History of Ultrasound
Ultrasound consists of elastic waves with a frequency of between around 16 kHz and several hundred Megahertz..
Frenchman Paul Langevin carried out the first work on ultrasound production techniques for SONAR at the end of the First World War. The applications and industrial uses of ultrasound developed towards the end of the 1950s. At this time a number of laboratories also demonstrated the extreme diversity of the applications of ultrasound.
Some of this work led rapidly to applications in the area of medical diagnostics and control, whilst aside from cleaning and welding, the use of high-power ultrasound in industry was not as widely spread. During the 1980s, the rapid development of modelling techniques led to significant progress being made in ultrasound technology.
The applications of ultrasound are generally classified into two categories:
- Low-power ultrasound for diagnostics, measuring and control,
- High-power ultrasound.
Ultrasound is considered to be high-power when it changes the environment in which it is propagated. The main actions are mechanical, thermal and/or chemical.
The application of ultrasound in solids can produce a rise in temperature when welding, or to vibrations, leading to a change in the apparent friction coefficient or the unclogging of sieves.
Ultrasound in fluids :
Cavitation is the predominant phenomenon generated by ultrasound. Cavitation is produced by emitting intense ultrasound waves which create areas of alternate compression and dilation. Above a certain threshold, they induce the implosion of micro-bubbles causing a significant localised heat increase and a wave of pressure several thousand times greater than atmospheric pressure. Ultrasonic cleaning and the treatment of surfaces, dispersion, homogenisation, the reduction of particle sizes, plant extraction, eco-extraction (essential fats, micro-algae, cannabis, polyphenols…), cellular lysis (cell disruption) and sonochemisty are based on this phenomenon. All of the applications of ultrasound in fluids, with the exception of cleaning, can be summarized under the heading of «Liquid treatment with ultrasound» or « Ultrasonic Liquid Processing ».
Les Ultrasons dans les poudres :
The action of ultrasound on powders induces effects that can be put to use in the production, handling and use of powders.
Ultrasound prevents sieves from clogging and is particularly effective when sieving fine particle sizes. The production of powder by means of atomization diminishes the dispersion of particle diametres, thus improving yields. Surface treatment by ultrasound facilitates the unclogging of particles maintaining clean sieves. This method is used in the cleaning of blocked sifting mats, the removal of sedimentary deposition in pipes. The compaction of powders by means of ultrasound considerably increases the density of sintered materials.
Ultrasounds in gas :
In gases, over short distances, it is possible to create very intense acoustic fields which enable liquids to be sprayed, powders to be dispersed, or suspended particles to be agglomerated. When filling and bottling, defoaming is another application (bottles, beer cas, dairy products…)
Ultrasound in solids (warmth, vibration):
A material is put in vibration by means of:
- Heating, characterized by the absorption of vibrations into the medium,
- Dispersal of waves on the surface which reduces the friction coefficients while under certain circumstances the surface layers of materials may be impaired.
The purpose of heating is mainly the welding (plastics, thermofilm and technical textiles …) of hot-melt materials.
The welding of metals is principally achieved through the exploitation of vibrations brought to bear on two parts simultaneously exposed to contact pressure. This technique offers a significant advantage : welding is performed at temperatures below the material’s melting point.