# Video of “Buoyancy – floating soap bubbles”

Here are the videos of the “floating soap bubbles” demonstration. The GIF titles link to the full (higher resolution) videos.

Forming the carbon dioxide layer.

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.

# Buoyancy – floating soap bubbles

A slightly more dramatic demonstration of buoyancy than simply having a series of balls float in water is to float soap bubbles on a layer of carbon dioxide.

Equipment

1. Fish tank
2. Large bag of baking soda
3. Gallon jug of vinegar
4. Soap bubble blower

Demonstration

1. Pour the baking soda into the fish tank and then pour in the vinegar. It will bubble vigorously creating the CO2 layer. You need quite a lot of each.
2. Wait for all the bubbling to stop. You may need to mix it up a bit, but not too much so that you do not mix up the CO2 layer too much.
3. Blow the bubbles into the fish tank. If it all worked out then the soap bubbles should appear to float in mid-air.

Analysis

This is a largely qualitative demonstration, though you can make up some numbers to do a simple calculation. Take a 1 cm radius soap bubble floating in the CO2 layer.

If you assume that the bottom half of the bubble is in the CO2 layer and the top half is in the air then you can calculate the buoyancy force acting on the bubble due to the CO2 and the air

FB=  (4/3)πr3 ((½)ρCO2 g + (½)ρair g)

This is balanced by the weight of the air in the bubble plus the weight of the soap.

W= (4/3)πr3 ρair g + 4πr2soapg

where T is the thickness of the soap film. The balance then becomes

FB= W or (4/3)πr3 (½ρCO2 g + ½ρair g)= (4/3)πr3 ρair g+ 4πr2soapg or (½)r (ρCO2  + ρair )=r ρair + 3Tρsoap

T=(½)r( ρCO2  – ρair)/3ρsoap

Substituting material properties (ρCO2=1.98 kg/m3 ρair=1.23 kg/m3 and ρsoap=900 kg/m3 ) into the equation give T=1.4 μm. This is consistent (at least in order of magnitude) with thin-film Interference estimates.

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 Bernoulli, head loss and siphoning water from a fish tank

Here is a video of “Bernoulli, head loss, and siphoning water from a fish tank” demonstration. The full video is here.

I keep cutting my head off in the videos, but the equipment and demonstration is more the point. You will need to turn up the volume for the full videos to here me.

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.

# Bernoulli, head loss and siphoning water from a fish tank

This is a really simple demonstration that I typically use twice a semester. I do it first when we are discussing Bernoulli’s equation and cavitation. I then repeat it when we are covering head loss in pipes. The basic activity of the demonstration is to measure the time taken to siphon water out of a tank.

Equipment

1. Two fish tanks (mine are about 25 x 50 cm and 30 cm deep),
2. a table,
3. a length of tubing that will comfortably reach from the bottom of the tank on the table to the top of the tank on the floor below (the lower tank is only to collect the water so it can be a bucket as long as it is big enough),
4. a tape measure,
5. a stopwatch, and
6. a couple of student helpers.

Put a vertical strip of tape on one edge of the top tank and draw two horizontal lines separated vertically by 10cm so that you can measure depth change.

Demonstration

Set up one fish tank on the table and one on the floor below.

1. Fill the upper tank to the upper line on the vertical scale.
2. Measure the dimensions of the upper fish tank including the depth of water, the distance from the bottom of the upper tank to the top of the lower tank (bucket), the tube length, and the tube diameter.
3. Flood the tube with water in the upper tank.
4. Hold one end of the tube near the base of the upper tank and, with your thumb over the other end of the tube, place it on the lip of the lower tank. This can be tricky and you may want to enlist another student to hold the upper end of the tube in the tank.
5. Take your thumb off the tube allowing the water to flow into the lower tank. Have a student measure the time taken for the water level to drop from the upper line on the scale to the lower line on the scale.

Class discussion

There is only limited opportunity for discussion as the demonstration does not result in any unexpected behavior. The water level drops as the water drains. You can discuss how siphons work and some applications, it is also an opportunity to talk about cavitation which limits the height you can siphon over. Incidentally, the plot of the world war two movie ‘the battle of the bulge’ turns on someone realizing the Germans are running out of fuel because the German prisoners are all carrying hoses to siphon fuel into their tanks.

Analysis

The payoff in the demonstration is in the analysis and the analysis all hangs on drawing the correct diagram (see figure below).

The main points are that the reference height for Bernoulli’s equation (or the energy equation if including head loss) is at the tube outlet and that there is a control volume around the whole upper tank and the tube. The analysis is as follows:

1. Write down Bernoulli’s equation and show that the velocity out of the tube is given by (2gz1)1/2 and that the flow rate out of the tube is Atube(2gz1)1/2.
2. Write down conservation of volume dV/dt+Qout-Qin=0 and note that the volume of fluid in the upper tank is V=Atankh= Atank(z1-H). Therefore, conservation of volume can be written as

This is a separable first order ODE that can be solved for the variation of depth with time. Separating and integrating from the initial to the final depth gives the time taken for the depth change.

You will find that the calculated time is a lot less than the measured time due to the head loss in the pipe.

If you want to do a follow up on the head loss then the work energy equation leads to

.

The main problem is that you do not know the friction factor, f, and f will vary during the flow as the flow rate, and hence Reynolds number, decreases with z1. As a first approximation assume that f is fixed and estimate the Reynolds number based on the average flow rate from the experiment. The final result is that the time is increased by a factor of

.

This gives a much better estimate of the draining 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.

# Video of viscosity of different fluids

Here are some videos of the “viscosity of different fluids” demonstration. The full videos are linked from the GIF titles. you may need to turn the volume up on the full videos.

Equipment

Demonstration

Projection

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.