Dependence of the breakdown of water drops on the parameters of a CO2 laser pulse

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.

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.

Time-resolved shadowgraphs of large individual water and ethanol droplets vaporized by a pulsed CO(2) laser

Carbon dioxide laser-induced explosive vaporization of water and ethanol droplets at high laser fluence has been observed with time-resolved shadowgraphs. The asymmetry seen in the droplet vaporization can be qualitatively explained by comparison to the internal-field intensity distribution. A central green spot observed in the shadowgraph is attributed to the near-field distribution just outside the shadow face of the droplet when the droplet is illuminated by a visible laser. This spot can be used to probe the shape deformation and optical inhomogeneity of the droplet. The energy dependence of the explosive vaporization of water was also studied. Increasing the CO(2) laser fluence increases the rate of explosive vaporization.

Water droplets irradiated by a pulsed CO2 laser: comparison of computed temperature contours with explosive vaporization patterns

The theoretical basis of a method which allows the computation of time-dependent temperature distributions within aerosol droplets irradiated by a laser pulse is given. The laser pulse irradiance can be time-dependent, and the temperature dependence of the droplet's conductivity, density, and specific heat is included. The algorithm is used to compute isothermal temperature contours at 305 degrees C within metastable water droplets, and these contours are compared to patterns of explosive vaporization shown in photographs of disintegrating water droplets. This comparison indicates that disintegration patterns can be explained by resistance heating if the temperature dependences of material properties are considered.