  As shown by Ganan-Calvo and co-workers, a free liquid jet can be compressed
in iameter through gas-dynamic forces exerted by a co-flowing gas, obviating
the need for a solid nozzle to form a microscopic liquid jet and thereby
alleviating the clogging problems that plague conventional droplet sources of
small diameter. We describe in this paper a novel form of droplet beam source
based on this principle. The source is miniature, robust, dependable, easily
fabricated, and eminently suitable for delivery of microscopic liquid droplets,
including hydrated biological samples, into vacuum for analysis using vacuum
instrumentation. Monodisperse, single file droplet streams are generated by
triggering the device with a piezoelectric actuator. The device is essentially
immune to clogging.
