Читать книгу The Rheology Handbook - Thomas Mezger - Страница 56

When setting the upper plate in motion at a constant speed, during a certain start-up time not all flowing layers of the sample are already shifted to the same extent along the neighboring layers. Initially, the resulting shear rate is not constant in the entire shear gap then since at first, only those layers are shifted which are close to the moving upper shear area. It takes a certain time until all the other layers are also set in motion, right down to the fixed bottom plate. Of course, this process will take a considerably longer time when presetting a considerably lower velocity to the upper plate.

The full shear force representing the flow resistance of the whole and homogeneously sheared sample is measured not before the shear rate is reaching a constant value throughout the entire shear gap as illustrated in Figures 2.3 and 2.9 (no. 2). Only in this case, steady-state viscosity η = η( γ ̇ ) will be obtained. Until reaching this time point, the lower flow resistance of the – up to then only partially flowing – liquid will be detected. Up to then, measured are still the clearly lower values of the still time-dependent, transient viscosity η+ = η( γ ̇ , t). Considering the entire shear gap in this period of time, the shear gradient still will be not constant and therefore, the shear process will be still inhomogeneously.