Model Description
This is a simple demonstration of the difference between density and viscosity in fluid mechanics. Two equal size containers, one containing shampoo and the other corn syrup, are used to show the difference between the two concepts. This demonstration should take 5-8 minutes.
Engineering Principle
Most students are familiar with density and viscosity separately, but erroneously assume the two are always related. Density is simply mass over volume, and can be thought of as the “heaviness” of a material. Viscosity (μ) is a measure of a fluid’s resistance to flow and is used to relate shear stress to the rate of deformation:
In general, as viscosity increases, so does density. However, in this demonstration the denser liquid (corn syrup) is less viscous than the less dense shampoo. This demonstration serves as a good lead into a discussion of viscosity as an independent property.
What You Need
| Item | Quantity | Description/Clarification |
|---|---|---|
| Graduated Cylinder | 2 | They must be of equal height. |
| Shampoo | 1 | Not that much is needed and the material can be recycled when complete. |
| Corn Syrup | 1 | Not that much is needed and the material can be recycled when complete. |
| Marbles | 2 | The two used must be the same size and weight. |
| Scale | 1 | Optional |
How It’s Done
Before Class: Fill each graduated cylinder with equal amount of shampoo and corn syrup. Have the marbles in your pocket or readily available.
In Class: Pour equal amounts of corn syrup and shampoo into the cylinders. You can either use the scale to weigh the cylinders and prove that the corn syrup is more dense, or simply tell them. Then show them the marbles and tell them you are going to simultaneously drop a marble in each beaker. If desired, you can measure the marbles to demonstrate they are the same weight. Ask the students to predict which marble will reach the bottom first. Most will state that the marble in the shampoo will hit first because it is the less dense fluid. You can play up the demonstration by having 1-2 students time the corn syrup beaker and others time the shampoo beaker. When you release the marble, the corn syrup marble will fall quickly, while the shampoo marble will take considerably longer – the difference will be obvious.
Observations: The students should observe that a denser fluid is not necessarily more viscous. After the demonstration, you can show this by pouring the fluids out into jars. This demonstration can then transition into the discussion of Newtonian fluids. If the demonstration will be repeated in back to back classes, have enough materials for 2 demonstrations since emptying the shampoo/corn syrup back into their containers can be a messy affair. Also, cleaning the marbles sufficiently to put them back into your pocket can be time consuming as well.