I have been trying to think about how to have students measure the viscosity of a fluid in their kitchen. The problem is that you need to make a lot of measurements (not least of which is the fluid layer thickness) that are (1) hard to make and (2) have potentially very large error. However, if the errors are consistent then it should be possible to quantitatively compare the viscosity of two different fluids using the identical test. I think that this may be a way around the problem.
The instructions given below are for one of the first take home labs that I use in a semester. The goal is to find the ratio of the viscosity of two common fluids you can find in a kitchen. There are a number of possible ways to approach this problem as long as you are not too hung up on accuracy beyond order of magnitude. At this stage in the semester the students have been down to the lab to measure the viscosity of a range of fluids including oil, molasses, corn syrup so they have data they can use for comparison. It is also possible to look up a lot of viscosity values at http://en.wikipedia.org/wiki/Viscosity.
The first time I ran this lab I got a range of results. It was the first lab of the semester and so the students were a little unsure how to approach the write up. However, the range of experiments they ran were very impressive. One team even built a simple viscometer using a cup, some string, a pencil, and a pile of pennies. I also had students researching online how to measure viscosity. That was all great. The actual quantitative results were less great. It is actually very hard to do well. It is also hard to get a simple theoretical model for some of the tests they ran. For example, you can place equal volumes of different fluids at the top of a slope and time how long each takes to travel a certain distance. However, the time taken is a function of the fluid density, the thickness of the slug of fluid flowing down the slope, and the viscosity of the fluid. It is, therefore, hard to control for all these parameters in order to get a viscosity ratio. There was also the age old problem of students not actually reading the instructions and instead of a viscosity ratio giving estimates of the actual viscosity which were obviously way off. I also had students picking non-Newtonian fluids which complicates things a little. In future I may move this lab to later in the semester.
As with all the take home lab write ups I will not publish detailed methods for conducting the tests. I still use them in class and want my students to figure it out on their own.
Instructions to students
On your first visit to the fluids lab you used a viscometer to measure the viscosity of various fluids. In this take home lab you will need to calculate the ratio of the viscosity of 2 common household fluids such as oil, honey, syrup, or molasses. Ideally you would use fluids that you also used in the viscosity lab.
- Identify 2 fluids that you will be using to measure viscosity.
- Run a series of experiments to establish the ratio of the viscosity of the two fluids you selected. That is, develop an experiment that will allow you to compare the two fluids viscosity’s without necessarily accurately measuring the viscosity of the individual fluids.
- Write a brief report that
- Is 3 pages max including photos of you running your experiments
- Details how you made the measurements including clear diagrams showing your setup
- Details of how you used the measurements to calculate the viscosity ratio (include any appropriate free body diagrams)
- Compares your measured ratio to that based on tabulated data from textbooks or online (or you can use your results from the first lab).
- Quantifies potential sources of error that explains any discrepancy between your measurements and your lab or tabulated data.
- You may not use equipment in the fluids lab or any other scientific lab equipment
- You should only use items that are commonly available in your home.
- If you need to go to a store to buy something please come and see me first. I may be able to lend you something or I will buy it and then lend it to you. You will need to explain why you need it and it should be cheap.
Due in 2 weeks
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.