Initial Droplet size vs. Final Droplet size
The frequency of an ultrasonic nozzle determines droplet size. The higher the frequency the smaller the “initial” droplet. Most customers however are more concerned with the size of the droplet that comes in contact with their part. Take the example of a thin film coating on a wide substrate. The ideal functional coating may not be so much a film but more like a dusting pattern, such as shown in the attached image. To achieve this kind of coating many factors should be considered, one of which of course is the initial droplet size a given ultrasonic nozzle frequency generates. What many customers perhaps overlook is the importance evaporation plays in the ultimate droplet size that comes in contact with your part. It is not uncommon for customers to think they need the smallest possible “initial” droplet (i.e. highest frequency ultrasonic nozzle) but when they set up their system, the coating is depositing dry. All the solvent or carrier is flashing off before the solids in solution have a chance to coat the part. In other words, they lose all the film building attributes of a liquid film and instead are powder coating! It is important to think in terms of the “final” droplet size. Don’t get me wrong, the frequency of an ultrasonic nozzle is very important, but highest frequency is not always better. Considerations to the type of solvent (low vs. high boiling point), distance the nozzle is from the substrate and flow rate also all play a role in determining the final droplet size and thus quality of coating.