Spreading and retraction dynamics of a dye doped smectic liquid crystal domain at the air-water interface

We have studied the spreading and retraction dynamics of a dye doped smectic multilayer domain in the collapsed state of a Langmuir monolayer at the air-water interface. We find that the domain undergoes shearing when excited with an appropriate wavelength in the epifluorescence setting of the microscope. The shearing leads to displacement of the stack of layers and results in asymmetric spreading of the domain. Eventually, due to line tension, the domain transforms into a circular shape. Here, the domain area was about twice that of the initial area. Under reflection setting of the microscope, with white light, we observe that the domain retracts to a smaller area. During the course of retraction, we find successive generation and evolution of edge dislocation loops leading to thickening of the domain. Our analysis on the variation of the normalized area of the domain with time yields different characteristic time constants for spreading and retraction. The spreading and retraction of the domain can be understood by invoking changes in interfacial tension due to photobleached surface active products and the depletion of photobleached products, respectively.