Viscosity is a measure of the ability of a fluid to resist shear forces. Simpson P.E., in Practical Onshore Gas Field Engineering, 2017 4.1.1 Viscosity In learning chemistry or physics, the mobility or fluidity (reciprocal of viscosity) should follow the Boltzmann energy distribution law, fluidity (П†) в€ќ e -E/RT.David A. The relation of О· with temperature is given by, О· = A e E/RT, where E = activation energy of the viscous flow and A = constant for liquid. This is due to the fact that at rising temperatures, the average thermal energy of its molecule increases. Intermolecular attraction decrease with increases in temperature, hence the viscosity of a liquid also decreases by 2 percent with the rise of temperature per degree Celsius. The relation between the two units, 1 pascal sec = 10 poise. In SI system, the unit of О· is Pascal sec (kg meter -1 sec -1). The unit of viscosity coefficient (О·) in CGS system is poise (g cm -1 sec -1). Coefficient of viscosity unit and dimension According to the above mathematical formula, viscosity is defined as the force per unit area originated between the two layers moving with the unit velocity gradient. The returning force originates due to internal friction between moving layers with the velocity gradient = (dv/dx) and area = A is expressed by the equation, Fv = вЂ“ О· A (dv/dx), where О· = symbol of viscosity coefficient of liquid. The coefficient О· is the measured viscosity of liquids and gases and is defined from the Newton law of viscous flow in science. In gas, the viscosity originates due to the exchange of momentum between two moving layers and no effect of pressure unless it is very low, but in liquid, it increases with an increase of pressure. This is due to the different origins of the term viscosity in the liquid and gas phases. Therefore, viscosity in liquid increases with the rise of temperature but in gases, it increases with the increase of temperature. The fraction between two layers moving with different velocities originates from the intermolecular attraction in liquid. The capillary tube is used in viscosity measurement to maintain a streamlined flow of liquid since the diameter of the tube is very small. Therefore, when Re 1500, turbulent flow persist in the liquid. Reynolds numberДЄ dimensionless quantity, name as Reynolds number (Re) uses to define different types of liquid flow having the value, (dПЃu/О·), where, О· = viscosity coefficient, ПЃ = density, d = diameter of the tube and u = velocity of the liquid. The velocity of the liquid at a point varies with time irregularly. When the rate of discharge of moving liquid is high, the condition of streamline flow is not maintained and turbulent flow is started. NewtonвЂ™s law of viscous flow only applicable to the laminar flow of the liquids. The streamlines do not cross each other at any point in the flow. The velocity of the fluid (liquid and gas) is always the same in magnitude and direction at any particular point. In Laminar or streamline flow, the velocity of the discharge of a liquid is low and all the layers of liquid move parallel to each other.

Laminar or streamline flow and turbulent flow are the main types of fluid flow. The resistance that one part of a fluid flowing with one velocity offers to another part of a fluid flowing with different velocities is known as viscosity.

Therefore, there is a velocity gradient, (dv/dx) of the flowing liquid.Д®ach layer of the liquid surface flowing with the higher velocity experience a retarding effect due to the internal friction of the two layers.

The velocity of the successive layers from the wall of the pipe increases and remains maximum for the central layer. A thin layer immediately in contact with the wall of the pipe remains almost stationary. When the fluid flows within the pipe, all the parts of the fluid do not move with equal velocity.

Recently it was used in the determination of the molecular weight of polymers. The measurement of viscosity data of fluids is important in the study of biology, chemistry, and physiology, it also uses to dealing with various problems solve in fluid mechanics, especially in paint, ink, colloids, rubber, textiles, etc. It is defined as kinematic viscosity = viscosity coefficient (О·)/ density (ПЃ) and expressed or calculate in the Stoke unit for the honor of the scientist Stoke. Kinematic viscosity in physics sometimes is used in engineering studies to compare the following property of the different liquids.

Viscosity unit