Laser-induced photothermoacoustic pressure-wave pulses in a polystyrene well and water system used for photomechanical drug delivery

A linear time-domain thermoelastic (photothermoacoustic) theory of a composite solid–liquid geometry has been developed. The theory includes multiple interreflections at all interfaces, acoustic diffraction and viscosity effects, and natural mixed, rigid, and free boundary conditions at the solid surface where laser-pulse incidence occurs (air–polystyrene interface). The theory was applied to experimental pressure-wave pulses from a Nd:YAG laser in a polystyrene well target and water system used for photomechanical drug delivery studies. Good fits of the linear theory to tripolar experimental pressure waveforms were possible at laser-pulse irradiances $\lesssim 100\mathrm{MW}∕{\mathrm{cm}}^2$, especially at distances $⩽5\mathrm{mm}$ from the solid–fluid interface. It was further determined from the combined theoretical and experimental approach that the onset of significant hydrodynamic nonlinearity in the water appears for laser-pulse irradiances in the $165\text{-}\mathrm{MW}∕{\mathrm{cm}}^2$ range, especially at axial distances $z\sim 8\mathrm{mm}$, as expected theoretically from the laser-ablation-induced nonlinearity of stress-wave propagation in the solid–water system.