In a previous post, Ultrasonic Cleaning - Soluble Contaminants, I discussed the effect that ultrasonic cavitation and implosion play in enhancing the removal of soluble contaminants from surfaces. Today's blog will introduce the effect that ultrasonic cavitation and implosion have on the removal of insoluble particles from surfaces. There are several mechanisms that can cause particles to be attracted to and/or remain attached to surfaces. Obviously, if these mechanisms didn't exist, particles would simply fall off without any outside intervention and cleaning would not be an issue. Probably the most common mechanism bonding particles to surfaces is adhesion. In some cases the adhesive attraction is directly between the particle and the surface. More commonly, however, there is an intermediary "glue" such as oil or another agent present in or prior to the manufacturing process that adheres to both the particle and the surface thereby bonding the two together. In this case, removing the particle may require first removing the "glue" using a suitable solvent and then displacing the particle as either a simultaneous or secondary step. In another scenario, the particle may be embedded in the "glue" requiring that the cleaning means must first remove the soluble material encasing the particle to gain access to it for removal and then proceed to remove the remaining "glue" beneath the particle to expose additional particles which are, in turn, removed to provide a clean surface. Since particles seldom travel alone, there are limitless permutations of this progression that involve particle/adhesive and particle/particle interactions. In any event, mechanical force is needed. In other cases it may be static forces, molecular attraction or ionic attraction that make particles adhere to surfaces. These forces, although very weak, are somewhat more difficult to deal with than the "glue" scenario described above. This is because even though the initial attraction between the particle and the surface may be temporarily overcome by some mechanical means, the potential for re-attachment remains since whatever force attracted the particle to the surface is seldom eliminated by the mechanical displacement. In order to remove a particle, sufficient mechanical force must first be imparted to break whatever bonding force is holding the particle in place. The displaced particle must then be distanced sufficiently from the surface to which it was initially attracted so that re-attachment does not take place. The implosion of cavitation bubbles produced by ultrasonics is a very effective means of initially displacing particles. The degree of effectiveness depends on a number of things including the size of the cavitation bubbles as the force of the implosion is related to bubble size. In addition, however, some means must be provided to provide the required distance between the particle and the surface to prevent re-attachment. The the force delivered by the implosion of cavitation bubbles is seldom adequate to provide this distance as these forces are very localized and are randomly directed. Secondary means of providing separation may include part agitation, turbulation, spraying or other sources of mechanical force applied in either the cleaning tank or in a subsequent rinse. Once sufficiently distanced from the surface, particles can be collected by filtration for disposal. The above has "broken ground" so to speak for the ongoing discussion of the effective removal of particles from surfaces using ultrasonics. In future blogs I will discuss the effect of using various ultrasonic frequencies as well as several other means to enhance the effectiveness of particle removal using ultrasonics. Meanwhile, if you haven't already read some of the posts regarding magnetized particles, now may be a good time to type "magnetism" in the search box. Magnetic attraction of ferrous metals is far stronger than the bonding forces referenced above and should be considered as a special case.
- FJF -