Patent ID: 8882226

Claim:
A method of ejecting droplets from a plurality of fluid reservoirs, the method comprising: moving at least one of the fluid reservoirs and a plurality of acoustic ejectors with respect to each other so that focal point positions of the acoustic ejectors change with respect to fluid surfaces of corresponding fluid reservoirs; sending interrogation pulses from the acoustic ejectors towards the fluid surfaces of the corresponding reservoirs; receiving time series of measurements of the fluid surfaces using echoes from the interrogation pulses; determining, based on the time series of measurements, at least one of: an ejection time when each fluid surface will be within a threshold distance of a focal length of the corresponding ejector or a multiple of the focal length, and a distance of each fluid surface from the corresponding ejector; and ejecting the droplets from the fluid reservoirs with the ejectors in an order based on at least one of the times of arrival of the fluid surfaces and the distances of the fluid surfaces.