Micron-sized droplets irradiated with a pulsed CO(2) laser: measurement of explosion and breakdown thresholds

Measurements of minimum CO(2) laser fluence required to explode or disintegrate 10-60 microm radius droplets of water, ethanol, diesel (hexadecane), CCl(4), bromoform, and ethyl bromide are reported. Threshold fluences range from 0.4 J cm(-2) for 10-microm radius ethanol drops to 20 J cm(-2) for 30microm bromoform drops. Threshold fluences for water droplets are ~3 J cm(-2) independent of drop size. Comparison of the measurements to calculations of laser fluence required for considered absorbing droplets to reach superheat temperature is in good agreement for cases where liquid material properties are known, suggesting that superheating of droplets is the dominant mechanism causing explosion/disintegration. Measured droplet-induced laser breakdown thresholds are considerably higher than explosion thresholds and have less dependence on droplet size and composition. The highest breakdown threshold values are for water drops, which range from 150 to 280 J cm(-2) (0.9-1.7 x 10(9)W cm(-2)) compared with 670 J cm(-2) (4.0 x 10(9) W cm(-2)) for clean air breakdown for the laser pulse length and spot size.