Fog Droplet Vaporization and Fragmentation by a 106-μm Laser Pulse

A pulsed CO(2) laser is used to irradiate single fog droplets. The droplets are photographed before, during, and after irradiation by a high speed back-illuminated photomicrograph system. The laser pulse has an energy of about 0.5 J, and the pulse width is about 300 nsec (full width at half-maximum); the power density in the region of the water droplet is about 10 MW/cm(2). In the larger droplets (~20-,microm radius) front surface vaporization is observed; this is accompanied by spallation of the droplet at its back surface. Presumably, a shock wave is produced within the droplet in reaction to the rapid blowoff of the vapor at the front surface, and the spallation is produced when the shock wave is reflected from the back surface of the droplet. Smaller droplets (<12-,microm radius) undergo volume heating rather than front-surface heating. The photographs are taken with a 20-nsec exposure, and the interval between photographs can be varied in steps of 10 nsec, a typical interval being ~1 microsec.