What cavitation does to water

Physical effects: shear, micro-jets and shock waves

The physical effects are the most general. A review of HC reactor modeling summarized HC as generating intense shear, localized hot spots and hydroxyl radicals Ranade, 2022. Shear is the tearing force between layers of liquid moving at different speeds; in a cavitation device it comes from the fast flow through the constriction and from the cavities themselves. The collapse adds strong local turbulence, micro-jets and shock waves of a few thousand atmospheres, according to another review Zheng et al., 2022.

A micro-jet forms when a cavity collapses close to a wall: a jet of liquid crosses the cavity and strikes the surface with a high, brief impact pressure. Reported jet speeds ranged from 30 to 877 m/s Krella, 2023. The same impacts cause cavitation erosion, described in What hydrodynamic cavitation is.

A shock wave is a sharp pressure front radiated by the collapse. Experiments with laser-generated single bubbles, with a maximum radius of 1 to 10 mm in demineralized water, showed how much energy it carries: highly spherical collapses emitted up to about 90% of the bubble's initial energy as shock waves, while bubbles deformed near a surface produced several shocks, including one from the jet's impact. The paper's introduction notes shock pressures of the order of gigapascals, able to wear metal surfaces Supponen et al., 2017.

Local hot spots

A collapsing cavity releases a large amount of energy into a tiny volume, with extremely high local temperatures and pressures. Light emitted by collapsing bubbles (sonoluminescence) suggests about 5000 K Zheng et al., 2022. Those measurements come from acoustic cavitation, driven by ultrasound; no source cited here measured a hot-spot temperature inside hydrodynamic cavitation. Simulations suggest that hydrodynamic cavities collapse over longer time scales, which hinders collapse Arrojo & Benito, 2008.

The heat is confined to the collapsing cavity; the bulk water stays near ambient temperature, as explained in What hydrodynamic cavitation is. Review estimates of up to 15,000 K are discussed in Advanced oxidation processes (AOPs), and proposed radical routes in Key physico-chemical properties.

Hydroxyl radicals: how much is made

Under hot-spot conditions, water vapor inside the cavity splits into chemically active radicals, hydrogen (•H) and hydroxyl (•OH) Zheng et al., 2022. The hydroxyl radical is a highly reactive member of the reactive oxygen species (ROS) and the basis of cavitation as an advanced oxidation process (AOP).

Hydroxyl radicals are short-lived: they react within nanoseconds, so they are difficult to detect directly Nöpel & Ayela, 2023. Studies add a probe chemical that reacts with •OH to form a stable product, a method called dosimetry. The method matters: the classic iodide test was judged unreliable for HC, and in terephthalic-acid dosimetry, oxygen in the water raised the product yield while nitrogen sparging lowered it Nöpel & Ayela, 2023.

The measured amounts were small. After 90 minutes in a Venturi, with salicylic acid at 50 and 300 mg/L in acidified tap water, hydroxylated products totaled 0.97 and 1.81 µg/mL. At the higher probe concentration the cavitation itself became gentler, so the probe can bias the result Zupanc et al., 2020. In micro- and millimeter-scale devices, radical production corresponded to about 2.5 to 6 × 10⁻¹⁷ moles of •OH per joule, and a review concluded that the rare quantitative data point to a low radical efficiency for HC Nöpel & Ayela, 2023.

When the chemistry matters

Because •OH lasts nanoseconds, the chemistry happens close to where cavities collapse. Three findings define when it matters.

• The target. In simulations, a compound's volatility and hydrophobicity determined whether HC chemistry could act on it Arrojo & Benito, 2008.
• The water. In a vortex device, acidic pH favored •OH generation, and sulfuric acid was more than three times as effective as hydrochloric acid for adjusting it De-Nasri et al., 2022.
• The collapse. When researchers looked for •OH from oxygen nanobubbles standing in water at ambient conditions, benzoic acid (an •OH probe) did not degrade over 24 hours and no •OH signal was detected; a fluorescent probe gave a false positive Chae et al., 2023.

Radical chemistry therefore comes with violent collapse inside the device, not with fine bubbles left standing in the water.

What changes the effects

Pressure. The relationship between pump pressure and effect was not a steady rise. For the pesticide dichlorvos, an optimum inlet pressure within 3 to 6 bar gave the most degradation Joshi & Gogate, 2012. For rhodamine B in a 4 L Venturi loop, degradation first rose and then fell as inlet pressure went from 0.1 to 0.5 MPa, with an optimum at 0.4 MPa Ye et al., 2021. In a vortex device, lowering the inlet pressure from 200 to 100 kPa raised per-pass removal of coumarin 7.3-fold, and raising the downstream pressure from 0 to 100 kPa raised it nearly threefold De-Nasri et al., 2022. How these pressures enter the cavitation number is covered in What hydrodynamic cavitation is.

Temperature. A review states that higher temperature raises the cavitation number, delays inception and lowers cavitation intensity Zheng et al., 2022. In the rhodamine B study, degradation rose up to 30 °C and fell above it; the authors explained that warmer water has a higher vapor pressure, and vapor filling the cavities lowered cavitation efficiency Ye et al., 2021. For dichlorvos, lower temperature was favorable within the 31 to 39 °C tested Joshi & Gogate, 2012.

Dissolved gas. Gas works both ways. Higher gas saturation was reported to add nuclei and lower the threshold for cavitation Zheng et al., 2022. In a Venturi tunnel with controlled air content, large cavities were mostly water vapor, but dissolved air left more gas bubbles behind after collapse, and the point where cavitation disappeared was highly sensitive to air content Tian et al., 2025. In the vortex study, runs started at lower dissolved oxygen (DO: 1 and 3.5 ppm) formed about 30% less of the •OH product than runs in oxygen-saturated water (11.5 ppm). The low-oxygen runs were prepared with dry ice, which also adds carbon dioxide, so the effect of oxygen alone is uncertain De-Nasri et al., 2022.