rheometer

A rheometer is an instrument used to measure the rheological properties of materials, i.e., how they deform and flow under applied forces. It characterizes viscosity, elasticity, and time-dependent behavior by applying controlled deformation or flow and recording the material’s response. Rheometers can operate under steady shear, oscillatory motion, or extensional flow, and they often provide temperature control and, in some cases, humidity control.

In rotational rheometers, samples are placed between two geometries such as parallel plates, cone-and-plate, or concentric cylinders. The device can impose a fixed shear rate and measure the resulting shear stress, or apply a fixed torque and measure angular displacement, allowing calculation of shear stress and shear rate and thus viscosity. For non-Newtonian fluids, the relationship between stress and rate is nonlinear and may be described by models such as the power-law or Herschel-Bulkley. Oscillatory tests apply a sinusoidal strain or stress to obtain viscoelastic moduli, including the storage modulus G’ and the loss modulus G”, as functions of frequency, which reveal the material’s solid-like and liquid-like behavior. Capillary rheometers measure melt flow properties by forcing fluid through a capillary under controlled pressure or piston speed and compute apparent viscosity at high shear rates. Additional tests include time-dependent and temperature-dependent measurements to assess thixotropy and thermal sensitivity.

Applications span polymer processing, foods, cosmetics, pharmaceuticals, coatings, and other soft matter systems. Rheometry provides flow curves, viscosity spectra, and viscoelastic spectra that inform processing design, quality control, and fundamental research.

Calibration requires appropriate geometry constants, gap settings, and temperature control, typically using standard reference materials.