Comparing Oil Viscosity in the Classroom

Fig.1 - Comparing Viscosity Using Air Bubbles

Viscosity is defined as a fluid’s internal resistance to flow. The correct amount of viscosity is important in oils used as lubricants. Motor oils, for instance, will not prevent friction between the engine parts if they are too thin but, conversely, will not allow parts to move freely if they are too viscous. Heat reduces the viscosity of a liquid and must therefore be taken into account when selecting appropriate oils.

The viscosity of fluids can be gauged by simply observing how easily they pour or spread along a solid surface or by determining the time taken for an object to move vertically through it. The following experiments are suitable for a middle secondary class, with methods ranging from simple to moderately difficult.

A Simple Method for Comparing the Viscosity of Oils – Materials and Teaching Method

In this experiment the viscosity of different oils is compared by determining the time taken for air bubbles to rise to the top of an inverted container of the oil (as demonstrated in figure 1).

Fig. 2- Determining the Viscosity of Engine Oil

The following materials and equipment are required per group of around 4 students:

•                four empty 500ml clear plastic drink bottles

•                200ml of each of the following: olive oil, canola oil, low viscosity motor oil, high viscosity motor oil

•                stopwatch

Students should be instructed to copy down the following directions, which could be followed by a teacher-led explanation.

1.             Pour each of the four oils into the clear plastic bottles and screw on the lids.

2.             For each oil, turn the bottle upside down and time how long it takes for an observed air bubble to reach the top of the fluid.

3.             Repeat this procedure twice and obtain an average for each oil.

4.             Record results in a table and draw a column graph of oil type v. average time for bubbles to rise.

Viscosity Follow-Up Activity

This experiment could be repeated after standing the four bottles of oil in a water bath or container of hot water for around 5 minutes. Students should observe that the viscosity of each oil decreases with increasing temperature. An alternative, yet still simple, experiment could involve timing how long each type of oil takes to actually pour out of the bottle when the lid is removed and it is turned upside down (see a similar method in figure 2).

A More Involved Method for Comparing the Viscosity of Oils – Materials and Teaching Method

This method, which involves calculating the time taken for a ball bearing to reach the bottom of a vertical column of the oil, can be used to actually calculate the numerical value for the viscosity,η, of each type of oil, in g/cm.s.

Data needed for this calculation include the radius and density of the ball bearing, the density of the oil and the speed of the ball bearing as it moves through the fluid. Alternatively, students may choose to simply compare the time taken for the ball bearing to travel through each type of oil.

The following materials and equipment are required per group of around 4 students:

•                four one litre measuring cylinders

•                one litre of each of the following: olive oil, canola oil, low viscosity motor oil , high viscosity motor oil

•                stopwatch

•                electronic balance

•                twelve ball bearings, each 3mm in diameter

•                10ml measuring cylinder of water to measure volume of the ball bearing by displacement

Students should be instructed to copy down the following directions, which could be followed by a teacher-led explanation.

1.             Pour each of the four oils into the measuring cylinders.

2.             Using the electronic balance and the measuring cylinder of water for the ball bearing, determine the density of the ball bearing and the oils by dividing their mass by their volume. The results should be in g/cm3.

3.             Calculate the density of the oil by dividing its mass by its volume.

4.             Drop a ball bearing into each of the four types of oil and record the time in seconds it takes to reach the bottom of the cylinder.

5.             Repeat this procedure twice and obtain an average for each oil.

6.             Measure the distance travelled by the ball bearing in cm and divide this distance by the average time taken in order to determine the average velocity (in cm/s) of the ball bearings in each cylinder.

7.             Record all results in a table.

Students can use the following equation, derived from Stokes’ Law, to determine the viscosity, η, of each type of oil in g/cm.s:

Viscosity (η) =2(Δρ)gr2 / 9v,

where Δρ is the density difference between the oil and the ball bearing, g is acceleration due to gravity of 980cm/s2, r is the radius of the ball bearing and v is the average velocity of the ball bearing.

Follow-Up Activity: Motor Oil Viscosity and Temperature

A good motor oil will not change markedly over a wide range of temperatures, as this could affect engine wear and tear. The above experiment could be repeated by standing the measuring cylinders in water baths that have been adjusted to specified temperatures.

Further work could involve students researching Stokes’ law, fluid dynamics and terminal velocity.The ball bearings should have reached a constant, or 'terminal' velocity when the force of gravity became balanced by buoyancy and viscosity forces within the oils.

References

Olsen, Andrew. " The Viscosity of Motor Oil." Science Buddies Science Fair Projects, 2010.

The Nuffield Foundation. "Falling Through a High Viscosity Liquid." Practical Physics, 2004.