We demonstrate that planar stagnation flow can be achieved using molten polystyrene in a die with walls lubricated by silicone oil. The boundary conditions and flow in the lubrication layer are discussed. Wall pressures drop by nearly an order of magnitude under lubricated conditions. Pressure values were below the accuracy of our transducers. However, both normal stress differences were measured by birefringence and at low extension rate were found in good agreement with the values expected for a Newtonian fluid. Even unlubricated flow gave similar stress differences at the same flow rates, including that the shear is confined to a thin layer near the die wall. Tracer pictures confirm this. Extrudate well was also measured and found to be in a reasonable agreement with total recoverable strain predicted from shear measurements.
Bibliographical noteFunding Information:
This work was supported by grants from the Mobil Oil corporation and the National Science Foundation, CME-79-07045. Silicone oil was donated by the dow Corning Corporation. We are grateful to Professor Janeschitz-Kriegl for suggestingb irefringence measurementst hrough the exit slits of the die.