What is Viscosity in Chemistry?

by Lucy Bell-Young

Viscosity is the tendency of fluids, in either liquid or gas form, to resist forces that act against them. For example, if you tried to rapidly stir pure water while rapidly stirring molasses, you would find it much more difficult to stir the latter. With this in mind, viscosity can also refer to how thick a fluid is.

Viscosity can either be an inherent physical property of a compound in fluid form in standard conditions, or it can be the result of a high concentration of solute in a solution.

For instance, viscosity of seawater in the Dead Sea is about ten times that of freshwater’s. The reason for this is the high concentration of dissolved salt in the seawater. Meanwhile, honey is a naturally viscous liquid; even when heat is used to thin it, honey will always return to its original thickness once it’s cooled down.

What Does Viscosity Mean?

Viscosity is the resistance of particles in a fluid to flow or move in response to an external force exerted on them. This resistance is because of the way the particles are arranged. If the particles are closer together, the fluid tends to be more viscous. The resistance is also correlated with the intermolecular forces. Molecules of the same type have cohesive force among the particles, which is difficult to break.

Fluids are substances that flow, and they can either be liquids or gases. In some instances, solids can also exhibit fluid-like properties, such as the ‘flow’ of wheat grains moving inside in a pneumatic conveying system. Conversely, the solid analogues of liquid and gaseous fluids also exhibit a solid-like structure when the individual particles are too crowded in a container.

For example, very fine particles of sand can easily flow through the narrow opening inside an hourglass. But if you were to use bigger sand particles, the sand would either move slower or become stuck. Similarly, the actual viscosity of fluids is dependent on how the particles (molecules) of the fluids move against each other. Therefore, viscosity can be considered as internal friction between particles of a fluid.

How to Measure Viscosity

There are two types of viscosity you can measure: dynamic and kinematic:

  • Dynamic viscosity: This is otherwise known as the absolute viscosity, and is related to the force under which a fluid is subjected. It’s the ratio between the shear stress applied and the area of the sample fluid. The unit of measure for absolute viscosity is millipascal seconds (mPa-s).
  • Kinematic viscosity: This is measured against the force of gravity. It’s basically how a fluid resists the pull of gravity. The measurement is dependent on the density of a fluid. The unit of measure for kinematic viscosity is square metre per second (m2/s)

Viscosity is the resistance of fluids to flow or deform at a given rate. Since the resistance force of the fluid cannot easily be measured in a direct manner, flow rate can be used as a surrogate. Therefore, viscosity is technically a measure of the speed of the flow of fluids under a given set of standard conditions.

External factors, such as pressure and temperature, must be maintained at a constant to have an accurate and standardised measurement of viscosity. The conventional or standard temperature and pressure, however, may actually vary; they’ll depend on what you want to measure and the international convention that you want to use. For example, the IUPAC standard conditions for liquids and gases are defined as: 273.15 K (0 °C) and 100 kPa (roughly 1 atm).

Various types of techniques and instruments can be used to measure either the dynamic (absolute) or the kinematic viscosity of a fluid. Choosing which one to use will depend on the type of fluid and your purpose of measuring. Here are five measurement methods that you can use:

  1. Capillary tubes

Also known as capillary viscometers, these are vertical U-shaped tubes with a narrow passageway for the fluid. Fluid flows into the tube with the aid of capillary action. The viscosity can be calculated based on the time it takes for the fluid to flow through the tubes. 

This method is used to measure the kinematic viscosity of a fluid. You must know the fluid’s density and the exact diameter of the tubes. See the illustration below:

Ostwald's Viscometer diagram
  1. Rotational rheometry

This method utilises the application of weak torque or rotational force to a sample of the fluid to be tested. The dynamic viscosity is measured by plotting the flow curve of the substance. It responds to the varying levels of shear force as the torque deforms the fluid. See the illustration below:

Rotational Rheometry diagram
  1. Vibrating viscometers

Subjecting a sample fluid to motion in order to measure the viscosity as it moves can also be done through vibrations. You can apply oscillations to the sample and then measure the resonance. Some amounts of energy will be dissipated. The energy loss in the resonating frequency is directly proportional to the viscosity of  the fluid being tested. 

  1. Non-contact rheology

This is otherwise known as microrheology. Compared to other methodologies, it’s a more complex measurement. Although it’s similar to rotational or torque-based rheometry, it’s designed for fragile fluids like gels and pastes. Multi-speckle diffusing wave spectroscopy is applied during this method. 

How to Find Fluid Viscosity

If you want to learn how to calculate the viscosity of a fluid, you need to understand fluid dynamics. You also need to have a good foundation in calculus because many of the problems involved in fluid dynamics require differential equations and integrations. 

However, there are several simple formulas that can be used based on the empirical data that you collect.

For instance, one of the most common formulas is based on the steel ball bearing drop experiment:

Viscosity = g(2Db-Dl)r2/9v

g = 9.8 m/s2 (acceleration due to gravity)

Db = density of the ball

Dl  = density of the liquid 

r  = radius of the ball

v = velocity of the ball when dropped through the liquid

The velocity of the ball as it falls through the fluid is inversely proportional to the fluid’s viscosity. Slower velocity means higher viscosity.

How Does Temperature Affect the Viscosity of a Fluid?

Temperature is inversely proportional to viscosity because the latter is mainly to do with how the molecules move and how they are bonded to each other. High temperature causes the molecules to move faster, making fluid less viscous.

On the other hand, pressure is directly proportional to viscosity: higher pressure slows down molecular motions. For instance, LPG is more viscous inside the tank because of higher pressure, which turns the gas into liquid.

To have an accurate and standardised measurement of viscosity, both the temperature and the pressure are kept at standard conditions based on the 1982 IUPAC conventions:

  • 273.15 K (0 °C) temperature
  • 100 kPa (roughly 1 atm) pressure

This means viscosities of various fluids can be accurately compared.

Examples of a Viscosity Chemistry Experiment

Even without the use of expensive and complex viscosity measuring equipment, you can calculate and compare the relative viscosities of various fluids by performing the classic ball bearing drop experiment

The materials you will need are:

  • Graduated cylinders of the same volume
  • Ball bearings of the same size and density
  • Different fluids for comparison

You simply need to drop a ball bearing in each graduated cylinder containing a fluid to be tested. Make sure you drop the ball at the same height for each test. Clock the speed of the drop, and then calculate the viscosity based on the formula given above. 


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