Bubble, Bubble – Less Toil and Trouble

Action or Force are essential for effective critical and precision parts cleaning. As we explained in a recent Clean Source, while spray cleaning can be effective, it is line-of-sight. Ultrasonic cleaning and cyclic cavitation (also referred to as vacuum cycle nucleation or cyclic nucleation) are omni-directional. While both involve “bubbles,” the two techniques are very different. Learn how the two techniques can solve difficult, seemingly impossible manufacturing problems.

How ultrasonic cleaning works
Ultrasonic cleaning employs high frequency, high energy sound waves (above approximately 20 kHz) to agitate a liquid bath. As sound propagates through a fluid (gas or liquid), there is sequential compression and decompression (or rarefaction) of the density. In liquid, if the energy (amplitude or loudness) is strong enough, the liquid tears apart during rarefaction. What are commonly referred to as ultrasonic bubbles are, in fact, tears or voids or cavitations. During the subsequent compression, the “bubbles” collapse, creating violent, omnidirectional shock waves. These shock waves are what produce the powerful cleaning force (1-2) that can dislodge even fairly tightly bound soils and particles from a surface.

Ultrasonics increases cleaning efficacy and has the economic benefit of reducing process time. There are many variables to be considered. Frequency is just one variable. Lower frequency voids have more time to expand and have more energy when they collapse. Higher frequencies, while not as powerful, can be more effective at dislodging small particles and are less likely to damage sensitive surfaces. High surface tension fluids, like water, need more energy to create voids and consequently create stronger shock waves.

Ultrasonics actually delivers a “one-two punch” because in addition to cavitation, there is a line-of-sight mechanism, acoustic streaming. As the frequency increases, it gets harder to generate cavitation. Acoustic streaming becomes the dominant mechanism above about 400 kHz. At very high frequency, the term is megasonics (3).

How cyclic cavitation works
Cyclic cavitation uses variations in air pressure above a liquid to induce and quell boiling (4-5). When the pressure is below the liquid vapor pressure, the liquid boils and creates gas bubbles. When the pressure is raised above the vapor pressure, boiling stops and the gas bubbles collapse. The technique is also referred to as vacuum cycle nucleation or cyclic nucleation because the boiling cavitation starts or nucleates at particles, edges, or imperfections on the product surfaces. These imperfections are where the cleaning action is most needed. We tend to like the term cyclic cavitation, because the term nucleation could be confused with radioactivity; the technique does NOT use radionuclides.

Cyclic cavitation cleans both during the higher and lower portions of the pressure cycle. The expanding boiling bubbles in the lower pressure phase expel the liquid. The liquid rushes back in during the higher-pressure phase. In both phases the momentum of the moving liquid is what dislodges and carries away the contamination. The process can employ water, aqueous cleaning solutions or solvents. Please be aware that cyclic cavitation is very different from “jacuzzi” cleaning, where air bubbles are injected into the tank to agitate the liquid.

Features, attributes, limitations
Ultrasonics power comes from transducers, usually built into the bottom or edges of the tank. Immersible transducers are also used. Because the cleaning action emanates from the transducers, cleaning is from the outside in. While the technique is omnidirectional, there is an effect of distance and of blocking. You can’t clean the bow of a cruise ship by putting an ultrasonic transducer on the stern. In critical product cleaning, we’ve seen too many instances where a basket in the tank is completely filled with parts. The parts toward the center are unlikely to be cleaned effectively. Another kind of blocking is the result of materials that absorb sound, including soft plastics. As long as the cleaning agent can reach the surfaces, cyclic cavitation can be effective even in a tightly-loaded process bath. Cyclic cavitation works well in small tubing, like hypodermic needles, because the cleaning force both dislodges the soil and also carries the soil away from the surface.

Ultrasonics is a mature technology that is offered by a number of suppliers. While it is sometimes considered as a “commodity” item, there are many variables to consider. Very simple systems are readily available; some may be suitable for your application. Technical options provide nuances and flexibility, albeit sometimes at a higher cost.

Most ultrasonic processes are aqueous, but solvents can be used as well. Due to lower surface tension, solvent cleaning is generally gentler than aqueous. If the solvent is flammable or combustible, appropriate equipment and controls must be used.

One limitation is that while higher temperatures frequently increase cleaning efficacy, ultrasonic cleaning is not effective near the boiling point of the liquid where the vapor pressure of the liquid is high. The voids fill with the vapor from the liquid and the subsequent cavitation collapse is cushioned. We have seen numerous instances where an ultrasonic system is operated close to the boiling point. The increased heat, without ultrasonics, may be responsible for cleaning efficacy. Under such circumstances, the ultrasonics were a wasted expenditure.

Cyclic cavitation is a relatively new technique with only a few suppliers in the marketplace. It requires dedicated vacuum equipment so that the internal pressure can be varied above and below the liquid vapor pressure. The vacuum seal may allow cleaning with otherwise hazardous solvents as well as by water because the solvent vapors can be contained in the system.

Quelling of the boiling in cyclic cavitation does not produce strong shock waves; the cleaning is from the flushing of liquid into and out of the chamber. A major feature is that cyclic cavitation cleans from the inside out, so can be very effective for the interior of narrow tubes and blind holes. The cleaning equipment provider may not offer equipment for higher boiling solvents.

Which technique should you use?
Forget the cauldron! Consider the decrease in toil and trouble, the improvement in cleaning, facilitated by cyclic cavitation and ultrasonics cleaning. You have more choices! One, the other, or both may be useful in improving product quality. Feel very welcome to reach out to us for assistance with these and other choices in critical product cleaning.

References
1. F. John Fuchs, “The Fundamental Theory and Application of Ultrasonics for Cleaning,” Handbook for Critical Cleaning, 2nd Edition. Vol 1, B. Kanegsberg and E. Kanegsberg, editors, CRC Press, 2011.
2. Sami Awad, “Ultrasonic Cleaning Mechanism,” Handbook for Critical Cleaning, 2nd Edition. Vol 1, B. Kanegsberg and E. Kanegsberg, editors, CRC Press, 2011.
3. Mark Beck, “Megasonic Cleaning Action,” Handbook for Critical Cleaning, 2nd Edition. Vol 1, B. Kanegsberg and E. Kanegsberg, editors, CRC Press, 2011.
4. Gerhard Koblenzer and Hans Hauger, “Discover the Cyclic Nucleation Process (CNp) with LPW,” A Product Quality Cleaning Webinar (PQCW) Webinar, Jul. 2020. Available through https://mailchi.mp/585453f04d7a/pqcwebinar-with-lpw
5. Joe Schuttert and Don Gray, “Cleaning Tight Features and Small Lumen Tubing with Vacuum Cycle Nucleation (VCN),” A Product Quality Cleaning Webinar (PQCW) Webinar, Apr. 2022. Available through https://mailchi.mp/d26713f48246/vacuum-cycle-nucleation-webinar

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