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Meaning of Viscosity in Chemistry

Viscosity is the resistance to flow exhibited by fluids (liquid or gas) like glycerol, ester, oils, water, alcohol, ether and due to the property, the flow of some liquids is slower than the other.  For example, glycerol, ester, oil flow slowly while water, ether, alcohol flow rapidly on the solid surface due to viscosity. The former liquids are called high viscous liquid while the latter liquids are called high viscous liquids. The reciprocal of viscosity is called fluidity which measures the flowing tendency of the liquid to study physics and chemistry.

Kinematic viscosity in physics sometimes is used in engineering studies to compare the following property of the different liquids. It is defined as kinematic viscosity = viscosity coefficient (η)/density (ρ) and expressed or calculate in the Stoke unit for the honor of the scientist Stoke. 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. Recently it was used in the determination of the molecular weight of polymers.

Flow of liquids in Pipe

Viscosity is resistance to flowing of fluids like glycerol, ester, oils, water, alcohol, the property define flow of some liquid or gas is slower than other

When the liquids flow within the pipe, all the parts of the liquid does not move with equal velocity. A thin layer immediately in contact with the wall of the pipe remains almost stationary. The velocity of the successive layers from the wall of the pipe increases and remains maximum for the central layer. Therefore, there is a velocity gradient, (dv/dx) of the flowing liquid. Each layer of the liquid surface flowing with the higher velocity experience a retarding effect due to the internal friction of the two layers. The resistance that one part of a fluid flowing with one velocity offers to another part of a fluid flowing with different velocity is known as viscosity.

Types of Liquid Flow

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. The velocity of the fluid (liquid and gas) is always the same in magnitude and direction at any particular point. The streamlines do not cross each other at any point in the flow. Newton law of viscous flow only applicable for the laminar flow of the liquids. When the rate of discharge of moving liquid is high, the condition of streamline flow is not maintained and turbulent flow is started. The velocity of the liquid at a point varies with the time irregularly.

A dimensionless quantity, name as Raynolds number (Re) uses to define different types of liquid flow having the value, (dρu/η), where, η = viscosity coefficient, ρ = density, d = diameter of the tube and u = velocity of the liquid. Therefore, when Re < 1000, streamline flow occurs, when Re ≈ 1000 to 1500, turbulent flow starts, and when Re > 1500, turbulent flow persist in the 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.

Origin of Viscosity

The fraction between two layers moving with different velocity originates from the intermolecular attraction in liquid. Therefore, viscosity in liquid increases with the rise of temperature but in gases, it increases with the increase of temperature. This is due to the different origins of the term viscosity in the liquid and gas phases. 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.

Coefficient of viscosity (η)

The coefficient η is the measured viscosity of liquids and gases and is defined from the Newton law of viscous flow in science. 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 = – η A (dv/dx), where η = symbol of viscosity coefficient of liquid. 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 units of viscosity coefficient (η) are poise (gm cm-1 sec-1) in the CGS unit and Pascal sec (kilogram meter-1 sec-1) in the SI unit and the dimensions of the quantity = [M L-1 T-1]. The relation between the two units, 1 pascal sec = 10 poise.

Viscosity and Temperature

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. This is due to the fact that at rising temperatures, the average thermal energy of its molecule increases. The relation of η with temperature given by, η = A eE/RT, where E = activation energy of the viscous flow and A = constant for liquid. In learning chemistry or physics, the mobility or fluidity (reciprocal of viscosity) should follow the Boltzmann energy distribution law, fluidity (φ) ∝ e -E/RT.