(52a) A High Pressure Viscometer for the Assessment of Viscosity and Density of Lubricants and Polymer Solutions Under Pressure

A unique rotational viscometer has been developed to simultaneously determine viscosity, density, and phase behavior of fluids across a wide range of pressures, temperatures, and shear rates. This high pressure instrument utilizes a magnetically coupled inner rotating shaft for the measurement of viscosity across multiple shear rates. It uses two pistons with accompanying LVDT (Linear Variable Differential Transformer) sensors to apply pressure to the fluid being evaluated by altering its volume and thus generating density data across a range of pressures at a given temperature. The instrument uses two pairs of sapphire windows for direct visual observation of the phase behavior of a fluid being examined. The system is capable of reaching pressures up to 70 MPa at temperatures up to 100^oC and rotational speeds of the inner shaft up to 700 rpm. Using this versatile system, we have explored the effects of pressure, temperature, shear rate, and composition on the viscosity and density of mixtures of PDMS (polydimethylsiloxane) and CO₂. We have conducted similar measures also for poly (alpha olefin) base oils (PAOs) at high pressures. Volumetric data for both the polymer solutions and lubricants have been fitted to the Sanchez-Lacombe equation of state and thermophysical properties, such as isothermal compressibility, isobaric expansivity, and internal pressure, have been calculated. The viscosities of the examined fluids have been related to density by using free volume approaches, such as the Doolittle Equation, and scaling factors.