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

Brewster angle microscopy was used to study the microrheological response o f fatty acid Langmuir monolayers subjected to shear flow. Slippage of domai ns past each other was observed during shear, and elastic recovery of domai nshape was monitored following the cessation of shear. Experiments were per formed as a function of shear rate, total strain, and temperature in two ti lted hexatic monolayer phases, L-2 and Ov. Three regimes were found as a fu nction of temperature. At low temperature (T = 17 degreesC), the degree of slippage and recovery was shear rate independent. At high temperature (T gr eater than or equal to 25 degreesC), the recovery decreased and the slippag e increased systematically with increasing shear rate. At an intermediate t emperature (T = 21 degreesC), a distinct transition from low-temperature to high-temperature behavior was observed at a shear rate of 0.35 s(-1). Thes e regimes correlated precisely to previous observations of parabolic or tri angular velocity profiles in monolayer channel flow. Taking all temperature s and shear rates into account, there was a one-to-one correspondence betwe en slippage and recovery, suggesting that the two are directly related. The se results suggest a fundamental correspondence between macroscopic monolay er theology and domain-level processes.