Surface tension, soap bubbles, and footballs

Surface tension can be visualized in a bunch of different ways. There are some nice videos of insects walking on water that use surface tension for support. However, these are hard to use as the basis for any sort of simple calculation. An easier way of demonstrating surface tension, that you can bring to class, is by using soap bubbles.

Equipment

  1. Soap bubble bottle
  2. American football / Rugby ball

Photo Jan 20, 9 03 00 AM

Demonstration

The demonstration is simply blowing bubbles around the room. The more the merrier.

Analysis

I start by writing down the equation for the change in pressure across a curved interface.

ΔP=σ(1/r1+1/r2).

where rand r2 are the two radii of curvature for the surface. The football comes in useful here because it can be used to illustrate that a curved surface can have two different radii of curvature. Looking end on the football is circular whereas from the side the surface is curved, but with a much larger radius of curvature.

football

The soap bubbles can be used to remind students that the pressure is higher on the inside of the curve. To form the bubble you blow onto the flat soap film which curves due to the stagnation pressure from the air jet coming from your mouth. Hence the pressure in the bubble is greater than the atmospheric pressure outside the bubble.

I then do a simple calculation of the pressure inside the bubble. Assuming that the radius of the inside of the bubble is R and the thickness of the soap film is T then you can write that the pressure in the soap film is

Pf= σ(1/(R+T)+1/(R+T))=2 σ/(R+T).

The pressure on the inside of the bubble (Pi) is given by

Pi= Pf + σ(1/R+1/R)= 2 σ/(R+T)+2 σ/R

Provided that R>>T this reduces to

Pi= Pf + σ(1/R+1/R)= 4 σ/R

(note that there is a typo in Crowe et al. 9th edition for the above result, they drop the example for the 10th edition). It would be easy to make numbers up for this calculation but I stick with the general result for this example. Finally, this result is also easily obtained using the standard force per unit length interpretation of surface tension though I prefer the pressure change formula for a curved surface.

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. If you do not wish to register with twitter or wordpress to get updates then send me an email and I will add you to the list I send update notifications to.

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2 thoughts on “Surface tension, soap bubbles, and footballs

  1. Pingback: Video of surface tension, soap bubbles, and footballs | Teaching Fluid Mechanics

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