 Rheometer

A rheometer imposes a controlled stress or deformation and measures the complementary response. In a rotational instrument, a sample fills a gap between surfaces such as concentric cylinders, parallel disks or a cone and plate. The directly controlled or measured quantities include torque, angular displacement and angular velocity; the geometry converts these into shear stress, shear strain and shear rate.^[ref]

For example, a small-angle cone-and-plate rheometer produces approximately uniform shear rate $\dot{\gamma }=\Omega /\alpha$ and shear stress $\tau =3\mathcal{T}/(2\pi R^3)$, with cone angle $\alpha$, radius $R$, rotation rate $\Omega$ and measured torque $\mathcal{T}$. A parallel-plate rheometer instead has a radius-dependent shear rate. The conversion therefore requires the appropriate flow model. Wall slip, edge failure, temperature drift and the instrument's own inertia can mimic a material response, especially for very weak or rapidly varying signals.

A rheometer is an instrument used to measure the flow and deformation behavior of materials, particularly in terms of their viscosity and viscoelastic properties. It is commonly employed in various industries, including food, pharmaceuticals, materials science, and polymers, to understand how a substance behaves under applied forces. Rheometers can measure both the steady flow and oscillatory rheological properties of materials.