RHEOLOGY: Types of Viscosity, Factors affecting viscosity

 RHEOLOGY Part 3

Link for the Video Demonstration of this topic 👇

https://youtu.be/qMlkKuFO_ak

TYPES OF VISCOSITY

1. Kinematic Viscosity

The kinematic viscosity of a liquid is its absolute viscosity divided by the density at a definite temperature.

Kinematic viscosity = ŋ/ρ

The CGS unit for kinematic viscosity is the stokes (S), named after George Gabriel Stokes. It is also sometimes expressed in terms of centistokes (CS). The S.I. unit of kinematic viscosity of m²/s.

1 stoke = 100 centistokes = 1 cm²/s = 0.0001 m²/s

2. Relative viscosity

Relative viscosity (ŋr), also known as viscosity ratio is the ratio of the viscosity of a solution (ŋ) to the viscosity of the solvent used (ŋs).

3. Specific viscosity

Specific viscosity (ŋsp) may be defined as the relative increase in the viscosity of the dispersion over that of the solvent(vehicle) alone.

4. Reduced viscosity (of a polymer)

Reduced viscosity (of a polymer) or viscosity number is defined as the ratio of the specific viscosity to the concentration of the polymer (c) :

FACTORS INFLUENCING THE VISCOSITY

• Intrinsic Factors

Chemical nature, i.e., molecular size, shape and intermolecular forces, influences the viscosity. The molecular weight-the heavier the molecule of the given liquid, the greater will be the viscosity. Liquids with large and irregularly shaped molecules are generally known to be viscous compared to small and symmetric molecules. Molecular collisions between larger molecules are not elastic, i.e., involve loss of kinetic energy. Smaller particles shows better collision. As a result, intermolecular interactions are stronger in large particles and the molecules tend to stick to each other thereby increasing the viscosity of the liquid. The higher the intermolecular forces, the higher is the viscosity. Molecules with spherical shape are expected to slide past one another, and thus have low viscosity.

• Extrinsic Factors

Pressure, temperature and added substances also influence the viscosity. An increase in pressure enhances the cohesive forces of interaction, leading to an increase in the viscosity. In general, small quantities of nonelectrolytes like sucrose, glycerine and alcohol when added to the water, the solution exhibits increased viscosity. Similarly, polymers and other macromolecules enhance the viscosity of solvents such as water. On the other hand, small amounts of strong electrolytes decrease the viscosity. Alkali metals and ammonium ions are a few examples. Temperature is an important factor that needs elaborate discussion.

Temperature: As the temperature increases, the system acquires thermal energy which facilitates the breaking of the cohesive forces. The viscosity of liquid decreases. In case of gases, an increase in temperature increases the viscosity owing to the increased molecular collisions and interactions and offer resistance to flow. The relationship between viscosity and temperature may be expressed in an equation similar to Arrhenius equation as:

where  A is a constant which depends on the molecular weight and molar volume, and Ev is an 'activation energy' required to initiate the flow between the molecules, R is universal gas constant, temperature T is taken in °K.