Microrheology of a sheared Langmuir monolayer:: Elastic recovery and interdomain slippage

Brewster angle microscopy was used to study the microrheological response of fatty acid Langmuir monolayers subjected to shear flow. Slippage of domains past each other was observed during shear, and elastic recovery of domainshape was monitored following the cessation of shear. Experiments were performed as a function of shear rate, total strain, and temperature in two tilted hexatic monolayer phases, L-2 and Ov. Three regimes were found as a function of temperature. At low temperature (T = 17 degreesC), the degree of slippage and recovery was shear rate independent. At high temperature (T greater than or equal to 25 degreesC), the recovery decreased and the slippage increased systematically with increasing shear rate. At an intermediate temperature (T = 21 degreesC), a distinct transition from low-temperature to high-temperature behavior was observed at a shear rate of 0.35 s(-1). These regimes correlated precisely to previous observations of parabolic or triangular velocity profiles in monolayer channel flow. Taking all temperatures and shear rates into account, there was a one-to-one correspondence between slippage and recovery, suggesting that the two are directly related. These results suggest a fundamental correspondence between macroscopic monolayer theology and domain-level processes.