Dilatational yielding of solid Langmuir monolayers.

In a previous work, Muruganathan and Fischer observed laser-induced local collapse of a methyl stearate monolayer. These experiments opened the possibility of studying the collapse mechanism in a highly controlled manner, since the laser intensity can be easily varied and collapse happens in a definite place (the laser focus). In this paper we extended the work presented by Muruganathan et al., describing the local yielding as an alternative pathway toward monolayer collapse competing with the global collapse of the monolayer. We first corroborated that the laser-induced collapse is a thermocapillary effect and afterward determined the threshold laser power necessary for the local pathway to win over the global collapse. We show that the laser threshold is determined more by the gradients in temperature and pressure than by the global pressure and temperature. We propose that the flow of material into the focus of the laser is observed after the yield stress of the monolayer is overcome. The higher the yield stress, the higher the temperature gradient that is necessary for the monolayer to yield. The local pathway opens only when the derivative of surface pressure with temperature is negative such that stress gradients point toward the laser focus and a sink of material is generated. In such a case we are able to give estimates of the dilatational yield pressure of the solid monolayer.