A preceding blog discussed the possible effect of too many available sites for the formation of cavitation bubbles to the point that there isn’t enough energy available to grow a significant enough portion of them to sufficient size to implode. This leads to the question that the reverse may be possible as well. In short, what if there aren’t enough cavitation “seeds” available. The answer is yes and although there is significant support for this premise it is seldom an issue in industrial cleaning.
The example that comes to mind immediately is the difficulty encountered in trying to cavitate extremely pure water that has been processed by Reverse Osmosis (RO) and De-ionization. Reverse Osmosis removes very small particles (down to molecular size) and De-ionization removes any ions that may be present. The result is the purest water we encounter in industrial cleaning.
Note – It is a common but erroneous belief that “distilled” water is the purest form of water. In fact, distilled water is low in organics and particles but is likely to have both dissolved gas and dissolved solids. Distilled water is necessary in applications, notably medical, where organic and pathogenic contaminants must be avoided. Distilled water finds little application in industrial cleaning except for in the medical field. In general, water processed by reverse osmosis and/or de-ionization provides superior performance and is more economical than distilled water.
It generally agreed that water processed in this way is very difficult to cavitate, and, that once cavitation is achieved, the implosions are extremely energetic and aggressive to the point that they may cause damage to sensitive substrates. The question arises, “So what does this mean for users of de-ionized water.” The answer, in many cases, “Not much.”
High purity de-ionized water (18 megohm or better) is not a reality under normal conditions of industrial cleaning. Ion hungry water rapidly acquires ions from its surroundings unless kept under extraordinarily controlled conditions. A resistivity of 7 to 10 megohms is achievable in an open tank under controlled conditions but not without considerable effort which usually includes continuous “re-polishing” by aggressive and repeated circulation through de-ionizing resin beds. One to two megohm resistivity is all that is required for rinsing in most industrial cleaning applications and even that is “over-kill” in many cases. “Spot-free” drying for most applications is achieved using 500kOhm to 1 megOhm water.
But there are processes where 18+ water IS a requirement. As it happens, ultrasonics is seldom required in these applications. The purpose of the high purity water is to absorb the final remnants of previous rinsing steps. This happens on a molecular scale where the major “actor” is ionic attraction. Ultrasonic activity serves little purpose in this.
So, in the overall scheme of things, although the difficulty in cavitating de-ionized water is recognized, it is seldom an issue. In rinses, there is usually enough carry-over from pervious process steps to provide the “seed” sites needed for effective ultrasonic cavitation making the only real “challenge” the initial preparation of tanks for use after refilling. In applications requiring water above the 1 to 2 megOhm range, cavitation doesn’t add much anyway.
Although each application is different and presents its own challenges, the difficulty in cavitating de-ionized water usually plays a small role in process development overall. In most cases, measures taken to improve cavitation will lessen the quality of the water sufficiently to counteract any benefit that ultrasonics may add.
– FJF –