WHAT IS VISCOSITY?

Viscosity is a property of fluid, which offers resistance to the movement of one layer of fluid over another adjacent layer of the fluid. When two layers of a fluid, a distance 'dy' apart, move one over the other at different velocities, say u and u+du as shown in fig.1, the viscosity together with relative velocity causes a shear stress acting between the fluid layers.

fig. 1.Velocity variation near a solid boundary

The top layer causes a shear stress on the adjacent lower layer causes the shear stress on the adjacent top layer. This shear stress is proportional to the rate of change of velocity with respect to y. It is denoted by symbol τ (Tau).

Mathematically,

τ∝ du/dy
or τ= μ du/dy

where μ(called mu) is the constant of proportionality and is known as the co-efficient of dynamic viscosity or only viscosity. du/dy represents the rate of shear strain or velocity gradient. Units of viscosity- SI unit of viscosity is Ns/m2 or Pa s.

Variation in viscosity with change in temperature-

Temperature affects the viscosity. The viscosity of liquids decreases with the increase of temperature while the viscosity of gases increase with the increase of temperature. This is due to reason that the viscous forces in a fluid are due to cohesive forces and molecular momentum transfer. In liquids, the cohesive forces predominates the molecular momentum transfer, due to closely packed molecules and with the increase in temperature, the cohesive forces decrease with the result of decreasing viscosity. But in case of gases the cohesive forces are small and molecular momentum transfer predominates. With the increase in temperature, molecular momentum transfer increases and hence viscosity increases.

Telegram channel- Join Now