Dynamics of rims and the onset of spinodal dewetting at liquid/liquid interfaces

Atomic force microscopy was used to investigate dewetting instabilities of liquid films from solid and liquid substrates in three limiting cases. We e xamined dewetting of a poly(styrene-co-acrylonitrile) (SAN) copolymer of vi scosity eta(T)(SAN) from a solid substrate, SiOx, and from a liquid polysty rene (PS) substrate whose viscosity eta(s)(PS) much greater than eta(T) (SA N)I The third case involved the dewetting of PS liquid films from liquid SA N substrates of varying thickness, Hs. In this latter case, the viscosity o f the top PS liquid layer eta(T)(PS) much greater than eta(s)(SAN), the vis cosity of the SAN substrate layer. SAN films dewet SiOx substrates by a spi nodal process when they were of thickness H < 300 Angstrom. For H > 300 Ang strom the SAN films were stable. For the situations in which the substrates were liquid PS films, SAN films of thickness H-T(SAN) < 1000 Angstrom dewe t via a spinodal mechanism whereas SAN films of H-T(SAN) > 1000 Angstrom de wet 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 R-SAN similar to t and the width of the ri ms l(SAN) similar to t(1/2). The height of the rims, q(SAN), increased at a more rapid rate with time than the width of the rims and l(SAN) much great er than q(SAN). In the final situation examined, where the PS films dewet t he liquid SAN substrates (eta(T)(PS) much greater than eta(S)(SAN)) of vary ing thickness, we identified a transition from low-viscosity liquid substra te-like behavior to solid substrate-like behavior when the thickness of the SAN substrate layer became sufficiently thin. Finally, we discuss signific ant differences in the mechanisms by which holes grow when the substrate vi scosity is liquidlike and when it is solidlike.