This is a few drops of water sitting on the surface of an ultrasonic transducer. The high-frequency vibrations cause the water to move around violently (on a tiny scale) and as it does, it smashes into itself and one result is that tiny droplets are formed. It's kind of like a nebulizer in a way.
Those tiny droplets are so small that they waft and float in the air they way smoke does. But since they're formed by all kinds of violent collisions, they're often emitted from this roiling, turbulent surface with what seem like small jets of motion going off in random directions.
I think that's what causes the neat formations. You do get a lot of tiny "mushroom clouds" and "smoke rings" as well as fast-flying larger droplets.
In some of the images, you can see where the water is slapping against itself and you can see splatters of very high velocity droplets shooting out away from the collision point. It's kind of bizarre to watch.
So the tiny particles are just very small water droplets. They're so small that they waft on the air currents like smoke before they evaporate. It's things like this that make me want to have a movie mode in my next DSLR.
