A method of controlling evaporation of droplet cloud resulting from nebulization of a liquid expelled from a liquid droplet spray device is disclosed, wherein the liquid droplet spray device comprises a reservoir containing liquid, a perforate membrane plate, a fluid interface for feeding liquid from the reservoir to the membrane plate, an ultrasound generating mechanism for acting on the supplied liquid to cause it to be expelled as a spray of droplets through orifices of the membrane plate, and an electronic control device for controlling activation of the ultrasound generating mechanism.