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# Centistokes

Solid, liquid and gases are three physical states of matter. They have distinct properties due to different arrangement of constituent particles. Solids have definite shape and volume. This is due to strong intermolecular force of attraction between constituent particles in solid state. The strong force of attraction contributes to least intermolecular distance between particles. Such arrangement of particles makes the solid state less compressible and denser compare to other physical states.

Gaseous state has weakest intermolecular force of attraction so the particles are quite far from each other that increases the intermolecular distance between gaseous particles. It makes the gases more compressible and diffusible. Liquid state can be considered as the intermediate state of matter in which the particles are close to each other but the intermolecular force of attraction is not strong enough like solid state.

That is the reason; liquid state exhibits different properties than solid and gaseous state. For example; surface tension, viscosity are some unique properties of liquid stare.

Liquid or fluids are one of the most common physical states which we can observe in our surroundings. Water is one of the most common examples of liquid. Other examples of liquids are honey, oil, alcohol etc. Liquids show unique property of flow which can be explained on the basis of laminar flow of liquids.

The laminar flow of liquids states that liquids flow in layers which have some forces (adhesive and cohesive) between them. These forces are responsible for capillary action of liquids. Surface tension is also due to unequal distribution of intermolecular forces between surface molecules. The molecules in the bulk are equally attracted with each other whereas surface molecules are attracted only towards bulk that creates a tension at surface called ass surface tension. The spherical shape of water droplet is also due to this surface tension.

## Viscosity of Water in Centistokes

Viscosity can be defined as the property of liquid that exhibits the tendency of flow of liquids. It expresses the magnitude of internal friction in liquid. It is measured by the force per unit area resisting uniform flow of layers of liquid.

According to laminar flow, the bottom layer is held fixed, while the top layer is moved to the right. He top layer moves with velocity v and F indicates the force that requires continue the flow of layer which is proportional to the area A. If the distance between two layers is L,

F =  $\frac{\eta .VA}{I}$

So $\eta$ = $\frac{F.I}{V.A}$

Here Î· represents the viscosity with SI unit Nâ‹…m/[(m/s)m2] = (N/m2) s or Paâ‹…s.  The CGS unit of viscosity is  stokes (S or St) which is named after George Gabriel Stokes. It can also express as centistokes (cS or cSt). For example; the viscosity of water at 25 Â°C in different units are given below;

• 8.90 Ã— 10âˆ’4 Pa. s
• 8.90 Ã— 10âˆ’3 dyn. s/cm2
• 0.890 cP
• 0.0091 poise
• 0.91 cSt
• 1 centipoise

## Centistokes Definition

Viscosity can be classified as dynamic or absolute viscosity and kinematic viscosity. Here absolute viscosity is measurement of internal resistance. It can be expressed as;
$\eta$ = T$\frac{dy}{dc}$

Here
τ =  shearing stress in fluid (N/m2)
dc =  velocity (m/s)
dy = distance between layers (m)

## Centistokes to Centipoises

The relations between different units of viscosity are listed below.

 centipoises (cP) poise (P) centistokes (cSt) stokes (S) 12) 0.01 1 0.01 2 0.02 2 0.02 4 0.04 4 0.04 7 0.07 7 0.07 10 0.1 10 0.1 15 0.15 15 0.15 20 0.2 20 0.2 25 0.24 25 0.24 30 0.3 30 0.3 40 0.4 40 0.4 50 0.5 50 0.5 60 0.6 60 0.6 70 0.7 70 0.7 80 0.8 80 0.8 90 0.9 90 0.9 100 1 100 1 120 1.2 120 1.2 140 1.4 140 1.4 160 1.6 160 1.6 180 1.8 180 1.8 200 2 200 2

So we can say that
•    1 St (Stokes) = 10-4 m2/s = 1 cm2/s
•    1 St = 100 cSt
•    1 cSt (centiStoke) = 10-6 m2/s = 1 mm2/s
•    1 m2/s = 106 centiStokes

## Centistokes Units

The viscosity can be two type; absolute and kinematic viscosity. Absolute viscosity can be expressed as poise that is CGS unit of it. It can also express as centipoises (cP) or millipascal seconds (mPa. s) in SI unit.

The CGS unit of  centistokes (cSt). It is equivalent to 0.01stokes (St). It is purposed after the name of Sir George Gabriel Stokes who contributed to fluid mechanics and also helped develop the Navier-Stokes equation. Here 1 stoke is equivalent to ratio of 1 poise and density of the fluid in g/cm3.

$\nu$ = $\frac{\eta}{\rho}$

Kinematic viscosity can also expressed as square meters per second (m2/s) in SI unit. No doubt the most common unit is square centimeters per second (cm2/s) that is equivalent to 100 cSt. The dynamic or absolute and kinematic viscosity values of some fluids are listed below.

 Fluid Dynamic Viscosity Kinematic Viscosity Temperature Name [cP] [cSt] [C] Water 1 1 20 Water 0.894 0.894 25 Air 0.0018 13.9 27 Honey 5000 3500 25 Mercury 1.526 0.11 25 Ethanol 1.074 1.36 25