Multiple superheating thresholds of micrometer-sized droplets irradiated by pulsed CO_2 lasers

Two distinct superheating fluence thresholds have been measured for micrometer-sized droplets of liquids irradiated by pulsed CO(2) lasers. The lower, deformation, threshold results in minimal droplet mass loss, whereas the higher, disintegration, threshold (which is well defined only if observed several tens of microseconds after the heating laser pulse) leads to droplet fragmentation into many microparticles and vapor. Deformation thresholds are nearly coincident for either long (10-micros) or short (0.4-micros) laser pulses. Disintegration thresholds are higher for long-pulse irradiation and increase with decreasing absorption. A qualitative explanation is given for these phenomena based on the effects of surface tension, thermal conduction, and thermally induced optical inhomogeneities.