FORMATION OF BILAYER DISKS AND 2-DIMENSIONAL FOAMS ON A COLLAPSING EXPANDING LIQUID-CRYSTAL MONOLAYER

We have visualized the collapse of a monolayer film of the thermotropic liquid-crystal 4'-n-octyl-4-cyanobiphenyl (8CB) at the air-water interface using Brewster angle microscopy. First, the film collapses into small circular domains that represent fluid multilayer structures under the influence of a line tension. These multilayer disks grow in both area and number with further compression. Coalescence of domains occurs at high are a fraction and is coupled with flow of the underlying monolayer. At still higher compression, additional coalescence leads to a homogeneous multilayer film. When the multilayer is allowed to expand, circular holes form. As these holes grow on further expansion, a two-dimensional foam is formed, stabilized by thin lamellae of the collapsed 8CB phase. The foam eventually breaks apart as the film expands back into the monolayer regime. Our results confirm the proposal by Xue et al. [Phys. Rev. Lett. 1992,69,474] that this collapse is a transition between the monolayer and a uniform trilayer composed of an interdigitated bilayer on top of a monolayer.