Pulsed laser ablation of aqueous medium under confined stress conditions was described based on time-resolved measurements of laser-induced acoustic waves with simutaneous imaging of ablation process by laser-flash photography. Stress transients induced in aqueous solution of K 2 Cr0 4 by ablative nanosecond laser pulses at 355 nm were studied by a broad- band lithium niobate acoustic transducer. Initial thermoelastic stress and recoil momentum transfer upon material ejection were measured from the temporal profile of the acoustic transducer signal. The important role of tensil phase of thermoelastic stress that produces an efficient cavitation in irradiated volume and drives ablation at temperatures substantially below 100° was revealed. Ablation process in aquesous media upon short laser pulse irradiation is proposed to include three stages of mass ejection. Based on experimental data, the following ablative forces are suggested: (1) expansion and explosion of cavitation bubbles, (2) collapse of single separate bubbles producing shock- wave-induced spallarion, and (3) collapse of cavitation bubbles after fusion into large bubbles producing liquid jets of larger volume. When the temperature of the irradiated volume exceeds 100° cavitaiton can proceed at normal pressure (1 bar) and be described by previously reported model of thermal explosion (intense boiling).
A. A. Oraevsky, S. L. Jacques, F. K. Tittel, "Pulsed laser ablation of aqueous media studied by time-resolved detection of stress transients and laser-flash photography," Workshop on Dynamic Response of Materials to Pulsed Heating, , 65-77 (1993).