Laser-induced pattern formation in liquid sulfur

Liquid sulfur is a well-known liquid which exhibits a polymerization transition at T_p=159 °C. Recently, it was found from our experiments that such a transition can be induced below T_p through laser illumination and that an iridescent pattern appears under strong illumination with a pulsed laser of more than 60 mJ/cm² pulse. It is proposed that the visible change in iridescence is due to a macroscopic reconstruction of laser-generated polymers and that a laser-induced phase transition takes place from a freely expanded polymer phase to an ordered polymer phase when increasing the laser illumination. To further examine this possibility, the time variation of the iridescent pattern has been fully investigated using a macro lens, a polarized microscope and an optical microscope. In an analysis of the iridescent pattern, a rapid decrease in the area was observed after an initial slow decrease, suggesting a type of phase transition. Results from the observation of a quenched sulfur sample with a polarized microscope gave evidence that the iridescent region consists of polymers. Through observation of the liquid with a microscope, a striped pattern with micrometer sized spacing was noted in the iridescent pattern. A drastic color change was observed in the pattern from its generation to its disappearance. Sample thickness dependence of the pattern was also observed. These results were well explained by assuming the self-arrangement of laser-generated colloidal polymers.