The apparatus includes a motor (20) which drives a first compound gear (28) having first and second portions (26, 32), the second portion driving a second compound gear (36). The second compound gear has one portion (38) which has teeth over only one half the circumference thereof. The apparatus further includes an air cylinder (12) and a rack (62) and a spring assembly (66) a sealing end portion (64) of the rack fitting in a fluid-tight relationship with the air cylinder, wherein the rack member is engaged by the one portion of the second compound gear, compressing the spring when the teeth thereon engage the rack member. When the no-teeth portion comes adjacent the rack member, the rack is released and the sealing end of the rack member is forced by action of the spring into the air cylinder, producing bursts of air. The second compound gear also drives a peristaltic pump, producing liquid in bursts which are directed to a mixing chamber 58, from which a stream of liquid droplets exit through exit 16.
