Transient Measurements in Rate Controlled Rheometers: A New Method of Standardization

Abstract

Abstract Rheological measurements in response to transient shear rate exhibit coupling effects between mechanical characteristics of rheometer and unsteady (thixotropic and viscoelastic) properties of fluid. This coupling is observed if the characteristic times of the latter lie in the range of those associated to mechanical properties of the apparatus. Damped oscillations observed in the stress relaxation curves obtained with some standard oils submitted to stress steps are an example of this type of anomalous responses. This work introduces a new method of standardization for a low shear rate rheometer (Contraves LS40) under unsteady flow conditions. This “standardization under transient conditions” is developed in order to obtain a correct interpretation of unsteady measurements performed on elastic and thixotropic fluids, hence to permit modelling of the latter. A second order differential operator is supposed capable to describe the fluid-rheometer coupling. A series of measurements on standard silicone oils, which include electromechanical characteristics of the instrument, allow us to determine the parameters involved in this operator. Then, a preliminary test of this method is performed on a typical system (Red Blood Cell suspension with Dextran) which presents viscoelastic and thixotropic behavior.