Comparison of Steady-State Shear Viscosity and Complex Shear Modulus in Langmuir Monolayers

Abstract

Langmuir monolayers of simple fatty acids exhibit a wide range of nonlinear flow behavior. One issue for such materials is whether or not there exists a simple relationship between the steady-state shear viscosity, η , and the complex shear modulus, ) ( * ω G . In particular, one might expect to find a relationship analogous to the Cox-Merz rule that exists for viscoelastic polymers relating η as a function of shear rate to ) ( * ω G as a function of frequency. We investigate η and ) ( * ω G for two different materials and a number of different phases. As expected, due to the strongly nonlinear features of monolayers, the measured values of η and ) ( * ω G differ by orders of magnitude. Furthermore, there is no simple relationship between η as a function of shear rate to ) ( * ω G as a function of frequency.