Viscosity is a physical property of a fluid that limits its ability to flow, Note, that it only applies to fluids (liquid and gases) because solids don’t flow. The higher a fluid’s viscosity, the slower it flows. The lower a fluid’s viscosity, the faster it flows. A fluid’s viscosity depends on its internal friction or the strength of molecular bonds.
During the measurement of viscosity, the following aspects have significant importance:
- If the bonds between a fluid’s particles are strong, then viscosity is high, which means there is high resistance to flow, and the fluid does not flow easily (flows slowly).
- If the bonds between particles are weak, then viscosity is low, which means there is low resistance to flow, and the fluid flows easily (flows quickly).
There are two types of viscosities based on fluidity:
Dynamic Viscosity: when the viscosity of the fluid is studied with reference to force.
Kinematic viscosity: When the viscosity of the fluid is studied with reference to the motion of the fluid.
How to measure Viscosity?
There are several methods to measure viscosity. The choice of method is based on the type of fluid under consideration. The most frequently used are discussed below:
The earliest methods for measuring viscosity were based on using capillary tubes and measuring the time it took for a volume of liquid to pass through the length of the tube. It is also known as Ostwald or Ubbelohde viscometers.
Made up of a small container with a handle, and a small hole in the bottom. The time it takes to empty the cup through the hole is correlated to viscosity. Note that, the Zahn cup is often used to measure the viscosity of paints.
Falling Sphere Viscometer
A sphere of known density is dropped into the fluid sample and the time it takes for the sphere to fall to a specified point is recorded. This method has been used on ships to monitor the quality of the fuel going into the ship’s engine.
It measures the viscosity by the value of damping of an oscillating electromechanical resonator immersed in a fluid. This technique is often used in-process to give continuous readings in a product stream.
In the method, the viscometer measures the torque required to turn an object into a fluid as a function of that fluid’s viscosity. This method is frequently used in quality control and production laboratories.
How Is Viscosity Measured?
Viscosity is measured by different methods including the Zahn cup, and Viscometer (vibrational, rotational, capillary, and falling sphere).
What is A Zahn Cup and how to use it?
Zahn cup is a method to measure viscosity, it is made up of a small container with a handle, and a small hole in the bottom. The time it takes to empty the cup through the hole is correlated to viscosity. It is to be noted, that the zahn cup is often used in the paint industry.
What 4 things determine the viscosity?
Temperature: The viscosity of liquids decreases by about 2% for each degree rise in temperature.2
Colloid Systems: The viscosity of the lyophilic colloid solution is generally relatively high.
Chemical Composition: The viscosity of liquids generally depends upon the size, shape, and chemical nature of their molecules. It is greater with larger than with smaller molecules; with elongated than with spherical molecules. Because large amounts of dissolved solids generally increase viscosity. Whereas small amounts of electrolytes lower the viscosity of water slightly.
What is the unit of viscosity in KG?
The SI unit of viscosity is the pascal second (Pa·s) or kg·m-1s-1.
Is high viscosity thick or thin?
The higher the viscosity of a liquid, the thicker it is, and the greater the resistance to flow.
How to measure viscosity with a falling sphere viscometer?
A sphere of known density is dropped into the fluid sample and the time it takes for the sphere to fall to a specified point is recorded. This method has been used on ships to monitor the quality of the fuel going into the ship’s engine
References Journals & Books
- Andrew W Hamilton, Francis Quail, Detailed State of the Art Review of the Different On-Line/In-Line Oil Analysis Techniques in the Context of Wind Turbine Gearboxes, Journal of Tribology (Oct. 2011)
- Kanishka Kobbekaduwa, Data Acquisition System for a Falling-Sphere Viscometer, Conference: Proceedings of the 28th Technical Sessions, Institute of Physics Sri Lanka, Volume: 28 (March 2012)
- Amalina Amir, Making of graphene fiber composites using pressurized gyration, Thesis for: Ph.D. New Materials and Mechanical Engineering Advisor: Prof. Mohan Edirisinghe, (June 2019)