Existe uma estreita relação entre viscosidade e taxa de cisalhamento, which is the rate at which a liquid deforms under shear force.

Shear rate is a physical quantity that describes the deformation rate of a liquid. It represents the deformation velocity per unit time. It is the ratio between the Shear velocity and the geometric constraint on the liquid. The shear rate is usually expressed as the ratio of the displacement difference per unit time (velocidade de cisalhamento) à distância entre duas camadas paralelas (distância de cisalhamento). The unit is usually the reciprocal of the second (e^(-1)).

Viscosity is the internal viscous resistance of a liquid, which measures the degree of stickiness or fluidity of a liquid. The greater the viscosity, the stickier the liquid and the more difficult it is to flow. Viscosity is usually expressed as the ratio of force per unit area to the difference in velocity per unit time (taxa de cisalhamento). The unit is usually PASCAL seconds (Pa·s) or millipascal seconds (mPa·s).

Viscosidade versus taxa de cisalhamento

The relationship between viscosity and shear rate can be described by Newton’s laws of fluid. A Newtonian fluid is an idealized fluid with a constant viscosity that is independent of the shear rate. According to Newtonian fluid laws, a viscosidade é igual à razão entre tensão e taxa de cisalhamento.

Em fluidos newtonianos, changes in the shear rate do not affect the magnitude of the viscosity, se a taxa de cisalhamento é pequena ou grande. The viscosity of a Newton fluid remains the same, no matter how fast or how it is cut. No entanto, para fluidos não newtonianos, such as shear-thinning and shear-thickening fluids, a viscosidade mudará conforme a taxa de cisalhamento muda. Isto significa que a viscosidade de um fluido não newtoniano é uma função da taxa de cisalhamento, which can increase or decrease as the shear rate increases. A relação entre viscosidade e taxa de cisalhamento de fluidos não newtonianos pode ser descrita e prevista por testes reológicos e modelos reológicos apropriados.