Provision is made for selectively addressing individual crests (64) of traveling or standing capillary surface waves (62) to eject droplets from the selected crests on command. To that end, the addressing mechanisms (65, 85) of this invention locally increase the surface pressure acting on the selected crests and/or locally reduce the surface tension of the liquid within the selected crests. The preferred addressing mechanisms have sufficient spatial resolution to address a single crest substantially independently of its neighbors.
Discrete addressing mechanisms having a plurality of individual addressing elements (85) are especially attractive for liquid ink printing and similar applications, not only because their individual addressing elements may be spatially fixed, but also because the spatial frequency of their addressing elements may be matched to the spatial frequency (wavelength λc) of the capillary wave. Such frequency matching enables selected crests of the capillary wave to be addressed in parallel, such as for line printing. Preferably, the capillary wave for a printer is a spatially stabilized standing wave, so that the crests and troughs of the capillary wave are locked in predetermined spatial locations.