The Case for Micro-Bubbles in Cavitation Enhancement

In 2010, a patent was issued to Kaijo Corporation (US 7,726,325) for a method to efficiently and rapidly “degas” liquids.  In short, the process involves passing a liquid through a restriction such as the narrowing of a pipe at a sufficient velocity that rarefication downstream from the restriction (ala Bernoulli’s principle) creates negative pressure to draw dissolved gas out of the liquid.  The gas thus extracted forms bubbles of sufficient size to make them buoyant and float to the surface of the liquid without being reabsorbed into the liquid.

This idea was of great interest to the ultrasonic cleaning community as gas dissolved in liquid is detrimental to ultrasonic cavitation.  Typically, fresh liquid in ultrasonic processing tanks is “degassed” prior to use to maximize the cavitation effect.  Degassing may take from a few minutes to several hours depending on the size of the tank and other factors including temperature and chemistry and is usually aided by the use of ultrasonic energy as well.  The addition of chemistry, by the way, significantly and visibly accelerates the degassing of liquids.  The promise was that using the new process, liquids could be delivered to process tanks in a pre-degassed state thereby significantly reducing or eliminating the need for subsequent degassing using conventional means.  This would result in a considerable time savings when re-charging tanks.  The device was also intended to be used in a closed loop system to maintain low levels of dissolved gas in liquids during extended use.

As one does when some new concept is introduced, members of the ultrasonic community including Kaijo started playing with this idea to verify its efficacy and to find ways in which it might be applied on a broader scope and in other ways.  It was soon discovered that the device worked surprisingly well in enhancing the cavitation effect in extremely pure water (processed with Reverse Osmosis and De-ionization).  This was surprising because, first, the amount of dissolved gas is typically quite low in water processed in this manner and, second, the difficulty in cavitating RO/DI water was commonly ascribed to the lack of nucleation sites or “seeds” for the growth of cavitation bubbles, not the presence of dissolved gas.

The result was somewhat of a “voila” moment!  Consider the possibility that this new deaerating device was not only creating bubbles of sufficient size to float to the surface of the liquid, as it appears was its primary intent, but also a vast population of smaller bubbles (micro bubbles) that remained suspended in the liquid.  Could these micro bubbles act as sites for the birth and growth of cavitation bubbles?  The answer is likely yes.

Small bubbles, VERY small bubbles, it appears do act as nucleation sites for the growth of cavitation bubbles.  It is also widely believed that the micro bubble needs to be of exactly the “right” size to serve as a nucleation site.  Laboratory trials were not inconsistent with these beliefs.  It was, in fact, found that the addition of a small volume of water processed through this device to a much larger volume enhanced cavitation and implosion effects in the entire volume of liquid.

 – FJF –

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