What is Ultrasonic Cavitation?
Ultrasonic atomization occurs at the nozzle’s tip through the rapid mechanical upward and downward motion of the nozzle tip which causes a film of liquid to form into standing capillary waves. When the amplitude of the capillary wave, which is a function of the amount of wattage applied (typically within the range of 1 to 20 watts), peaks what is required for stability of the system, the liquid at the peak crests breaks away in the form of droplets. You can see an excellent video showing this concept.
If the energy level is excessive, cavitation will occur. Instead of forming an ideal film on the tip of the nozzle, excessive energy will cause liquid emerging from the nozzle to prematurely aerosolize and literally “rip apart” into unevenly sized droplets. Cavitation can be observed by observing how far a liquid “wets-out” onto the nozzle tip. In the following example, each image represents a view of a nozzle tip looking straight on. The center circle is the liquid hole or orfice from which a liquid wets-out onto the nozzle tip. The inner gray circle represents the liquid film. Let’s assume for both examples, the liquid and flow rate are the same. The only difference is the power level. In the low power example, you can see that most of the surface of the nozzle has a liquid film. This image represents the “ideal.” Notice it does not expand to the edge, which will be explained in a moment. The high power example, only a small amount of film build is created. Remember both examples have the same flow rate, so what happen to the missing liquid? It is being cavitated or ripped off the nozzle tip before it had a chance to properly atomize. Both examples are technically ultrasonic aerosolization. Ultrasonic cavitation produces uneven sized droplets, while ultrasonic atomization creates more evenly sized droplets since they are being formed by a more controlled mechanical process vs. brute force.
How to determine the ideal Power “Wattage” for Ultrasonic Atomization
The best way to set the ideal power level for a given nozzle frequency, flow rate and type of material is to set the power roughly one watt above the stall point. Let’s say you start at 3 watts and the liquid is atomizing and not cavitating. The film build on the tip looks like something between the high and low power examples. Dial slowly down your wattage until the nozzle stalls, let’s say it stalls at 1 watt. A stall is when a large drop forms on the tip and atomization ceases. From this point add one watt, setting your ideal power level to 2 watts. Now take a look at your nozzle tip, if it looks something like the low power example, this is a good set up. As mentioned, you don’t want to be too close to the edge or you run the risk of your film wrapping around the nozzle tip. This can cause unwanted side spray, as liquid is atomized off the side of the tip.