Sonomechanics Blog

Ultrasonic cavitation is a physical phenomenon where tiny vacuum bubbles form, grow, and then violently collapse in a liquid due to high-frequency sound waves (ultrasound).

How it works

• Ultrasound waves (typically above 20 kHz) travel through a liquid.
• These waves create alternating high-pressure and low-pressure cycles.
• During low-pressure phases, microscopic bubbles form (this is called cavitation).
• During high-pressure phases, the bubbles collapse or “implode.”

What happens when bubbles collapse?

The collapse of these bubbles releases:

• Intense local heat (thousands of degrees at a microscopic level)
• High pressure
• Shock waves and tiny jets of liquid

Why it matters?

This effect is useful in many applications:

• Cleaning: Used in ultrasonic cleaners to remove dirt from delicate items like jewelry or medical instruments
• Medicine: Used in therapies such as non-invasive fat reduction
• Chemistry: Enhances chemical reactions (called sonochemistry)
• Industrial processes: Helps mix, cut, or break materials at a microscopic level

Why does it matter in liquid processing?

Liquids exposed to high-intensity ultrasound undergo acoustic cavitation. It can be seen as a cloud of bubbles forming in the vicinity of an ultrasonic horn and heard as an intense hissing noise. Cavitation produces violently and asymmetrically imploding vacuum bubbles, causing micro-jets with extremely strong mechanical shear forces. These forces are responsible for the well-known ability of ultrasound to drive a multitude of physical and chemical processes forward.

The cavitation bubble dynamics, schematically presented in the above figure, shows the growth and asymmetric collapse of the low-pressure cavity, resulting in a micro-jet. Bubble oscillation that takes place simultaneously with its growth is not shown. As the bubble oscillates and grows, it draws the vapor of the surrounding liquid into its interior, along with any dissolved gasses. This process is called "rectified diffusion". The pressure in the bubble remains relatively low, which helps its final implosion. During the final stage of implosion, the speed of the bubble's wall can exceed the speed of sound in its gaseous interior. This creates a shock wave (similar to that created by an airplane when it crosses the sound barrier) in the bubble, breaking it up into tiny fragments, which subsequently become inception points for further cavitation events.

Shock-wave Model of Acoustic Cavitation

 A 2008 paper written by Sergei L. Peshkovsky and Alexey S. Peshkovsky titled “Shock-wave model of acoustic cavitation” further illustrate cavitation. A shock-wave model of liquid cavitation due to an acoustic wave was developed, showing how the primary energy of an acoustic radiator is absorbed in the cavitation region owing to the formation of spherical shock-waves inside each gas bubble. The model is based on the concept of a hypothetical spatial wave moving through the cavitation region. It permits using the classical system of Rankine–Hugoniot equations to calculate the total energy absorbed in the cavitation region. Additionally, the model makes it possible to explain some newly discovered properties of acoustic cavitation that occur at extremely high oscillatory velocities of the radiators, at which the mode of bubble oscillation changes and the bubble behavior approaches that of an empty Rayleigh cavity. Experimental verification of the proposed model was conducted using an acoustic calorimeter with a set of barbell horns. The maximum amplitude of the oscillatory velocity of the horns’ radiating surfaces was 17 m/s. Static pressure in the calorimeter was varied in the range from 1 to 5 bars. The experimental data and the results of the calculations according to the proposed model were in good agreement. Simple algebraic expressions that follow from the model can be used for engineering calculations of the energy parameters of the ultrasonic radiators used in sonochemical reactors. 

What liquid processing applications are affected by ultrasonic cavitation?

1. Particle Size Reduction

Cavitation breaks particles into smaller sizes, helping create:

• Stable suspensions
• Nanoparticles
• Fine emulsions

Examples:

• Paints
• Inks
• Battery slurries
• Ceramic suspensions

2. Emulsification

Oil and water normally separate. Cavitation creates very fine droplets and disperses one phase into the other.

Examples:

• Functional beverages
• Cosmetics
• Pharmaceutical active ingredient delivery
• Fuel emulsions

3. Cell Disruption and Extraction

The shock waves rupture cell walls and membranes.

Examples:

• DNA/RNA extraction
• Protein extraction
• Botanical extracts
• Essential oils

4. Faster Chemical Reactions (Sonochemistry)

Cavitation creates localized "hot spots" that accelerate some reactions.

Examples:

• Nanomaterial synthesis
• Catalysis
• Polymer processing
• Wastewater treatment

5. Degassing

Ultrasound helps remove dissolved gases and trapped air bubbles.

Examples:

• Coatings
• Epoxies
• Adhesives
• Laboratory solutions

What Controls Cavitation?

The most important factors are:

Why Amplitude Is So Important

For many industrial ultrasonic processors, cavitation begins to become significant around 20–30 microns of probe displacement. Heavy-duty applications often operate at:

• 40–60 microns: mixing and emulsification
• 60–100+ microns: aggressive cell disruption and particle size reduction

A processor may have a large wattage rating, but if it cannot deliver sufficient amplitude at the probe tip, cavitation effectiveness may be limited.

Summary: What Is Ultrasonic Cavitation and Why Does It Matter in Liquid Processing?

Ultrasonic cavitation occurs when high-frequency sound waves (typically above 20 kHz) pass through a liquid, creating alternating high- and low-pressure cycles. During low-pressure phases, microscopic bubbles form and grow; during high-pressure phases, they collapse violently. This collapse generates intense localized heat, pressure, shock waves, and high-velocity micro-jets.

In liquid processing, cavitation is the primary mechanism that allows ultrasonic processors to perform tasks such as mixing, emulsifying, dispersing, extracting, and accelerating chemical reactions. The imploding bubbles create powerful mechanical shear forces that break apart particles, disrupt cell walls, and enhance mass transfer and reaction rates.

Research, including the 2008 shock-wave model developed by Sergei and Alexey Peshkovsky, demonstrated that much of the energy delivered by an ultrasonic processor is absorbed through shock waves generated within cavitating bubbles. This model helps engineers predict and optimize ultrasonic processing performance.

Key liquid-processing applications include:

• Particle size reduction for paints, inks, battery slurries, and nanoemulsions.
• Emulsification of immiscible liquids such as oil and water for functional beverages.
• Cell disruption and extraction for proteins, DNA/RNA, and botanical compounds.
• Sonochemistry, where cavitation accelerates chemical reactions.
• Degassing of coatings, adhesives, epoxies, and laboratory solutions.

The effectiveness of cavitation depends on factors such as amplitude, power density, temperature, viscosity, pressure, and probe size. Among these, amplitude is often the most critical parameter. Cavitation becomes significant at approximately 20–30 microns of probe displacement, while demanding applications may require 40–100+ microns. As a result, a processor's ability to deliver sufficient amplitude is often more important than its wattage rating alone.