# Take home lab – Relative viscosity

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.

1. Identify 2 fluids that you will be using to measure viscosity.
2. 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.
3. Write a brief report that
1. Is 3 pages max including photos of you running your experiments
2. Details how you made the measurements including clear diagrams showing your setup
3. Details of how you used the measurements to calculate the viscosity ratio (include any appropriate free body diagrams)
4. Compares your measured ratio to that based on tabulated data from textbooks or online (or you can use your results from the first lab).
5. Quantifies potential sources of error that explains any discrepancy between your measurements and your lab or tabulated data.

Rules

1. You may not use equipment in the fluids lab or any other scientific lab equipment
2. You should only use items that are commonly available in your home.
3. 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 (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

# Definition of a fluid – sliding OHP transparencies

From now on this is how I am going to start each semester teaching fluid mechanics. The definition of a fluid that is use is

A fluid is a material that deforms continuously under the action of a shear stress.

This demonstration illustrates the continuous deformation.

Equipment

2. Two transparency sheets
3. A viscous liquid such as dish soap (easy to clean up) or honey (not so easy to clean up)
4. A text book (they have to be good for something)

Demonstration

1. Remind the class that a when you apply a shear force to a solid it deflects a finite amount
2. Demonstrate this by applying a shear to the text book. The book is a useful prop because you can get it to noticeably deflect.
3. Place one of the transparencies on the OHP and pour some of the soap onto the middle of the sheet
4. Place the second sheet on top of the first.
5. Slowly pull the two sheets apart horizontally while stating that a fluid differs from a solid in that a fluid deforms continuously under the action of a shear stress.

I then write the definition on the board and launch into fluid properties. Obviously there is no analysis to be done 5 minutes into the first class of a fluids course, but it can help students clearly visualize the difference right from the start.

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 (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

# Videos from “Viscous flow between parallel plates – lubrication and interfacial instabilities”

Here are the videos from the “Viscous flow between parallel plates – lubrication and interfacial instabilities” demonstration. The two GIFs as a single higher resolution video can be found here.

Lubrication demonstration

Taylor-Saffman instability

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 (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

# Video of “Non-Newtonian silly putty”

Here are a couple of videos from the “Non-Newtonian silly putty” demonstration. The full videos are linked from the GIF titles.

Watching the silly putty flow through the racquet. The full video is one minute long and is made from 60 photos taken at 2 minute intervals so that the video represents 2 hours of elapsed time.

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 (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

# Non-Newtonian silly putty

There are a bunch of different ways to demonstrate shear thickening fluids (eg. Cornstarch in a paddling pool). However, some of these are a little messy.  A tidier method is to use some silly putty. I can’t remember when I first saw this demonstration. I think it was in a chemistry class at UNSW when I was an undergraduate.

Equipment

1. A ball (or 2) of silly putty
2. A tennis or squash racquet.
3. Something to support the racquet so that you can see below it (for example, a glass tank).

Demonstration

The demonstration has 2 parts. In the first part you see the high viscosity behavior; in the second part the low viscosity behavior is shown.

Part 1: Roll the silly putty into a ball and bounce it on the racquet. This can be a bit of a challenge as it is hard to get the silly putty round enough to get it to bounce in a regular manner. You can also throw it at the wall and have it bounce back (though, again, it is hard to get a regular bounce. You may see it go off at an odd angle, so be careful not to hit anyone).

Part 2: Once you have finished various bouncing demonstrations mount the racquet with the strings facing up and place the silly putty ball on the strings. This part takes some patience. If you wait long enough, the silly putty will flow through the strings. It is best to do this at the start of class so that there has been some flowing by the end of the class.

Below are the before and after photos are shown below. The time gap between these two photos is several hours.

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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 (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.