Videos of “More on surface tension – floating Ping-Pong balls”

Here are the videos of the “More of surface tension – floating Ping-Pong balls” demonstration. The full videos are linked from the GIF captions.

Cup partially full with ball drawn to the edge of the cup

Video Apr 22, 11 17 01 AM 00_00_30-00_00_36

Cup over full with ball drawn tot he middle of the cup. 

 

Video Apr 22, 11 18 39 AM 00_00_27-00_00_34


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.

More on surface tension – floating Ping-Pong balls

This is not a new demonstration but rather an extension of a previous post “Surface tension – floating Ping-Pong balls” The equipment is just a ping-pong ball, a cup and some water.

Photo Apr 22, 11 16 28 AM

In the previous demonstration the ball was placed in the middle of the cup and was dragged to the side by a surface tension imbalance. Soap was then added as a surfactant to reduce the imbalance and allow the ball to float near the middle of the cup. In this extension (see, for example, various Martin Gardner books 1,2) the first part of the demonstration is the same as in “Surface tension – floating Ping-Pong balls“. However, in the second part, the ball is held in the middle by changing the curvature of the water surface rather than reducing the surface tension.  The water surface curvature is changed by overfilling the cup so that the water surface curves up above the lip of the cup. In this case the minimum area occurs when the ball is centered in the cup. Again, see John Bush’s lecture notes here for a more formal discussion of surface tension.


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 of “Surface tension Bernoulli, and soap bubbles II – bubble size”

Here are some videos from the “Surface tension Bernoulli, and soap bubbles II – bubble size” demonstration. The full videos are linked from the GIF headings

Blowing slowly for a large bubble.

slow

Blowing faster for smaller bubbles.

fast

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.

 

Surface tension, Bernoulli, and soap bubbles II – bubble size

The first demonstration on surface tension and soap bubbles was highly qualitative and the soap bubbles were merely a prop for talking about surface tension. There was a calculation of the gauge pressure in the bubble (Pi) which is given by

Pi= 4 σ/R

where R is the bubble radius and σ is the surface tension of the soap film, though this result was not taken any further. It is, however, possible to go a little further once you have gone over Bernoulli’s equation with your class.

Equipment

One container of soap bubble fluid and wand

Photo May 26, 11 55 45 AM

Demonstration

The demonstration is similar to the previous one. You blow bubbles. Except this time you vary how hard you blow. If you blow very slowly you can get bigger bubbles whereas if you blow a little harder the bubbles get smaller. For a given wand shape there is a limit to the range of bubble sizes you can get. I have found it hard to get really small bubbles. I have also found it hard to get the really big bubbles to pinch off. They often just burst.

Analysis

The analysis requires the result above on the pressure in the bubble and Bernoulli’s equation.

When you blow into the loop in the bubble wand the air stream from your mouth stagnates on the soap film creating high pressure point there. This produces a curvature in the soap film. Therefore, there is a balance between the pressure in the cured film and the stagnation pressure. Eventually, the film pinches off and a bubble forms with the pressure in the bubble related to the stagnation pressure from your breath.

Recalling that the stagnation pressure is given by

P= ½ρV2

one can write an expression for the bubble radius as a function of the air speed (related to how hard you blow)

R=8σ/ρV2.

That is, the harder you blow (larger V) the smaller the resulting bubbles.

It would also be possible to leave this result unstated, run the demonstration and ask the students to discuss the relationship between pressure, surface tension, and radius. If the demonstration is done well you should be able to lead the class to discover that there is an inverse relationship between the pressure and radius. One could then introduce the exact relationship which they may have a greater ability to comprehend having observed it first.

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 “Surface tension – Parting of the pepper sea” demonstration

Here are the videos of the “Surface tension – Parting of the pepper sea” demonstration. The higher resolution videos are linked from the GIF captions.

Adding the pepper and trying to part it without the surfactant

p1

Parting the pepper with soap under your nail

p2

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.