A High Shear-Rate Optical Rheometer for Complex Fluids

A parallel-plate optical rheometer in a magnetic-disc drive configuration has been designed and constructed of optically transparent materials and operating with a very small gap to obtain rheological and structural measurements at very high shear rates.

The friction force at the disk-slider interface has been measured as a function of sliding speed while the film thickness was monitored in situ using a capacitance technique. The shear rate is calculated from the film thickness and the sliding speed. A thin film can be applied on the disk, which allows very high-shear-rate measurements at low sliding speeds with negligible viscous heating.

Both disk and slider have been made of optically transparent material to allow optical measurements simultaneously with the rheological measurements. In the present mode the apparatus is set up for simultaneous rheometery and birefringence measurements on a thin film of polymer lubricant. Rheology and birefringence measurements were made on a perfluoropolyether lubricant over a range of strain rate from 10³ s^-1 to greater than 10⁶ s^-1 with different film thicknesses.

The rheological results of PFPE Fomblin^â YR under steady-state shear conditions reveal a shear-thinning transition at shear rate about 10⁴s^-1. The rheological results were in agreement with the flow-induced birefringence. The birefringence measurements where first time that results have been carried out at high shear rate.