Rheology is the study of flow and deformation of materials under applied forces which is routinely measured using a rheometer. The measurement of rheological properties is applicable to all materials – from fluids such as dilute solutions of polymers and surfactants through to concentrated protein formulations, to semi-solids such as pastes and creams, to molten or solid polymers as well as asphalt. Rheological properties can be measured from bulk sample deformation using a mechanical rheometer, or on a micro-scale by using a microcapillary viscometer or an optical technique such as Microrheology.
Many commonly-used materials and formulations exhibit complex rheological properties, whose viscosity and viscoelasticity can vary depending upon the external conditions applied, such as stress, strain, timescale and temperature. Internal sample variations such as protein concentration and stability, and formulation type for biopharmaceuticals, are also key factors that determine rheological properties.
Rheological properties impact at all stages of material use across multiple industries – from formulation development and stability to processing and product performance. The type of rheometer required for measuring these properties is often dependent on the relevant shear rates and timescales as well as sample size and viscosity. Examples of rheological measurements include: