What is Viscosity?

Viscosity is defined as the resistance of a fluid to flow. Dynamic viscosity’s unit is Pa.s in the SI system and poise (g/cm.s) in the CGS system. Viscosity varies with temperature. Because of the difference in their molecular structure, viscosity of liquids and gases changes by temperature. Viscosity of most of the liquids decreases when temperature increases. In the liquids the cohesive forces between the molecules predominates the molecular momentum transfer between the molecules, mainly because the molecules are closely packed. When the liquid is heated the cohesive forces between the molecules reduce thus the forces of attraction between them reduce, which eventually reduces the viscosity of the liquids. While the viscosity of gases increases with increasing temperature. The reason behind this is again the movement of the molecules and the forces between them. In the gases the cohesive forces between the molecules is lesser, while molecular momentum transfer is high. As the temperature of the gas is increased, the molecular momentum transfer rate increases further which increases the viscosity of the gas (Rheology). 

Viscous fluids tend to deform continuously under the effect of an applied stress. Viscous fluids are categorized as Newtonian and non-Newtonian fluids. Newtonian fluids are that follow Newton’s law of viscosity. Gases, oils, water, tea, coffee, fruit juices, milk and honey are the examples of Newtonian fluids. However, fluids which do not follow Newton’s law of viscosity are called as non- Newtonian fluids. Shear thinning and shear thickening fluids are included this group. 

Viscosity is measured by using some devices like capillary flow viscometers and Brookfield viscometers. Capillary flow viscometers are generally U shape devices. They are simple, inexpensive and suitable for Newtonian fluids. In capillary flow viscometers, the time for a standard volume of fluid to pass through a known length of capillary tubing is measured. The flow rate of material due to a known pressure gradient is determined. The driving pressure is usually generated by the force of gravity acting on a column of the liquid although it can be generated by the application of compressed air or by mechanical means. The diameter of a capillary viscometer should be small enough to provide laminar flow. Capillary viscometers are calibrated with Newtonian oils of known viscosities because the flow rate depends on the capillary radius which is difficult to measure. For the viscosity measurement the viscometer is accurately filled with an accurately known volume of test fluid and the apparatus is immersed in a constant temperature bath until equilibrium is reached. Then fluid is sucked up from the other limb through the capillary tube until it is above the marked level. Then suction is removed and fluid flows through the capillary tube under the influence of gravity and the time for the fluid to flow between marked levels is recorded. This time is a direct measure of the kinematic viscosity since it depends on both viscosity and density of fluid. Brookfield viscometers are used for characterizing the rheological behavior of fluids in terms of shear stress-shear rate relationship. A spindle attached to the instrument with a vertical shaft is rotated in the fluid and the torque necessary to overcome the viscous resistance is measured. The results are presented in the form of apparent viscosity against rotational speed. By using this viscometer it is possible to determine whether the fluid is time dependent or not. (Sumnu & Sahin, 2006)