Ultrasound - a brief introduction
Ultrasound is used in science and technology for a wide range of applications. In the following, the physical principles of ultrasound will be briefly described.
When ultrasonic waves are applied from a frequency of 20 kHz and a sound intensity of 0.1 W / cm² to liquid media, small cavitation bubbles occur due to the strong alternating stress. These finest cavities with low internal pressure are due to the inertia of the medium and the induced mechanical vibrations. Due to the external pressure of the medium, the unstable bubbles impulse after short growth under high pressure and temperature peaks. This results in high shear forces at the boundary layer. This effect occurs particularly on air bubbles or dirt particles, the so-called cavitation nuclei. The cavitation in the water triggers the acoustically perceptible noise of high-performance ultrasound equipment.
In ultrasonic cleaners cavitation promotes the removal of dirt particles from the solid object to be cleaned. Further applications are the homogenization, in which emulsions are produced as well as the degassing of liquids. Due to the very low pressure within the cavitation bubbles, dissolved gases dissolved in the medium are converted into the gaseous state. When the cavitation bubbles are impregnated, they are then taken up again to the point of the solution equilibrium by the medium. Excess gas emerges from the medium. The apparatus used for the sonication of e.g. Biogas substrate utilizes the shear forces generated by the cavitation at the solid-liquid boundary layer and the resultant attack of the surface, which leads to a comminution and disintegration of contiguous particles in liquid media.
The demonstration of this cavitation effect can be easily achieved with the so-called "foil test": an aluminum foil of 0.01 to 0.02 mm thickness is stretched on a wire frame, introduced into the liquid and sonicated for 60 seconds. The cavitation causes perforations of the foil, where the intensity and distribution of the cavitation can be seen.
The Foil test
Use of cavitation for cleaning
The interest in the cleaning effect is based on the fact that cavitation predominantly occurs at points of inhomogeneity, ie where contaminants adhere to surfaces. The dirt itself attracts the cavitation, which eventually dissolves it from the surface.
This process is often referred to as 'micro-scrubbing' or 'electronic brushing'. The ultrasound is, however, an equally gentle and thorough brush, because the effect of the cavitation allows - even in the short term - sensitive surfaces unaffected, and unfolds wherever cleaning liquid is present.
Ultrasonic cleaning is an ideal method to clean blind holes, knurls, cracks and all kinds of cavities that are manually poorly accessible. Whole assemblies, such as gearboxes, need not be disassembled for cleaning, but can be inserted as a whole into an ultrasonic cleaning bath. Ultrasonic cleaning is also suitable for sensitive parts, eg electronic components, circuit boards, inaccessible electrostatic filter cells, diesel valves or fine car injection nozzles. Also in medicine, the dirt-dissolving effect of cavitation in hospitals and practices is used for the cleaning and time reduction in the disinfection of fine cutting instruments, endoscope parts and in particular microinstruments.