Viscosity

Viscosity

Viscosity is a fluid property that describes its internal resistance to shear stresses. The Viscosity of a fluid is characterized by its resistance to flow. Viscosity is a material property of fluids and is influenced by factors such as molecular structure, intermolecular forces, fluid composition as well temperature and pressure. Knowledge of Dynamic and Kinematic Viscosities is needed for the design of fluid transportation as well as heat and mass transfer systems. Many common fluids are termed Newtonian, as they obey Newton’s equation, according to which, the resulting shear of a fluid is directly proportional to the force applied and inversely proportional to its viscosity. The relationship between shear-stress and shear rate for Newtonian fluids is linear and the viscosity is constant at a given temperature. On the other hand, non-Newtonian fluids demonstrate non-linear and also time dependent relationships between shear stress and shear rate. Emulsions, suspensions, slurries, gels, waxy oils, polymer melts, toothpaste are all examples of non-Newtonian fluids

Measurements of Viscosities are generally indirect and Viscometers and Rheometers are used to determine Viscosities for various types of fluids. These instruments measure either shear rate (velocity) or shear stresses, to obtain calculated Viscosities. The simplest and commonly used Viscometers that use velocity are Ostwald (or U-Tube), Falling Sphere and Redwood Viscometers, all of which measure the time of travel of a liquid column which is converted to kinematic viscosity. Corresponding density values are used to obtain Dynamic viscosities. Gas Viscosities can be measured using Capillary Viscometers.

A Brookfield Viscometer is an example of shear rate and applied force measurement to obtain Viscosity. It uses a rotating sensing element at a defined shear rate and the Viscometer measures the torque needed to overcome viscous resistance of the fluid resistance to rotation, reporting a Viscosity value.

Common Uses of Viscosity Tests

• Viscosity is a key property for Hydraulic design of fluid transportation systems
• Used in fluid dynamic characterization for process and product design including heat and mass transfer
• Polymer processing system design
• Packaging of gels and materials displaying non-Newtonian flow behaviour

 Advantages of Viscosity Tests

• Accurate Viscosity measurement enables effective design and operation of many industrial important systems.

Limitations of Viscosity Tests

• Thorough cleaning of instrument is needed when changing fluids
• Special materials of construction needed for aggressive materials

Industrial Applications of Viscosity Tests

• Hydraulic design
• Polymer processing
• Pharmaceutical production
• Cosmetics production
• Paints and coatings
• Food processing
• Adhesives production
• Biotechnology
• Chemical reactor design
• Crude oil processing
• Oil well drilling muds