Hydrodynamic model for laser swelling

Abstract

The evolution of surface layers of a glassy material heated by a laser pulse above the glass transition temperature and cooled by heat diffusion is considered as the flow of a stretchable viscous fluid. The strong dependence of viscosity on temperature and pressure leads to the appearance of a hump with reduced density. This hydrodynamic model for laser swelling is formulated in general form. We present a 1D solution for laser swelling of a thin glassy polymer film on a strongly thermally conductive substrate for laser pulses long enough that the sound confinement effect can be neglected. It is shown that within this condition the evolution of the film thickness over time can be addressed using a second-order ordinary differential equation. It is also shown that in some cases this equation can be reduced to a first-order differential equation resembling the phenomenological equation of the previously published relaxation model of laser swelling. The main features of the dependence of laser swelling on laser fluence, namely the threshold at low fluencies and saturation at high fluencies, have been clarified allowing for the dependence of viscosity on pressure, which was not taken into account in the previous theoretical studies of laser swelling. Typical regimes of the film thickness evolution are considered and compared to existing experimental data.