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Viscosity is the ability of a fluid or solid to resist the change in its shape to force from the adjacent layer. In other words, it is the opposite of fluidity and its SI unit is pascal-second. Viscosity is divided into two types: dynamic viscosity and kinematic viscosity. The viscosity of any substance depends on the temperature and the state of matter. For liquids, viscosity decreases with an increase in temperature, and for gases, viscosity increases with an increase in temperature.
Newton’s Law of Viscosity
Newton’s law of viscosity tells us about the relationship between the shear stress and velocity gradient of fluids. Shear stress refers to the amount of force acting per unit area on a particular fluid parallel to the surface of the fluid. The velocity gradient is defined as the velocity difference between the layers of fluid adjacent to each other.
Newton’s law of viscosity states that the shear stress is directly proportional to the velocity gradient.
The equation of Newton’s law of viscosity is τ = μ du/dy
Where τ = shear stress, μ = viscosity, and du/dy = velocity gradient.
Types of viscosity
Viscosity is divided into two types; dynamic viscosity and kinematic viscosity.
Viscosity is divided into two types; dynamic viscosity and kinematic viscosity.
Dynamic viscosity- is a way to measure a fluid’s resistance to its flow under an external force. In other words, it is the force needed by a fluid to overcome its internal molecular friction so that it can flow. In simple words, it is a fluid’s viscosity that is referred to as dynamic viscosity. The SI unit of dynamic viscosity (μ) is the Pascal-second. Its formula is similar to that of viscosity.
The formula is τ = μ du/dy
Where τ = shear stress, μ = viscosity, and du/dy = velocity gradient.
The dynamic viscosity of a fluid is measured using a rotational viscometer. It rotates a probe in the liquid sample, and the viscosity is determined by measuring the force used to turn the probe.
The dynamic viscosity depends on temperature; as the temperature increases, the dynamic viscosity decreases for liquids. In the case of gases, the dynamic viscosity increases with an increase in temperature.
The significance of dynamic viscosity is to understand the behaviour of any fluid and its way of reacting and moving when it encounters any solid surface.
Kinematic viscosity- is the measure of a fluid’s internal resistance to flow under gravity. It is also considered the ratio of absolute viscosity to the density of the fluid. It is determined by measuring the time required for a fixed volume of fluid to flow across a known distance under the influence of gravity through a capillary action within a calibrated viscometer in a fixed temperature range. In simpler words, it measures any fluid’s internal resistance to the flow across the cross-section area per unit of time.
The formula for Kinematic viscosity is v = μ/ρ
here v = kinematic viscosity, μ = dynamic viscosity, and ρ = density.
The SI unit of kinematic viscosity is m2/s and the CGS unit is stokes. It can also be expressed in centistokes (1 stokec= 100 centistokes).
The physical significance is that it can measure how fast momentum can be transferred between the fluid layers. Its application is to prevent corrosion and lubricating objects.
Difference between Dynamic and Kinematic Viscosity
Conclusion
Viscosity is defined as the ability of a fluid or solid to resist the change in its shape and structure to the stress or force from the adjacent layer. According to Newton’s law of viscosity fluids, shear stress is directly proportional to the velocity gradient, and fluids can be divided into two categories, Newtonian fluids and non-Newtonian fluids, based on their properties. Viscosity is divided into dynamic viscosity and kinematic viscosity. Dynamic viscosity is the force needed by a fluid to overcome its internal molecular friction so that it can flow freely. Kinematic Viscosity is considered as the ratio of dynamic viscosity to the fluid density.
