Behaviour of Viscosity

The ability of a fluid (liquid or gas) to change shape or move adjacent parts relative to one another is measured by viscosity. In essence, the resistance to flow is referred to as viscosity. The behaviour of viscosity is a critical factor in determining the forces that must be overcome when fluids are used in lubrication or transported in pipes. In spraying, injection moulding and surface-coating activities, it controls the liquid flow. The tangential or shearing stress that causes flow in many fluids is directly proportional to the rate of shear strain or deformation. In other words, the shear stress divided by the shear strain rate is constant for a given fluid at a given temperature. 

Dimension and formula

Force, time and area are the three dimensions of dynamic viscosity. Accordingly, the unit of viscosity is newton-second per square metre, which is sometimes written in SI units as pascal-second (force × time ÷ area). 

Variance

The viscosity of liquids reduces rapidly when the temperature rises, but gases’ viscosity rises as the temperature rises. As a result, liquids flow more freely when heated and gases flow more slowly. The viscosities of water at 27 °C (81 °F) and 77 °C (171 °F) are 0.85 x 10-3 and 0.36 x 10-3 pascal-seconds, respectively, while the viscosities of air are 1.85 x 10-5 and 2.08 x 10-5 pascal-seconds at the same temperatures.

Types of Viscosity

Kinematic and dynamic viscosity are the two types of viscosity typically reported. The link between shear stress and shear rate in a fluid is known as dynamic viscosity. The link between viscous and inertial forces in a fluid is known as kinematic viscosity.

Common Symbols

γ:Shear rate
μ:Dynamic viscosity
ν:Kinematic viscosity
ρ:The density of the fluid
τ:Shear stress

Dynamic viscosity

The dynamic viscosity of a fluid is defined as the ratio of shear stress to shear rate.

μ = γ/τ

The force required for fluid to resist its internal molecular friction and flow is known as dynamic viscosity. In other words, dynamic viscosity is defined as the tangential force per unit area required to move a fluid in one horizontal plane. In fluid mechanics, dynamic viscosity is also known as absolute viscosity.

Kinematic viscosity

The ratio of viscous force to inertial force on a fluid is measured by kinematic viscosity and its behaviour. This is illustrated in the equation below, which may convert between dynamic and kinematic viscosity if the fluid density is known. Kinematic viscosity is the diffusivity of momentum, similar to mass and heat diffusivity.

ν=ρ/μ

Types of Fluids 

Newtonian Fluids

Newton’s viscosity law states that viscosity should remain constant regardless of stress. In a wide range of temperature and pressure, the most common fluids (liquids and gases) such as water, organic solvents, oils, air, steam, nitrogen or rare gases are Newtonian.

The viscous stresses generated by a Newtonian fluid are linearly proportional to the local strain rate or the rate at which its deformation varies over time, at every location. The most prevalent type of fluid is a Newtonian fluid. Newtonian fluids are the most basic mathematical models of fluids that account for viscosity. While no real liquid or gas fits the description completely, many common liquids and gases – such as water and air – can be considered Newtonian for practical calculations under typical conditions.

Non-Newtonian Fluids

It is the one that does not obey Newton’s viscosity law, which states that viscosity should remain constant regardless of stress. When non-Newtonian fluids are forced, their viscosity can change from liquid to solid. For example, when ketchup is shaken, it gets runnier. Thus, it is a non-Newtonian fluid.

Many molten polymers and salt solutions and many everyday items such as toothpaste, custard, starch suspensions, paint, corn starch, melted butter, blood and shampoo are non-Newtonian fluids. Most non-Newtonian fluids’ viscosity (the progressive deformation caused by shear or tensile stresses) is determined by shear rate history or shear rate. However, various non-Newtonian fluids with shear-independent viscosity show normal stress differences or some other non-Newtonian characteristics. 

Conclusion

A fluid’s viscosity measures its resistance to deformation at a specific rate. It corresponds to the informal sense of “thickness” in liquids; syrup, for example, has a higher viscosity than water. The internal frictional force between adjacent layers of fluid in relative motion is measured by viscosity.