Thermal motions in complex liquids: The 2D Lennard-Jones liquid

Collective thermal motions are studied in an aggregated 2D Lennard-Jones liquid at thermal equilibrium and under shear flow. As a means to resolve different temporal and spatial scales, smoothing over increasing times is used. On times of few to few tens of particles vibration periods, particles in-cage vibrations and highest frequency longitudinal and transverse Hypersound dominate the picture. On times up to many thousands of particle vibration periods, the liquid appears spatially heterogeneous. On these times, non-oscillatory currents involving many particles manifest the hierarchical dynamics of the heterogeneity. These currents result in slow changes in temperature, density, and velocity profiles across the system persisting for surprisingly long times. Heterogeneity fades, and a crossover to non-fluctuational Hydrodynamics is observed only for smoothing times approaching many tens of thousands vibration periods. On these asymptotically-large time-scales, the liquid is spatially homogeneous in the bulk; in thin layers near the boundaries the degree of crystallinity increases and the mobility decreases due to liquid-boundary interactions.