Fig.86 Areas of stable (potential’s holes) and unstable balance.

A package of standing waves (nodes and crests) emerges between synchronized ultrasonic emitters. If a cold steam (a tube to the right) is injected in the area, drops of water condense in the nodes being held in their places by the wave energy. A phase displacement between the emitters would cause the nodes to shift relative to their initial positions. The drops of water would follow in their wake.