Lu Zou

Gasous Hole Closing in a Polymer Langmuir Monolayer

The hole-closing phenomenon is studied in a polymer Langmuir film with coexisting gaseous and liquid phases both as a test of hydrodynamic theories of a two-dimensional fluid embedded in a three-dimensional one and as a means to accurately determine line tension, an important parameter determining size, shape, and dynamics within these and other membrane model systems. The hole-closing curve consists of both a universal linear regime and a history-dependent nonlinear one. Improved experimental technique allows us to explore the origin of the nonlinear regime. The linear regime confirms previous theoretical work and yields a value lambda = (0.69 +/- 0.02) pN for the line tension of the boundary between the gaseous and liquid phases. The observed hole closing also demonstrates that the two-dimensional polymer gas must be taken as having a small, probably negligible elasticity, so that line-tension measurements assuming that both phases are incompressible should be re-evaluated.

Hole dynamics in polymer Langmuir films

This article develops a model for the closing of a gaseous hole in a liquid domain within a two-dimensional fluid layer coupled to a Stokesian subfluid substrate, and compares this model to experiments following hole dynamics in a polymer Langmuir monolayer. Closure of such a hole in a fluid layer is driven by the line tension at the hole boundary and the difference in surface pressure within the hole and far outside it. The observed rate of hole closing is close to that predicted by our model using estimates of the line tension obtained by other means, assuming that the surface pressure in the gas is negligible. This result both supports the model and suggests an independent means of determining the line tension. Unlike most previous hydrodynamics models of Langmuir films, the closing of a hole necessarily involves vertical motion of the underlying incompressible fluid. Fluid is dragged along with the liquid monolayer towards the center of the hole, and must plunge away from the surface. An explicit expr...