Ultrasonic vibrations application increases the effectiveness of degreasing, etching and cleaning the parts in detergent solutions and water in a great way. In the propagation of ultrasonic waves in the liquid rises to the so-called sound (acoustic) flows which have a vortical nature, and the propagation velocity of up to hundreds of centimeters per second, vigorously facilitating surface cleaning. However, the most important factor in the influence of ultrasonic vibrations to the liquid medium is cavitation.
The phenomenon of cavitation is presence of microscopic gas bubbles and tiny solid particles, which serve as its center and embryos. Under the influence of ultrasonic field gas bubbles periodically expand, contract the surface and blow causing cavitating. Cavitation occurs when the bubble oscillations achieve a certain diameter. So, quite large bubbles under the influence of ultrasonic vibrations alternately compressed and expanded, not slamming, rise to the surface of the liquid and stand out from it. While the bubbles are less than 60 microns in diameter are not foldable, they perform oscillating movements and provide a cleansing effect on the wafer surface as a kind of mechanical scrapers. With the collapse of the bubbles appear instantaneous local pressure, reaching tens of thousands of kilopascals.
The removal processes in the ultrasonic field of soluble and insoluble impurities takes place differently. During the purification from soluble contaminants the movement of the wash liquids plays a vital role. In this case, the acoustic flows accelerate the dissolution of pollution and provide quick removal of contaminants from the surface being cleaned. When cleaning the insoluble impurities plays the main role of mechanical failure due to cavitation. In this case, under high pressures arising from the collapse of the bubbles, cracking occurs film surface dirt and partial peeling them from the surface being cleaned. Oscillating gas bubbles penetrate the exfoliated film, tear them away from the surface and the dirt particles are carried away by the acoustic flux.
An important advantage is the ability of cavitation bubbles to penetrate into the deep pores and grooves parts of complex design and configuration that can not be cleaned by any other means. Despite the high maintenance costs, some systems already demonstrate a considerable operational expenses decrease. Thus, Hilsonic, a prominent UK-based manufacturer, offers a comprehensive line-up of ultrasonic cleaner equipment, claims an average customer saves up to 18% on operational expenses.
Ultrasonic cleaning efficiency depends on the frequency and power of ultrasonic vibrations as well as the temperature of the solution composition, the extent and nature of the contaminants, as well as processing time. The most intensive ultrasonic treatment occurs at frequencies of 20-40 kHz. This is explained by the fact that at these frequencies, gas bubbles grow large and emit more energy cavitation. In addition, under the influence of these frequencies and to vibrate the samples themselves, which also helps clean the surface thereof. At frequencies below 20 kHz sound becomes audible, so it is not applied.
For the purification of small and easily deformable articles preferably ultrasonic vibration frequency of about 400kHz because at low ultrasonic frequencies samples may deform or collapse. At high ultrasonic vibrations cleaning occurs under the action of the vortex acoustic flow, while cavitation vibration-processed samples are negligible.
Increasing the frequency of oscillation reduces the length of the sound waves and hence enhances its penetrating ability. Therefore, high-frequency ultrasonic vibrations provide high quality in cleaning products with holes, grooves and other cavities.