Back To Basics - How Cleaning Really Works, Part II

John B. Durkee, Ph.D., P.E.

In the last C4 column, I started my contribution to compliance with Controlled Environment’s 2010 editorial calendar by communicating some of the basics behind cleaning science. I used two simple “laws” of cleaning, and showed how they described how cleaning works—that it is really soil management. Cleaning is essentially moving soil from where it is not wanted to where it can be accepted. In this column I will relate the two other “laws” and summarize all four.

CLEANING THERMODYNAMICS
As classical thermodynamics teaches that no process is perfectly and totally reversible, the “Third Law of Cleaning” is:

• One can never get something completely clean.

If a perfectly clean surface is contacted with a cleaning agent containing one molecule of soil, that soil molecule can be transferred (at some slow rate by diffusion) to the clean surface and infect it. Then the driving force is reversed, and the soil molecule can be transferred back (at some slow rate) to the cleaning agent. So, a perfectly soil-free surface is theoretically impossible.

Said another way, a surface can be no more clean than the cleanliness of the cleaning agents last in contact with it.

This concept of a limiting driving force explains why perfect cleaning cannot be obtained in any single stage of cleaning (or rinsing) work.

It’s not an abstract concept. It’s why vapor degreasing was developed so that surfaces could be rinsed with pure liquid solvent produced on the part surface from condensed vapor. It’s why the final contact on critical surfaces in aqueous cleaning is with the purest and most mineral-free water (DI water).

THE UNCERTAINTY PRINCIPLE
The uncertainty principle in cleaning science originates in nuclear physics. Applying Heisenberg’s Uncertainty Principle¹ to cleaning operations:

• To get a particle off a surface, first you have to find both.

Cleaning work, done with ultrasonic transducers, is usually omnidirectional. That’s one reason these transducers are popular, as the pressure waves from them can impact surfaces not in direct line with the pressure radiation. In other words, ultrasonic transducers have the least difficulty in complying with the “Fourth Law of Cleaning.” The limitation of ultrasonic transducers is that they can’t remove (resolve) particles whose major dimension is significantly less than 1 micron.

Cleaning work, done with megasonic transducers, is usually directional. Mechanical force is radiated only perpendicular to the transducer surface. So these transducers must be aimed at where the force is needed to be applied. Megasonic transducers can remove particles perhaps a decade smaller than do ultrasonic transducers.

• In other words, smaller particles on surfaces can’t be removed by pressure radiation until their location is better known, and compensated for. This situation was certainly valid for particle removal by laser means, which is one reason they are less commonly used today.²

Consider aqueous spray cleaning machines. If the nozzles aren’t aimed at the current position of the soil on the parts, the work has been wasted. Certainly, this is also true with wipe cleaning means.

SUMMARY
The four basic “Laws of Cleaning” can be reduced to one sentence: cleaning is soil management. After all, one wouldn’t try to manage people: (4) without knowing where they were, (3) while expecting them to be perfect, (2) while not expecting that some management reinforcement would be needed, and (1) without believing that some management wasn’t needed in the first place.

In subsequent C4 columns in 2010, we’ll cover the basics of specific technologies, problems, and opportunities.

References

1. Named for Werner Heisenberg who was one of the greatest physicists of the twentieth century. He wrote in 1927 “The more precisely the position is determined, the less precisely the momentum is known”
2. One technique developed to overcome the limitations of the “Fourth Law of Cleaning” is called Vacuum Cavitational Streaming (VCS). Additional commercial experience may show that it overcomes the limitations of this “law.”

John Durkee is the author of the book Management of Industrial Cleaning Technology and Processes, published by Elsevier (ISBN 0- 0804-48887). He is the author of the forthcoming book Solvent Cleaning for the 21st Century, also to be published by Elsevier, and is an independent consultant specializing in critical cleaning. You can contact him at PO Box 847, Hunt, TX 78024 or 122 Ridge Road West, Hunt, TX 78024; 830-238-7610; Fax 612-677-3170; or jdurkee@precisioncleaning.com.