Experimental signatures of a nonequilibrium phase transition near the crossover point of a Langmuir monolayer

We investigate the response of the two-dimensional (2D) continuous non-particulate film of surfactant sorbitan tristearate confined at the air-water interface under oscillatory shear deformation. The time dependence of various rheological parameters show critical-like behavior at a value of strain amplitude close to the crossover point of elastic () and viscous () shear moduli. Imposing oscillatory shear of different strain amplitudes () above and below the crossover strain amplitude () over a large number of cycles, we quantify the temporal dependence of interfacial viscous modulus, phase angle () as well as higher harmonic components of stress. The number of shear cycles () required for these quantities to reach the steady state value diverges near . The steady state values of the third harmonic () show order parameter like behavior indicating the importance of higher order harmonics near the nonequilibrium transition. We further show that the energy dissipation per cycle per unit volume has a marked change near , consistent with continuum level nonequilibrium shear-transformation-zone model of amorphous viscoplasticity.