A micromachined ultrasonic droplet generator is demonstrated for atomization of liquids for fuel processing. The device comprises a bulk ceramic piezoelectric transducer for ultrasound generation, a reservoir for the fuel, and a silicon micromachined array of liquid horn structures as the ejection nozzles. Since one piezoelectric actuator can drive multiple ejectors of a single array, the array size can be scaled to meet flow rate requirements for higher power applications. Furthermore, due to the planar configuration of the ejector array, it is ideally suited for integration with the planar design of fuel cells. Simulations of the harmonic response of the atomizer confirm that operation at cavity resonances and the use of acoustic wave focusing yield low power consumption. Device operation is demonstrated through atomization of water and methanol from 4.5 to 16 mu m diameter orifices at multiple frequencies between 0.5 and 2.5 MHz. The results of high -spatial-resolution visualization experiments combined with a scaling analysis of the fluid mechanics provide a basic understanding of the physics governing the ejection process and allow for a comparison of device operation with different fuels. A high degree of control of the atomization process and highly uniform atomization at low flow rates are achieved with a device that is extremely simple to fabricate, assemble, and operate.