(159c) Thermodynamics, Dynamics, and Rheology of Fuel Surrogates: Application of Time-Temperature Superposition Principle in Molecular Dynamics Simulations

Petroleum-based and bio-based fuels are made up of hundreds of different components, and predicting the physical properties, which are related to the combustion of these mixtures, is an extremely challenging task. For this reason, simplified mixtures known as surrogates are often used to gain insight into the effect of fuel composition on thermophysical properties. In this study, we show all-atom molecular dynamics (MD) simulation can be used as an effective tool to determine the thermodynamics and rheological properties of fuel surrogates, which are modeled as a mixture of n-hexadecane and methyl laurate. The volumetric properties of the studied systems, i.e., density and coefficient of thermal expansion, show excellent agreement with experiments. The temperature dependence of translational and rotational diffusion of the molecules follows an Arrhenius-type behavior, which is consistent with the zero shear viscosity results obtained from nonequilibrium simulations. At extremely high shear rates, the molecules align in the flow direction that gives rise to the shear-thinning behavior for these fuel surrogates. Finally, we will demonstrate how the time-temperature superposition (TTS) principle can be successfully applied to collapse the shear viscosity and translational/rotational motion of molecules in all systems. The application of TTS on the dynamics data obtained in equilibrium, which is accessible in the all-atom MD simulations, allows one to reduce the timescale gap between experiments and simulations, and predict the rheological response of complex fluids, especially mixtures of short alkanes and fatty acid esters, which are of interest in fuel surrogates.