The ultrasound technologyInduced effects › Liquid and cavitation

Ultrasounds in fluids: cavitation

The action of ultrasound in liquids is based on the phenomenon of cavitation: creation; growth and implosion of bubbles formed when a liquid is subjected to a pressure wave. This leads to a pressure shock around the bubble and the formation of a thermochemical micro-reactor inside the bubble.

 

 
In order for acoustic cavitation to take place, a power threshold must be reached. This threshold is around 0.5W/cm² at 20 kHz for water at atmospheric pressure and around a few W/cm² for organic solvents. The amplitude of the depression to be supplied to reach the cavitation threshold depends on several parameters: the higher the viscosity of the media (therefore the internal cohesion of the liquid), the harder it is to achieve cavitation as the particles are more difficult to separate.

 

In liquids, acoustic cavitation causes power ultrasound effects.

Ultrasonic cleaning

If, as they develop, the cavitation bubbles come into contact with a solid surface, they implode on this surface by forming very violent microjets of liquid (100 m/s) which strip the solid surface.

 

To achieve the best quality cleaning and to adhere to the constraints of the application, the following should be opted for:

 

  • an ultrasonic power of around 5 - 20 W / litre of bath liquid
  • ultrasonic frequency: 25 kHz (Powerful cleaning of hard and very dirty surfaces), 40 kHz (Fine cleaning of delicate surfaces), and 1 - 2 MHz for silicium wafer cleaning
  • detergent (neutral, acid or base) to encourage the dissolving of dirt without damaging the item to be cleaned
  • bath temperature - generally between 40 and 60 °C
  • la durée de nettoyage qui varie de quelques secondes pour du dégraissage à quelques heures pour le décapage

 

The benefits ultrasonic cleaning are that it is:

 

  • Ecological : both environmentally friendly and economical
  • an alternative to solvents biodegradable detergents
  • reduced cleaning time
  • accurate, effective and easy

Ultrasonic liquid processing and formulation

Under very high power (50 W - 1 kW/L), the mechanical effects of cavitation are used to process liquids:

 

  • degassing
  • ultrasonic homogenisation, 
  • ultrasonic emulsion or thorough mixing through convection currents, even for viscous products of up to 400 Pas
  • enzyme/DNA extraction
  • bacterial destruction or lysis
  • ultrasonic cell lysis (disrupter or disruptor)
  • ultrasonic nanoemulsion
  • ultrasonic degassing
  • formulation
  • electrochemistry
  • emulsification
  • extraction

Sonochemistry

Cavitation is also used for chemical reactions:

 

Applications