Patent ID: 7717544

Claim:
An improved method for acoustically ejecting a droplet of fluid from a reservoir contained in or located on a substrate by (a) acoustically coupling the reservoir containing a fluid to an acoustic ejector that produces acoustic radiation, (b) activating the ejector to generate at least one pulse of acoustic radiation which travels through the substrate and to a site at or near the surface of the fluid in the reservoir in a manner effective to eject a droplet from the fluid in the reservoir, wherein the improvement comprises: prior to acoustically ejecting a droplet of fluid, determining the wave form of a droplet-forming pulse of acoustic radiation which travels to the site in step (b) and adjusting the amplitude of the droplet-forming pulse of acoustic radiation to achieve an acoustic energy output level sufficient to eject a fluid droplet, and wherein the amplitude of the drop forming pulse for the site is determined by: (i) activating the acoustic ejector to generate a perturbation pulse of acoustic radiation which travels to the site such that no fluid droplet will be ejected as a consequence of that pulse; (ii) generating a perturbation interrogation pulse which travels to a zone on the fluid surface near the site; (iii) detecting the reflection of the perturbation interrogation pulse from the fluid surface; and (iv) processing the reflection detected in step (iii), wherein the processing in step (iv) includes using a frequency domain-based algorithm to identify the difference in frequency between two minima of the frequency spectrum of the reflection detected in step (iii).