Ultrasonics – Effect of Frequency Removing Soluble Contaminants
Earlier blogs have explained the effect of frequency on the size of cavitation bubbles and where they are formed. Today’s will explain the effect of bubble size and location on removing soluble contaminants.
Soluble Contaminants –
The removal of soluble contaminants requires that solvent saturated with the contaminant being removed must be displaced from the solvent/contaminant interface and replaced with unsaturated solvent in order for the removal process to continue. The micro-agitation or mixing at the solvent/contaminant interface is the most useful effect of cavitation implosion. This, combined with some form of macro-agitation such as part movement or solvent agitation to further separate the saturated solvent from the interface zone is an extremely powerful means of removing soluble contaminants.
Since the force of the implosion of a cavitation bubble releases a lot of energy which is preferentially directed toward a discontinuity (in this case the solvent/contaminant interface), cavitation bubble implosions are ideally suited for creating micro-agitation at the solvent/contaminant interface. The larger the cavitation bubble, the greater the force released on its implosion. This indicates that a larger bubble created by a lower ultrasonic frequency will be more effective in removing soluble contaminants than a smaller bubble created by a higher frequency as shown in the illustration below.
In most cases this is true. However, there are also a couple of other issues to deal with. First, with the same energy input, there are more cavitation bubbles produced at a higher frequency than at a lower frequency. The combined energy of the implosions of all the bubbles in both cases must be the same. Secondly, there is the issue of the boundary layer. Depending on the topography of the part or substrate, the more powerful “jetting” force of the implosion of larger cavitation bubbles produced at a distance away as dictated by the barrier layer may be effective in removing the contaminant from generally smooth surfaces. However, if the substrate topography is rough or “spongy” in nature such that the larger bubbles produced at a lower frequency are not able to form close enough to the convoluted surface due to the barrier layer effect, removing the soluble contaminant may require resorting to a larger number of implosions of less energy produced by smaller cavitation bubbles at a higher frequency which are better able to penetrate the surface configuration.
In fact, this is a bit of a “balancing act.” The above provides some guidelines for selection of frequency but the proof, as they say, is in the pudding. The one sure thing is that extremely “gooey” contaminants like heavy grease, paint, adhesives will probably be better removed using lower ultrasonic frequencies.
An upcoming blog will address the effect of bubble size and location on the removal of insoluble contaminants or “particles.”
– FJF –
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