Particle Size Removal vs. Ultrasonic Frequency

With the availability of ultrasonic cleaning equipment operating at frequencies from 20kHz up to over 250kHz and the capability to use more than one frequency in a single piece of ultrasonic cleaning equipment, users inevitably and rightly question the optimum frequency or frequencies for their cleaning applications.  The popular and most touted notion is that the size of particles being removed dictates the optimum operating frequency.  Although physical laws support this theory as seen in the blog  Effect of Frequency Removing Particles and others referenced from it, there remains only one documented instance in published literature that validates this experimentally.  This was in a paper entitled “The Impact of Ultrasonic Frequency on Particle Removal”  by Jonathan Harman & Edward W. Lamm published in the 2002 Clean Tech symposium proceedings.  Despite the lack of rigorous verification, however, the anecdotal correlation is undeniable.

The following chart has been widely published in various iterations and links frequency with particle size removal and also with particular application families.

Although far from complete, this chart may serve as a starting point for frequency selection when it comes to particle removal.  The removal of large amounts of contamination or extremely large particles (the size of grains of sand for example) and heavy buildups of other contamination like caked-on oil, grease and road dirt is logically relegated to pressure and spray washing.  The displacement requirement to efficiently remove these contaminants is beyond the realm of ultrasonic capability and practicality.  Removal of heavy contaminants using ultrasonics is usually a long process as contaminants are eroded layer by layer.  Also, the volume of contamination results in rapid fouling of the cleaning bath resulting in reduced ultrasonic overall process efficiency.  Pressure and spray washing, on the other hand, proceed rapidly with removed contaminants expelled with the cleaning solution which goes directly to disposal or, in the case of spray washing, is filtered prior to re-use.  Pressure and spray washing are recommended for pre-cleaning in cases where an ultrasonic specification is required but the contamination level is high.

One serious down fall with the above chart is that it applies almost exclusively to particle removal.  In the real world, particles are often accompanied by other, many times soluble, contaminants which can alter the relationships considerably.  One example might be very small particles held in place by animal fat or wax – buffing compound is an example.  There is evidence that lower ultrasonic frequency is more effective in dissolving soluble contaminants.  So, despite the small particle size, a lower frequency may provide better overall cleaning.  In the extreme case, a combination of lower and then higher frequency may be required to provide the required result.  Although I am not aware of any studies evaluating frequency vs. the removal of soluble contaminants, it would be difficult to say based on anecdotal evidence and repeated instances of a positive correlation between removal efficiency of soluble materials and frequency, that there is not a relationship between the two.  It’s just that I’m not sure we know exactly what it is, where it applies or how to predict it.

Selection of the most effective frequency for ultrasonic cleaning to this day remains largely a trial and error effort based on experience and “tribal knowledge.”  I think it would be to the benefit of the ultrasonic community to tie down these.  If I have missed something along the way and the relationship has been scientifically established I would be happy to report your findings in this forum.

 JF

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