Shear rate

Shear rate measures how quickly neighboring fluid layers move relative to one another, usually as the velocity gradient perpendicular to their motion. Its SI unit is reciprocal seconds (s⁻¹).

Velocity gradient: The spatial rate of change of a velocity component in a specified direction. Shear rate is the velocity gradient across layers moving at different speeds.
Shear deformation: A change in shape in which parallel layers or planes slide relative to one another. Shear rate describes how rapidly this deformation develops over time.
Rheometry: The experimental measurement of how materials deform and flow under applied forces. Rheometry often measures stress across a range of imposed shear rates.
Vorticity: A vector measure of the local rotation of a fluid element. Shear and rotation can coexist, but vorticity measures rotation rather than layer-speed change alone.
Apparent viscosity: The ratio of measured shear stress to shear rate for a specified flow condition. For non-Newtonian fluids, its reported value depends on the selected shear rate.
Shear stress: Tangential force per unit area acting across a surface within a material. Fluid viscosity relates shear stress to shear rate in simple flow.
Strain rate: The rate of change of strain, describing how quickly a material deforms. Shear rate is the shear component of a strain-rate description.
Rotational rheometer: An instrument that measures material response while one surface rotates relative to another. Its geometry and rotation speed set the sample’s shear-rate conditions.
Velocity field: A description assigning a velocity vector to each point in a flowing material. Shear rate is derived from spatial changes in the velocity field.
Extensional rate: The rate at which material elements stretch or contract along a flow direction. It characterizes stretching deformation, unlike the relative sliding measured by shear rate.

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