Viscosity Measurements

Viscosity can be simply explained in non-physical terms as the sticky and thick characteristics of a liquid. However, if we had to describe viscosity in terms of physics, it can be defined as the resistance shown by a fluid to permanent deformation. Viscosity can be understood better by studying liquids like oils, honey, syrup, etc. These liquids are thicker and stickier hence making them viscous liquids. The unit of viscosity is newton-second per metre square. The SI unit is hence called pascal-second.

Important Terms

Before understanding viscosity, one must understand two important terms.

• Shear stress: Shear stress is the force per unit area required to move one layer of fluid about another.
• Shear rate: The shear rate can be defined as the measure of change in speed at which intermediate layers move concerning one another.

Types of Viscosity

There are majorly two types of viscosity. 

• Dynamic viscosity

Dynamic viscosity is the resistance to the movement of one layer of fluid over the other. 

• Kinematic viscosity

Kinematic viscosity is the dynamic viscosity divided by the density at that exact temperature at which the dynamic viscosity is measured.

Formula of Viscosity

The formula for viscosity is 

Viscosity= Shear stress/Shear strain rate

The resulting solution is usually expressed in centipoise, which is equal to one millipascal second.

The unit of viscosity is newton-second per square meter, and the SI unit of viscosity is pascal-second. The cgs unit of viscosity is poise. 

Newtonian and Non-Newtonian Fluids

The fluids are mainly classified into two – Newtonian fluids and Non-Newtonian fluids. Newtonian fluids are the ones that have constant viscosity, which is independent of shear rate. While Non-newtonian fluids are the ones that have variable viscosity as they don’t follow Newton’s law of viscosity; hence the viscosity depends on the shear rate.

Viscosity Measurement

There are five methods to measure viscosity. These fundamental methods will be good enough to measure both dynamic and kinematic viscosity.

• Vibrating Viscometer

The viscosity of a particular fluid under study is measured by applying oscillating vibrations and observing the effects of damping vibrations on the fluid.  Assessment of power input, the decay time of the oscillation, or changes in the resonated frequency helps measure the viscosity.

• Capillary Viscometer

The instrument used in this method is the capillary tube. Mainly used to measure kinematic viscosity, it requires an understanding of the density and volume of the sample that is under study. The fluid is made to pass through a tube of very small diameter. The kinematic viscosity is measured by finding the time taken for the sample under study to travel through the capillary. 

• Rotational Rheometry

In this method, low levels of torque are applied to measure the viscosity of a liquid sample. The amount of torque necessary to cause rotation across a horizontal plane in the sample under study is measured and related to sample viscosity.  

• Microfluidic Rheometers

Microfluidic Rheometers are a new method to determine the dynamic viscosity of fluids in small volumes. A sample fluid of small volume is forced through a microfluidic channel in laminar flow. 

• Non-Contact Rheology

This method is a slightly complicated way of measuring rheological characteristics. It is similar to rotational rheometry; however, it is better adapted to more complex and delicate fluids like gels and viscoelastic materials.

Why Should Viscosity Be Measured?

Having the viscosity data of various fluids helps the industries and manufacturers to predict the nature and utility of the fluid for future applications. For example, if tomato ketchup doesn’t have the correct viscosity, manufacturers may witness a fall in demand. Knowing the viscosity of material also helps in predicting transportation processes.

Viscosity Test in Various Industries

The following industries are heavily dependent on the property of viscosity. 

• Food industry

Viscosity measurements are crucial in the food industry to maximise production and cost-effectiveness. Understanding viscosity is essential for the food industry as the products move a lot from one part of the machine to another. For example, the time taken for a food product to travel through a pipe determines the rate of production. Therefore the production blueprint must be drawn according to the viscosity of the food product. 

Viscosity also determines the texture of the food products. 

• Adhesive industry

The adhesive industry is another such industry that depends upon the viscosity of the adhesives. Adhesives with low viscosity will travel much faster compared to high-viscosity adhesives. 

• Petroleum and concrete industry

The viscosity measurement is necessary for both the petroleum and concrete industries as viscosity helps predict the fluid’s lubricating power. At the same time, the viscosity of concrete is crucial for self-levelling.

• Cosmetics

Like the food industry, the cosmetic industry also depends on viscosity measurement as it helps determine and predict the way the product will feel after being applied or kept unused for months.

Conclusion

Viscosity is a significant property that finds numerous applications in the industrial and manufacturing sectors. It is indicative of the quality of various products manufactured across various industries. Viscosity is defined as the resistance shown by a fluid to permanent deformation. 

It is of two types dynamic and kinematic viscosity and there are five methods to measure both – capillary viscometer, vibrating viscometer, rotational rheometry, microfluidic rheometers, and non-contact rheology.