Dynamics of Rims and the Onset of Spinodal Dewetting at Liquid/Liquid Interfaces

Abstract

Atomic force microscopy was used to investigate dewetting instabilities of liquid films from solid and liquid substrates in three limiting cases. We examined dewetting of a poly(styrene-co-acrylonitrile) (SAN) copolymer of viscosity ηT(SAN) from a solid substrate, SiOx, and from a liquid polystyrene (PS) substrate whose viscosity ηS(PS) ≫ ηT (SAN). The third case involved the dewetting of PS liquid films from liquid SAN substrates of varying thickness, HS. In this latter case, the viscosity of the top PS liquid layer ηT(PS) ≫ ηS(SAN), the viscosity of the SAN substrate layer. SAN films dewet SiOx substrates by a spinodal process when they were of thickness H < 300 Å. For H > 300 Å the SAN films were stable. For the situations in which the substrates were liquid PS films, SAN films of thickness HT(SAN) < 1000 Å dewet via a spinodal mechanism whereas SAN films of HT(SAN) > 1000 Å dewet the PS substrates by a mechanism involving the nucleation of holes with outer rims and the subsequent growth of these holes. Two regimes of growth were identified: an early stage regime where the hole initially nucleates and a rim is yet to form and a later stage regime where the rim exists. In this later stage, the hole radii RSAN ∼ t and the width of the rims lSAN ∼ t1/2. The height of the rims, qSAN, increased at a more rapid rate with time than the width of the rims and lSAN ≫ qSAN. In the final situation examined, where the PS films dewet the liquid SAN substrates (ηT(PS) ≫ ηS(SAN)) of varying thickness, we identified a transition from low-viscosity liquid substrate-like behavior to solid substrate-like behavior when the thickness of the SAN substrate layer became sufficiently thin. Finally, we discuss significant differences in the mechanisms by which holes grow when the substrate viscosity is liquidlike and when it is solidlike.