High Pressure Rheology

An understanding of the rheological properties of lubricant components is vital to the petrochemical industry. In particular, the properties of alkanes in the C20-C40 mass range are of fundamental importance in industrial applications as they are important constituents of lubricant base-stocks. Depending upon the given application a lubricant can be subjected to a wide range of operating temperatures and pressures and so a consideration of not only the viscosity, but also the viscosity-temperature and viscosity-pressure behaviour is vital to determine if a fluid would make a good lubricant component.

Although viscosity is a function of both temperature and pressure, the effect on the viscosity of a small change in pressure from atmospheric is quite small compared with the effect of changes in temperature. For that reason much less time has been devoted to examining the influence of pressure, and experimentally its effect is usually ignored. However, the performance of machine elements such as gears and roller bearings depends on lubricants that routinely are subjected to pressures in the GPa range. Relative to atmospheric conditions, these pressures can induce viscosity increases of several orders of magnitude.

In the literature there is very little experimental data available on the high-pressure rheological properties of alkanes, especially in the mass range of interest. The most comprehensive experimental study of the viscosity of lubricants to high pressures was undertaken at Harvard University (ASME 1953) in the late 40s. Viscosity and density data were reported for 40 lubricants, including pure hydrocarbons, mineral oils and synthetic lubricants, to pressures of 1GPa and temperatures up to 200°C. Hence additional experimentation is needed in order to further investigate the influence of pressure and molecular architecture on viscous behavior. However experimental measuremnets can be very costly and time consuming.

Computer simulation is proving to be an attractive and valuable means with which to “fill in the gaps” in the experimental literature and obtain this important information. Furthermore, the conditions commonly met in practical applications such as automobile engines and machinery, (i.e. giga-pascal pressures and nanoscale gaps between surface asperities), are very difficult to achieve and study experimentally, but pose fewer difficulties in a computer “experiment”.

Our work in this area focuses on the development of simulation techniques that can be used to predict the high pressure behaviour of lubricant components, with the ultimate goal of using simulation to guide lubricant design. Click here to see a simulation of 9-octylheptadecane at giga-pascal pressure undergoing shear. From such simulations we can obtain information on the shear rate dependant viscosity of the molecule at the stated temperature and pressure.

Supported by ACS/PRF

Relevant Publications*

C. McCabe, S. T. Cui, P. T. Cummings, Peter A. Gordon, and Roland B. Saeger, "Examining the Rheology of 9-Octylheptadeacne to Giga-Pascal Pressures," Journal of Chemical Physics, 114, 1887-1891 (2001).

C. McCabe, S. T. Cui, and P. T. Cummings, "Characterizing the Viscosity-Temperature Dependence of Lubricants by Molecular Simulation," Fluid Phase Equilibria, 183, 363-370 (2001).

C. McCabe, D. Bedrov, G. D. Smith, and P. T. Cummings, "Discriminating Between Correlated Experimental Viscosity Data Using Molecular Simulation," Industrial & Engineering Chemistry Research, 40, 473-475 (2001).

S. Bair, C. McCabe, and P. T. Cummings, "Comparison of Non-Equilibrium Molecular Dynamics with Experimental Measurements in the Nonlinear Shear-Thinning Regime", Physical Review Letters, 88, 058302 (2002).

C. McCabe, C. W. Manke and P. T. Cummings, “Predicting the Newtonian Viscosity of Complex Fluids from High Strain Rate Molecular Simulations”, Journal of Chemical Physics, 116, 3339 – 3343 (2002).

S. Bair, C. McCabe, and P. T. Cummings, "Calculation of Viscous EHL Traction for Squalane Using Molecular Simulation and Rheometry”, Tribology Letters, 13, 251-253 (2002).

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