A new method for determination of normal‐stress differences in highly viscoelastic substances using a modified Weissenberg rheometer

Abstract

This paper presents a new experimental approach for determining both normal‐stress differences using a modification of the Weissenberg rheometer. The method eliminates both wall slippage and rim fracture that so far have been major obstacles in attaining steady‐state flow conditions with highly consistent fluids in rotational instruments. This is achieved by extending the upper plate with a separately moving exterior ring that is closed on the outside. For determination of both normal‐stress differences with this modification, the wall pressure at the radius of separation is to be measured in addition to the integral force on the inner plate, yielding a set of linear differential equations in N2 with linear terms in N1. Combining two geometrical configurations from the cone‐and‐plate, cone‐plate‐distance, and parallel‐plates systems, a differential equation in N2 only is obtained whose solution can be determined from a quadrature of the measured mechanical quantities. N1 results from the equations after reintroduction of the solution for the second normal‐stress difference.