Role of Intermolecular Interactions in Pattern Formation in Unstable Thin Films

Abstract

A mini-review of the recent theoretical and experimental work of the present authors on the spontaneous evolution of morphological patterns in thin (<100 nm) unstable liquid films on homogeneous solid substrates is presented. Theoretically, the film stability, dynamics, morphology and the sequence of dewetting are resolved based on numerical solutions of the nonlinear 3-D equations of motion, in which the excess short and long range intermolecular interactions ar incorporated as the body forces. experiments are reported for a novel system composed of thin polymer (PDMS) films sandwiched between water and a bi-modal PDMS brush chemically grafted on silicon wafers. both the simulations and experiments show two distinct pathways for the evolution of instability leading to dewetting. Initially, a small amplitude bicontinuous structure emerges, which either grows and fragments into a collection of microdroplets (for relatively thinner films), or leads directly to isolated circular holes (for thicker films) which dewet the surface. The characteristics of a pattern, and its pathway of evolution are found to depend crucially on the form of the intermolecular potential in ana extended neighbourhood of the initial thickness. The linear 2-D nonlinear analysis use hitherto fail completely in predicting morphological patterns, but can predict their length scales rather well. Based on the matching of the simulated and experimental patterns, a novel Thin Film Force Microscopy (TFFM) concept is proposed and applied, which can determine the unknown intermolecular interactions of the film pattern on micrometer scales.