Morphological Phase Transitions in Spontaneous Dewetting of Thin Films on Homogeneous and Heterogeneous Surfaces

Abstract

An overview of recent work on the stability, morphology and intermolecular forces in thin liquid films is presented. Various stages of evolution of the surface instability and pattern formation are investigated for unstable thin (similar to 100 nm) films subjected to a long-range van der Waals attraction and a shorter-range repulsion. Numerical solutions of the nonlinear thin film equation provide 3-d morphologies, which are compared with experiments on thin ( < 300 nm) films of polydimethylsiloxane (PDMS) sandwiched between water and bimodal brushes of PDMS chemically grafted on silicon wafers. Initially, random disturbances are quickly reorganized into a small-amplitude undulating structure consisting of long "hills" and "valleys". Two distinct pathways of morphological evolution and dewetting are found thereafter - depending on the initial film thickness vis-a-vis location of the minimum in the spinodal parameter (intermolecular force per unit volume curve). While dewetting of relatively thick films occurs by the growth of isolated circular holes, thinner films dewet by the formation and growth of droplets. Based on the matching of the simulated and experimental patterns, we propose and apply a novel concept of thin film force microscopy (TFFM), which can determine the unknown intermolecular interactions based on the observations of the film pattern on micrometer scales. Finally, novel dynamical and morphological features are uncovered for films on chemically heterogeneous substrates where dewetting occurs by a chemical potential gradient (microscale wettability gradient) along the solid-fluid interface. Clear and unambiguous criteria are presented for differentiating the spinodal and heterogeneous instabilities. The phenomena of dewetting on homogeneous and heterogeneous surfaces have similarities with spinodal phase separation and heterogeneous nucleation, respectively.