Brewster angle microscopy studies of Langmuir monolayers under flow
Description
Interfacial rheological properties of Langmuir monolayers of fatty acids are studied using Brewster angle microscopy (BAM). We explore the origin of the non-Newtonian flow behavior within the liquid crystalline (LC) phases. A channel flow experiment allows us to systematically study the monolayer flow throughout the various tilted LC phases as a function of flow rate, temperature and alkyl chain length. The BAM images show a gradual evolution of the velocity profile across the channel from a parabolic to a triangular and sometimes back to a parabolic shape with increasing flow-rate. The critical flow rates for these transitions are dramatically different in the different phases suggesting that the flow response is related to the particular molecular packing in the various mesophases. Additionally, we observe transient behavior of the velocity profile, which indicates the importance of monolayer elasticity. By performing shear 'creep' experiments in a constant shear cell we observe the domain structures under shear deformation and domain relaxation after cessation of shear. The slippage and recovery measurements obtained at all temperatures and shear rates fall on to a universal curve, suggesting the degree of elastic recovery depends directly on the amount of inter-domain slippage during shear. The behavior of slippage and recovery as a function of temperature and shear rate correlates directly to the measurements of parabolic and triangular velocity profiles in channel flow. Our results indicate a fundamental correspondence between domain-level processes and macroscopic rheological measurements