The blog The Effects of Liquid Properties on Ultrasonic Cleaning discussed what effect(s) liquid properties might have on ultrasonic cleaning effectiveness. These are summarized here – –
Many physical properties are inherent to the liquid. De-ionized water, for example, due to its very high surface tension and tensile strength (as a result of its lack of impurities) is difficult to cavitate. But the cavitation bubbles that do form implode violently releasing significantly more energy than tap water under the same conditions. In the case of de-ionized water, increased surface tension and tensile strength contribute significantly to the effectiveness of ultrasonics.
The physical properties of any given liquid may also change with changes in temperature. In the above chart which is typical of water, all of the physical properties listed change with temperature. As a result, the cavitation properties of liquids also change with temperature. In ultrasonic cleaning we can use these changes to our benefit, and of course if managed improperly, they become our nemesis.
In the chart above, increasing the temperature produces the listed effect in all cases except for Vapor Pressure which increases rather than decreases with increased temperature. In the chart below, the effect of Vapor Pressure has been “flipped” so that all properties reflect the result from an increase in temperature.
Arrows have been added for visual purposes indicating a positive or negative effect on cleaning resulting from an increase in temperature. The preponderance of green arrows indicates a positive effect on cleaning. It is true that higher temperature, in general, results in better cleaning results. But there are some “surprises” that require a little more interpretation. For example, in the case of Dissolved Gas, three red vs. one green arrow would imply that less dissolved gas would have a negative effect on cleaning. If we could weight these effects (which is very difficult), it would become clear that the benefit of higher implosion energy eclipses the negative effects of more difficult cavitation, smaller cavitation bubbles and less cavitation bubbles. All the easily produced, large cavitation bubbles in the world are of no value unless they implode to release energy at the surface of the part being cleaned. A similar thing happens with Vapor Pressure. Although, higher vapor pressure results in more, larger, easily produced cavitation bubbles, the bubbles tend to fill with diffused vapor of the liquid rather than maintaining a vacuum. The bubbles containing vapor rebound from the internal pressure of the vapor rather than imploding violently. They contribute only minimally, if at all, to cleaning. In the extreme condition, when the liquid is boiling, cavitation bubble implosions due to ultrasonics do not occur at all.
Again, the chart above is offered as a guideline only and should not be applied literally for all cases and under all conditions. The point here is that liquid properties are important but not easily sorted out because of the interactions of additive and cancelling effects. The recognition of these effects, however, can give us some insight into the selection of temperature. Temperature, because if its impact on both the physical properties of liquids and the performance of cleaning chemistry, has a larger impact on cleaning than any other single variable. The other important variable is chemistry. Chemistry will be discussed in the next blog.
– FJF –