The principle of ultrasonic cleaning

Ultrasonic cleaning is a highly efficient physical cleaning method.

The core process involves immersing the workpiece to be cleaned in a tank containing a cleaning agent and irradiating the tank with ultrasonic waves. When ultrasonic waves propagate in the cleaning liquid, the sound pressure exhibits periodic changes.

In the negative pressure region of the sound wave, the liquid inside tears due to the pressure dropping below its vapor pressure, forming tiny cavities (cavitation nuclei) containing gas or vapor. When the sound pressure amplitude exceeds a certain critical value (cavitation threshold), these cavitation nuclei rapidly grow into micron-sized bubbles visible to the naked eye.

In the positive pressure region of the sound wave, the bubbles are violently squeezed and rapidly contract and finally collapse and close. The moment the bubbles collapse and the collision process releases enormous energy, producing local microjets and shock waves with extremely strong impact force and extremely high acceleration. Although the macroscopic displacement and velocity changes caused by this process are small, the instantaneous acceleration produced is extremely large.

This key physical phenomenon is called the "cavitation effect," which is the main mechanism by which ultrasonic cleaning efficiently removes dirt.

I. Ultrasonic Cleaning Principle

1. Cavitation Effect

Under the action of ultrasonic waves (typical frequency 20-400 kHz), the liquid medium produces periodic rarefaction and condensation changes, forming negative pressure cavitation bubbles. When the cavitation bubbles collapse, local high temperatures (>5000 K) and high pressures (>50 MPa) are generated,

resulting in:

Chemical bond breakage: Shock waves cause the dissociation of pollutant molecular bonds (e.g., Si-O bond energy is approximately 452 kJ/mol)

Microjet scouring: Bubble collapse produces microjets with speeds >100 m/s, peeling off surface particles

2. Secondary Effects

Acoustic streaming: High-frequency vibrations form boundary layer acoustic streaming (Reynolds number >2000), enhancing mass transfer efficiency

Thermochemical effects: Cavitation heat promotes cleaning solution activation, such as the decomposition of H₂O₂ to produce ·OH free radicals

Capillary penetration: Bubble oscillations enter micron-level gaps, producing a "micro-brushing" effect

3. Key Process Parameters

II. Technological Limitations

1. Submicron Particle Removal Bottleneck

Experiments show that the removal rate for 100 nm particles is only about 65% (compared to 92% for 1 μm particles), due to:

Stokes force dominance: The proportion of small particles and surface adsorption forces (such as van der Waals forces) increases

Cavitation bubble size limitation: The optimal cavitation bubble diameter is about 2-10 μm

2. Surface Damage Risk

High-frequency vibration leads to increased surface roughness (Ra value can increase by 0.2-0.5 nm)

Cavitation erosion forms nano-scale pits (depth <5 nm)
 

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