lets cavitation reach microscopic crevices that neither hand instruments nor the scaler tip itself can physically access.
This matters especially for dental implants, which have microscopically rough surfaces designed to promote bone integration. Scraping these surfaces with metal instruments can damage the texture and interfere with healing. Cavitation-based cleaning disrupts bacterial biofilm without gouging the surface. Research has shown that operating scaler tips in carbonated water, which provides pre-existing dissolved gas nuclei for bubbles to form around, significantly increases cavitation activity and removes more biofilm compared to still water in the same timeframe.
Safety Limits in Medical Ultrasound
Because cavitation can damage tissue, diagnostic ultrasound machines are designed to stay below the threshold where it becomes dangerous. Since 1992, all ultrasound machines sold in the U.S. have been required to display a Mechanical Index (MI) on screen, which indicates the likelihood of cavitation occurring in tissue.
The FDA sets the maximum MI for diagnostic imaging at 1.9. Below an MI of 0.5, bubble growth essentially doesn’t occur. Between 0.5 and about 2 to 3, cavitation becomes increasingly possible. Animal studies have reported capillary leakage at MI values above 0.4 when contrast agents (pre-formed microbubbles injected into the bloodstream) are present, which is why sonographers pay closer attention to MI during contrast-enhanced exams. In routine diagnostic scanning without contrast agents, the risk of cavitation-related harm at standard settings is extremely low.