# VISCOSITY : Flow Measurement

All we know about the friction force. When the term friction comes in a fluid, then it is called viscosity. Viscosity of a fluid is basically friction between the fluid and a surface. The viscosity of a fluid is the measure of its resistance to gradual deformation by shear stress and tensile stress. For liquids, the viscosity depends mainly on the thickness: for example butter has more viscosity than water.

Viscosity is a physical property of a fluid which resists the fluid to flow upon a surface that is moving at different velocities. In simple words, the viscosity means friction between the molecules of fluid and the particles of the surface.

We will discuss here the viscosity based upon the flow rate measurement.

Types :  there are two types of viscosity. Such as Absolute viscosity and Kinetic viscosity.

# Absolute Viscosity (µ):

Absolute viscosity or the Dynamic viscosity of a fluid is the measurement of the fluid’s internal resistance to flow.

Mathematically,

F V
F ∝ A
F ∝ 1/d
So,
F ∝ VA/d
F = µ VA/d

µ = Property of the liquid and known as absolute viscosity of the liquid.

Mathematically,

µ = (Fd)/(VA)

Unit,
µ = Newton . Meter / Meter / Sec . Meter2
= N. Sec2 / m2 = Pascal. Sec

1 CST (centistoke) = 10-3 Pascal. Sec

NOTE,
Absolute viscosity is highly temperature dependent and is different for liquid and gas.

µLiquid 1/T

And,  µGas T

Where, T is the temperature

µLiquid > µGas

Some Cases,
I)                   Absolute viscosity of Water, µWater = 1 CST
II)                Absolute viscosity of Water, µHoney = 300 CST
III)              Absolute viscosity of Water, µButter = 10,000 CST

Also,
F = (µVA)/d
µ = Fd/VA = (F/A)/(V/d)

µ = Shear stress / Shear rate

# Kinematic Viscosity (v):

The resistance experienced by the liquid measure when the fluid falls down by the virtue of gravity.

Liquid with more density will experience more force and will go down fast.

F = ma = v ρ g

The time taken (t) by the fluid to fall down is inversely proportional to the density (ρ).

t 1/ ρ
t µ
v µ / ρ
v = c µ / ρ , c is the proportional constant

When, c = 1
v = µ / ρ

It means,

Kinematic Viscosity = Absolute Viscosity / density of the fluid