You must have seen honey. It is also a liquid like water. How can you differentiate honey from water? Yes. It is a brown color liquid which is more viscous than water. That is the reason; it cannot flow like water. Now compare some other liquids with water like syrups, glycerin etc. They are defiantly different in their chemical properties but at the same time they are more viscous than water.
How can we decide that these liquids are more viscous than water? When you let them flow, water will flow easily whereas other liquids cannot. So we can say that viscosity expresses the tendency of liquid to flow. A more viscous liquid cannot flow whereas a less viscous like water will flow. The properties of liquids are due to intermolecular force of attraction between molecules. The intermolecular force of attraction between liquid molecules lies between solids and gases. Because of these intermolecular forces, they acquire the shape and volume of container. The particles of liquid state are not free to move like gaseous state but they do not have strong intermolecular forces like solids. The force of attraction in liquid molecules determines many properties of liquids.
Surface tension is another unique property of liquid which is due to unequal distribution of attraction force between molecules. The molecules in the bulk are attracted equally from all the sides whereas molecules at the surface can feel attraction towards bulk. This unequal distribution of force induces a tension at the surface that called as surface tension.
Viscosity can be defined as the fundamental property of all liquids which represents the internal resistance to flow in them. So it can be represented as the measurement of resistance of flow in liquids. It usually terms as the force which is function of temperature and pressure. The viscosity of liquids changes with temperature and pressure just like gases. It can be expressed as absolute viscosity and kinematic viscosity. Absolute viscosity is also called as dynamic viscosity which represents the force required per unit area to slide two layers with respect to each other. Absolute viscosity can be expressed as
$\eta$ = $\sigma \times v$
$\eta$ = viscosity coefficient
$\sigma$ = shear stress
Unlike absolute viscosity, kinematic viscosity depends on the density.
v = $\eta$ / $\rho$
$\eta$ = Absolute viscosity
ν = Kinematic viscosity
$\rho$ = Density of liquid
Poise and centipoises are most convenient units of viscosity.
Poise (P), Stokes (St) and Saybolt Universal Seconds (SSU) are some common units of viscosity. Absolute viscosity can be bets expressed as Centipoise (cP) which is named after French physician Jean Louis Poiseuille who purposed the Poiseuille experiment to explain the viscosity of liquids.
Unit of viscosity in SI system is N.s/m2 or Pa.s or kg/m .s for absolute viscosity. Here N stands for Newton and Pa is for Pascal.
1 Pa.s = 1 N.s /m2 = 1 kg / m.s
In CGS system the unit of absolute viscosity is g/cm.s or dyne.s / cm2 or Poise (P).
1 g/cm.s = 1 dyne.s / cm2 = 1 P = 1 / 10 Pa.s
In BSU (British system of unit) absolute viscosity is measured in lb/ft.s or lb.s/ft2.
The SI unit of kinematic viscosity is Stokes (St) or SSU ( Saybolt Second Universal) unit. 1 St = 10-4 m2/s. Since many of the liquids have very small value of viscosity, so in place of Stokes, we can use Centistokes (cSt),
1 St = 100 cSt
1 cSt = 106 m2/s
For example the kinematic viscosity of water is 1.0 cSt at 20 °C. So we can say that Poise and centipoises are units for absolute viscosity whereas Stokes and centistokes are units of kinematic viscosity. They can be converted into each other with the help of density of fluid.
1 Pa·s = 1 kg·m-1·s-1 = 10 P
1 cP = 10-2P = 10-3 Pa·s
Centistokes (cSt) is one hundreth of a stokes. It is symbolized as cSt. For example; the kinematic viscosity of water is 1.0038 centistokes. The relation between Centipoise and Centistokes is given below;
CentiPoises(cp) = CentiStokes (cSt) x SG (Specific Gravity)
The units of absolute and dynamic viscosity can be summarized as given below.
| Viscosity of fluids
|| Different units
| Absolute Viscosity
| Kinematic Viscosity
In a fluid, the absolute viscosity is the relation between the shear stress and the shear rate whereas kinematic viscosity is the relation between viscous and inertial forces of the fluid. Fluids can be classified as Newtonian fluids and non-Newtonian Fluids. Mostly fluids are Newtonian fluids which have constant viscosity is constant with shear stress and shear rate. For such fluids, the viscosity depends on pressure and temperature. Viscosity of Non-Newtonian fluids varies with time. On the basis, they can be classified as thisotropic fluids and rheopectic fluid. The variations of viscosity with time for these fluids are shown below.