Dissipative Structure Formation in Lipid/Lipopolymer Monolayers

We study the formation of dissipative microstructures in monomolecular films of surfactant mixtures, which occur near the three-phase contact line during Langmuir-Blodgett transfer onto a solid substrate. Continuous stripes parallel to the transfer direction are generated over several centimeters, indicating the phase separation of phospholipids and lipids with polymer head groups (lipopolymers). The systematic variation of transfer conditions revealed that transfer speed and subphase viscosity determine the stripe-to-stripe distance from several micrometers to submicrometers. To account for the physical mechanism of such pattern formation, we characterize the local film thickness and the membrane composition in the vicinity of the three-phase contact line using imaging ellipsometry and fluorescence microscopy. At relatively slow rates of substrate lifting, the power law exponent that we found between the interstripe distance and the transfer speed suggests that the stripe formation is due to spinodal decomposition, which can be accounted under the framework of the Cahn-Hilliard equation, whereas at relatively high rates, the distance is found to be proportional to the substrate speed, suggesting a dominant effect of the shear force on the stripe formation.