Ultrasonic vapor formation is really fascinating, particularly because nobody is really sure how it works. Here's a study from the Journal of Fluid Mechanics a couple years ago:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4428615/#!po=35.6383
The short version is this: at the fluid-gas boundary of a liquid agitated by an ultrasonic transducer, chains (or stacks) of drops form on top of pressure ridges. Within the topmost drop of each chain, bubbles are spawned. These bubbles erupt through the surface of the drop, generating a vapor of nanodroplets of fluid.
The most commonly-accepted theory to explain these bubbles is cavitation. Basically, the ultrasonic waves agitate the fluid enough to form tiny pockets of vacuum (which appear to us as "bubbles"). What
isn't well understood (and, spoiler alert, this study does not resolve) is what happens next. It may be that when enough cavitation bubbles form within a drop, the surface tension of the drop can no longer contain them. Breaking the surface, gas molecules rush in to fill the void with enough energy to obliterate the drop. Or, it may be that the fluid molecules crashing into each other as the cavitation bubbles collapse generate enough heat to make the fluid boil, causing the fluid drop to explode. Or it may be a little of both.
As you'll see in the study, viscosity is a factor in vapor formation, and the thicker the fluid the greater the amount of energy you need to produce vapor. To atomize something like wax, you'd need one hell of a transducer. Even undiluted hash oil would be challenging, but it would be very interesting to try it.
