(418f) Spatio-Temporal Pattern Formation in Liquid-Crystalline Langmuir Monolayers

A variety of physical and chemical systems exhibit macroscopic patterns and textures of high regularity. In general, these modulated phases can be classified as having competing interaction combinations of dipolar, interfacial, charge exchange, entropic, or geometric influences. Example systems that exhibit interesting pattern formation include type 1 superconducting films in their intermediate state, block copolymers, magnetic garnet films, Turing patterns in reaction diffusion systems, ferrofluids, and Langmuir monolayers of amphiphilic molecules that reside at an air-liquid interface. Our focus is on Langmuir monolayers primarily composed of the phospholipid DPPC which forms liquid-crystalline domains surrounded by a liquid-like matrix. We also incorporate either palmitic acid or 1-hexadecanol and small amounts of cholesterol. The former are known to co-crystallize with DPPC and increase the crystallinity of the domains. The later tends to absorb to domain perimeters and alter the line tension between the domains and their liquid-like background. The shape of the domains is due in part to the underlying crystal structure, while the size of these domains is believed to be governed by the competition between the domain line tension and the square of the electric dipole density difference. Line tension favors circular domains and the electric dipole density favors elongated stripes. In immiscible liquid-liquid systems near a critical point, the line tension is believed to disappear more rapidly than the dipole density difference and the result is a circle-to stripe transition. However, this type of morphological transition has not been reported for crystalline domains. We find that crystalline domains composed of DPPC, palmitic acid or 1-hexadecanol, and cholesterol can undergo a spatiotemporal circle-to-stripe transition at fixed surface tension or area per molecule. It is proposed that this time-dependent morphological transition is induced by epitaxial crystal growth of DPPC in the liquid-like phase onto DPPC-hexadecanol co-crystals. Cholesterol is believed to lower the domain line tension sufficiently so that the dipole density can provide the driving force for domain deformation from circles to stripes.