Unstable Thin Polymer Films: Rupture and Dewetting Processes

Abstract:

An extended study of instabilities of thin polystyrene films (thickness < 100 nm) is presented. Different processes are found if the films are annealed above the glass transition temperature. Initially smooth films become rough and eventually break up by creating cylindrical holes. The average distance between the holes is proportional to h2.05+/-0.08, where h is the thickness of the film. This result is interpreted by a theory describing the amplification of thermal fluctuations of the surface of thin liquid films on solid substrates. The holes grow and thereby dewet the substrate. The growth velocity is found to depend on the contact angle (theta) and to be constant in time. Eventually the holes merge and form polygons comparable to cellular patterns found in many other areas of science. Finally, droplets are formed. For large theta fingering instabilities of the moving rim at the edges of the holes generate droplets. For small theta the ribbons forming the polygons decay via Rayleigh instabilities into droplets of size proportional to the diameter of the ribbons.