Mechanism of laser ablation for aqueous media irradiated under confined-stress conditions

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Abstract
Pulsed laser ablation of aqueous medium irradiated under conditions of temporal confinement of thermal stress is described. Time‐resolved measurements of laser‐induced transient stress waves with simultaneous imaging of ablation process by laser‐flash photography were performed. Stress transients induced in aqueous solution of K2CrO4 by ablative nanosecond laser pulses at 355 nm were studied by a broad‐band lithium niobate acoustic transducer. Recoil momentum upon material ejection was measured from the temporal profiles of the acoustic transducer signal as a function of incident laser fluence. Cavitation bubbles produced in the irradiated volume during the tensile phase of thermoelastic stress were shown to drive material ejection at temperatures substantially below 100 °C. Experimental data are evident that nanosecond‐pulse laser ablation of aqueous media (when temporal stress‐confinement conditions are satisfied) include the following two main stages of material ejection: (1) ejection of water microdroplets due to expansion and rupture of subsurface cavitation bubbles; (2) ejection of liquid streams with substantial volume upon collapse of initial crater and large cavitation bubbles in the depth of irradiated volume (after coalescence of smaller bubbles).