Instability of thin polymer films on coated substrates: rupture, dewetting and drop formation

Abstract

We present a direct comparison of theoretical predictions and experimental results on instabilities and various stages of dewetting of thin (<60 nm) films on coated substrates. Thin (>10 nm) polystyrene films, prepared on silicon wafers with three different nanosized (˜1 nm) coatings, dewet spontaneously above the glass transition temperature by the growth of cylindrical holes with wavy rims. We could clearly distinguish, and theoretically explain, four different stages as dewetting proceeded: (a) rupture of the film; (b) expansion and coalescence of holes to form a polygonal "cellular" pattern; (c) disintegration of polymer ridges forming the polygon into spherical drops due to Rayleigh instability; and (d) fingering instability of hole rims during hole expansion witnessed only on low wettability coatings. The theory gives a clear understanding of roles of substrate (silicon) wettability, coating wettability, and the film thickness in various stages (phenomena). For films much thicker than the coating, the wettability (contact angle) of the coated substrate by polystyrene has no influence on the initial length scale of the instability (number density of holes). This is explained by the dominating influence of long-range Lifshitz-van der Waals (LW) interactions originating from the bulk substrate; the LW and short-ranged polar interactions with the coating determine only the contact angle after appearance of an ultrathin three-phase contact zone (rupture). In stage b also, the final polygon diameters are rather independent of the coating, which is explained by insensitivity of stage a to the coating properties and a competition between the Rayleigh instability (leading to droplets) and drainage from ridges (leading, to coalescence). Hole growth, fingering instability, and the drop diameters, on the contrary, are highly affected by the surface properties (wettability) of coatings. An increase in the film thickness (h) decreases the number density of initial holes (αh⁻⁴), increases the polygon diameter (αh²), increases the drop diameter (ahq,qvaries from 1 to 1.5 depending on the contact angle), and makes the fingering instability stronger. Theory and experiment are in excellent qualitative and quantitative agreement. The theory also suggests several interesting possibilities for the design of future experiments on the dewetting of thin films.