# Rheological measurements

Water, honey, toothpaste, bitumen or steel – each and every substance has rheological properties. To determine these you need a rheometer. We explain how rheometers work.

Rheology and its effects accompany us everywhere we go. Whenever a material flows or is being deformed, rheology is involved. Rheology is the science of the flow and deformation behavior of substances. When matter flows or is deformed there are three factors which must be considered: the internal structure, the external forces which act on the substance, and the environmental conditions, for example temperature. ### The rheometer

To measure the rheological properties of a material, Rheometers are used. What does a rheometer measure? The short answer is: the torque and the deflection angle of the measuring bob. And what does that mean? In a viscosity measurement the rheometer presets a certain current that correlates to a defined torque. The sample provides resistance or a reset torque to the setting and the resulting deflection angle is then measured very precisely by the rheometer’s encoder. The speed is calculated from the deflection angle and the time. In a measurement, modern rheometers can set either the torque or the speed – the other parameter is then measured. Actually it is always the current – and therefore the torque – which is provided and the parameter of either speed or torque is set via a quick controller. The speed is then measured. However, the rheometer user is usually more interested in the rheological parameters. Rheological parameters are calculated from the measured values torque, deflection angle and speed using conversion factors (see Table 1). All further parameters, such as viscosity, are not measured but calculated.

### The rheometer setup

The measuring head of a rheometer contains the drive motor and encoder which measure and set the previously mentioned torque, deflection angle and speed. Figure 1: Setup of a modern air-bearing rotational and oscillatory rheometer with the most important components

Basically there are two measuring methods available: Rotational tests and oscillatory tests. With rotational measurements the measuring bob turns in one direction. In an oscillatory test the measuring bob “oscillates” around the axis.

Let’s imagine that the measuring gap contains ketchup. The viscosity of the ketchup should be measured with a shear-rate-controlled rotational test. The user sets the shear rate profile – the different speeds of rotation – and the rheometer determines the required shear stress. The viscosity η is then calculated in the rheometer software according to the viscosity law from the quotients of the shear stress τ and shear rate .