A converging channel rheometer for the measurement of extensional viscosity

TLDR: In this article, an extensional rheometer for polymer solutions is described, where the test section is a converging channel through which a test fluid is pushed at high Reynolds numbers, and several pressure drops along the channel are measured with flush mounted transducers.

Abstract: The development of an extensional rheometer for polymer solutions is described. The test section is a converging channel through which a test fluid is pushed at high Reynolds numbers, and several pressure drops along the channel are measured with flush-mounted transducers. The high Reynolds numbers ensure a core flow of purely extensional motion and the channel is shaped to produce a constant rate of extension. Analysis of the stress field shows that the pressure drop is equal to the normal stress difference τzz - τrr in the core, plus other terms which are calculated. The calculations are based on an analytical solution for inelastic power-law fluids, and the calculations were verified by finite element computations. Extensional viscosity measurements were made for a solution of a copolymer of PMMA in an organic solvent. The data show that extensional viscosity increases with extensional rate to about the 3/2 power and, at a fixed extensional rate, extensional viscosity increases roughly as the total fluid strain.