Nonlinearity and slip behavior of n-hexadecane in large amplitude oscillatory shear flow via nonequilibrium molecular dynamic simulation

Molecular dynamic simulation is used to investigate the viscoelastic properties of n-hexadecane under oscillatory shear flow. Rheometric simulations of an ultra-thin molecular film are studied and compared with the results of a bulk simulation. Strain amplitude sweep tests at a fixed frequency show that strain thinning (the dynamic modulus monotonically decreases with increasing strain amplitude) exists at extreme strain for both bulk and thin film systems. Fourier analysis is performed to characterize the nonlinear behavior of the viscoelasticity. No even harmonic was found in our study even though wall slip occurs. Furthermore, we show that a Fourier series with odd harmonics can be used to perfectly describe the simulation results by plotting Lissajous loops. Shear wave propagation appears when the frequency is larger than a certain value. Moreover, the molecular orientation and molecular potential energies, including those for bonding potential, intra- and intermolecular van der Waals interactions are plotted against the strain amplitude to examine the changes in the microscopic structures with respect to the macroscopic thermodynamic states.