Negative pressure model for surface foaming of collagen and other biopolymer films by KrF laser ablation

A single KrF laser pulse of energy larger than 0.5 J/cm² is enough to create a microfoam layer on the surface of a collagen film and other related biopolymers. This is a new result that can be of interest for many new applications. The target material is excited in the radiation absorption depth of ∼17 µm and expands into a foam layer whose new surface is ∼5 µm above the original one. The estimated surface transient temperature of ∼83°C at threshold fluence does not account satisfactorily for this fast foaming process but consideration of the bipolar pressure variation ∼±200 bar, i.e. laser induced acoustic wave suggests that a cold homogeneous boiling is induced by the tensile part of the pressure wave in the laser excited volume. The classical nucleation theory predicts a spontaneous dense and homogeneous bubble formation when the pressure is negative in the inviscid liquid. These results constitute new examples of laser induced fast expulsion of liquid due to the hydrodynamic pressure wave which can also be considered as resulting from the surface acceleration/deceleration sequence.