Collapse and yield pressure of solid Langmuir monolayers

S. ALIASKARISOHI; TH. M. FISCHER ; N WILKE

JOURNAL OF PHYSICAL CHEMISTRY B - (Print)

AMER CHEMICAL SOC

Año: 2011 vol. 115 p. 11631 - 11637

1520-6106

In a previous work, Muruganathan and Fischer observed laserinducedlocal collapse of a methyl stearate monolayer. These experimentsopened the possibility of studying the collapse mechanism in a highlycontrolled manner, since the laser intensity can be easily varied and collapsehappens in a definite place (the laser focus). In this paper we extended thework presented by Muruganathan et al., describing the local yielding as analternative pathway toward monolayer collapse competing with the globalcollapse of the monolayer. We first corroborated that the laser-inducedcollapse is a thermocapillary effect and afterward determined the thresholdlaser power necessary for the local pathway to win over the global collapse.We show that the laser threshold is determined more by the gradients intemperature and pressure than by the global pressure and temperature. We propose that the flow of material into the focus of thelaser is observed after the yield stress of the monolayer is overcome. The higher the yield stress, the higher the temperature gradientthat is necessary for the monolayer to yield. The local pathway opens only when the derivative of surface pressure with temperature isnegative such that stress gradients point toward the laser focus and a sink of material is generated. In such a case we are able to giveestimates of the dilatational yield pressure of the solid monolayer.