Viscoelasticity of Maxwell's model and non-Newtonian viscosity revisited

The viscosity of fluids and melts is an important characteristic to steer processes and reactions of materials, to use lubricants and to fabricate diverse products of glass. To steer such fabrication processes reliably and free of failures one necessarily needs to understand and use the true data of the viscosity. The forces applied to measure the viscosity act also to accelerate parts of the samples and particularly to deform them elastically. To evaluate the viscosity one must necessarily consider such side reactions'. Cylindrical samples under longitudinal deformation are demonstrated to deform rather elastically than by viscous flow upon application of the load, e.g. Neglecting this effect the'viscosity' apparently decreasing with increasing load may be misinterpreted as non-Newtonian viscosity. In fact, it represents the change from viscous to elastic behaviour with decreasing time interval of the application of force. Furthermore, production of entropy during deformation for measurement has to be taken into account adequately. The sample warms up during heavy deformation. If the viscosity depends strongly on the temperature, one must take into account that temperature and viscosity change with the intensity of the load. Then the so called'shear thinning' is rather'sheer heating'. This is demonstrated quantitatively with data of the viscosity as a function of the load, which have recently been published using capillary rheometers.