If you have access to an overhead projector then you can use it to demonstrate the viscosity of different fluids and also that viscosity, or more precisely viscous shear stresses, act to retard the flow of a fluid.
- Overhead projector (OHP) and screen
- Overhead transparency
- A couple of syringes
- Honey, molasses, or some other viscous fluid
- Water and food coloring
- Mix the water and food coloring and put it in one of the syringes. Put the honey in the other syringe
- Prop the OHP up so that it slopes from back to front (so that the liquids flow down when projected).
- Place the transparency sheet on the top of the OHP and turn it on.
- Use the syringes to pour the two fluids out onto the transparency sheet. The water should flow down the slope relatively quickly whereas the honey should be quite slow. Eventually the water might stop flowing due to surface tension at the front of the water blob.
The obvious qualitative analysis is that the higher the viscosity the slower the flow of the fluid blob down the slope. A more quantitative analysis can be done by assuming that the flow is steady. Therefore, the average shear stress along the base is balanced by the component of the fluid weight down the slope. Denote the parameters of the blob by its area A, depth D, specific weight γ, viscosity μ, and velocity U. The OHP slope is denoted by S (assumed relatively small). The balance of shear stress and weight can be approximated by
γ ADS≈ μ AU/D
μ ≈ γ D2S/U
The velocity differences between the water and hone flow is substantial whereas the differences in γ and D are much smaller. Therefore, the higher the velocity the lower the viscosity. Hence, the honey has higher viscosity (no surprise).
An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (email@example.com). I also welcome comments (through the comments section or via email) on improving the demonstrations.