Example of an MCTP course log distributed to the listserv and the ensuing discussion. (Chem 121 is part of the introductory physical science course at College Park)

Date: Sun, 13 Nov 1994 08:52:00 EST
From: Tom O'Haver (College Park) Thomas_C_O'HAVER@UMAIL.UMD.EDU
Subject: Journal - Soap bubbles

Personal Journal
CHEM 121/122

Tom O'Haver


This report was delayed by the pressures of the NSF PIs meeting and the National Panel of Visitors meeting last week.

In a previous class, one of the student asked why bubbles and suds are formed by soaps and detergents. Simple enough, I thought. Our textbook did not have much to say about that, so I had promised to look into it in some other reference material. In my search, I ran across an interesting example of an apparent discrepancy in book explanations. Accordingly, I brought into this class two current books that attempted to explain soap bubbles. Both of the explanations are couched in terms of "surface tension", which our textbook did describe and which I amplified with a chalkboard drawing. (This would been a perfect time to let the students "float" paper clips and thumb tacks on the surface tension film of water, and then sink them by adding a drop of detergent, but I just didn't think of that in time - we'll try it later).

Anyway, here are the two statements (verbatim) from the two references:

James Birk, Chemistry, Houghton Mifflen, 1994.
Page 399: "It is possible to blow quite large bubbles as long as the surface tension of the liquid is sufficiently high. The higher the surface tension, the stronger the film that is the bubble's surface, and the larger the bubble can be before it bursts".

Dorling-Kindersly Science Encyclopedia, 1993.
Page 128: "Soapy bubbles can be stretched into strange shapes because soap weakens the surface tension of water".

I asked the students if they felt that these two explanations were contradictory. They all felt that they were. Then I asked them which one was the "correct" explanation, and exactly half of the students chose Birk's explanation and half chose the Dorling-Kindersly explanation!

We know from common experience that agitating pure water will make bubbles, even without soap, but the bubbles are very short-lived. That soap weakens the surface tension of water can be demonstrated experimentally, for example by observing how small metal objects supported on a surface tension film are sunk by adding soap. So that suggest that pure water a too high a surface tension to form stable bubbles. On the other hand, bubbles need SOME surface tension to hold their shape.

So what do you think? Are soap bubbles too complex a system to attempt to understand? The students readily accepted the notion that a sphere is the shape that has the smallest surface area for a given volume. Perhaps this is just something they have heard before. Then there is the idea that surface area would tend to be minimized by the intermolecular attractive forces - how can that be demonstrated or proved? Of course, a spherical drop of water has a much smaller surface area than a hollow sphere (bubble) that contains the same mass of water. So the bubble represents a sort of metastable state or local minimum, suggesting that there is an optimum range of surface tension for good bubble formation - too high and it collapses into a drop and too low and the bubble won't form in the first place or won't hold together if it does form.

I obviously need to research this in greater detail. Anyone out there have any good references?
Date: Mon, 14 Nov 1994 09:33:10 -0500
From: "Kenneth R. Berg" (College Park) krb@MATH.UMD.EDU
Subject: Re: Journal - Soap bubbles

I don't have any references, or even any clever ideas, but why should I let that stop me. It is a very interesting question. Suppose you blow up a balloon by pumping in a certain quantity (measured of course in mass, not voulume) of air. Different ballons would expand to different sizes, depending on the stretchability of the rubber. I believe that it would be fairly simple to write the mathematical relation between the size of the resulting balloon, the mass of the air, and the tension in the rubber. Possibly such an equation would show that any bubble of the type we usually see is impossible when using water. Or, more likely, we would see that a water bubble would require such enormous internal air pressure to counteract the tension that we could reasonably expect instability. Explanations come in various forms, and mathematicians tend to look for equations. Keep me posted on what you find.
Ken Berg
========================================================================= Date: Tue, 15 Nov 1994 12:41:05 -0500
From: Jay Zimmerman - Towson State University E7M2ZIM@TOE.TOWSON.EDU
Subject: Soap Bubbles

Dear Tom,

Soap Bubbles have a fascinating geometry and are a good way to interest students in Riemannian Geometry. I was at an MAA Workshop at Frostberg last year and Frank Morgan at Williams College spoke on the topic. He has written several books on the subject and he is an advocate of using hands-on methods to teach Geometry. This strikes me as a good topic for an interdisciplinary course. The two books of his that I have are:

Geometric Measure Theory - Academic Press

Riemannian Geometry, A Beginner's Guide - Jones and Bartlett Publishers
Sincerely, Jay Zimmerman
Date: Tue, 15 Nov 1994 15:20:00 EST
From: Tom O'Haver (College Park) Thomas_C_O'HAVER@UMAIL.UMD.EDU
Subject: Re: Soap Bubbles

Thanks for the tips, Jay. I checked VICTOR (the on-line card catalog) and found that both books are owned by the System, but that both are checked out at the College Park campus (which I guess means this is a hot topic, right?)
AUTHOR(s):       Morgan, Frank.
TITLE(s):        Geometric measure theory :  a beginner's guide /  Frank

                 Boston :  Academic Press,  c1988.
                 viii, 145 p. :  ill. ;  24 cm.
                 Includes indexes.
                 Bibliography: p. 135-137.

(material deleted)
AUTHOR(s):       Morgan, Frank.
TITLE(s):        Riemannian geometry :  a beginner's guide /  Frank Morgan ;
                   illustrated by James F. Bredt.

                 Boston :  Jones and Bartlett Publishers,  c1993.
                 119 p. :  ill. ;  24 cm.
                 Jones and Bartlett books in mathematics
                 Includes bibliographical references and indexes.

OTHER ENTRIES:   Geometry, Riemannian.

Owners: UMCP

UMCP    ENGIN  BKSTKS    STATUS: Due: 01/05/95
CALL #: QA611.M674 1993
Date: Tue, 15 Nov 1994 16:47:21 EST
From: Joe Hoffman (Frostburg State) j_hoffman@FRE.FSU.UMD.EDU
Subject: Re: Journal - Soap bubbles

Soap bubbles would seem to provide a wealth of activities that could lead into all sorts of interresting "highlights and sidelights" of science. There are several toys and kits available that you might want to employ. Cenco has a nice kit that enables you to make all sorts of interesting soap bubbles in the shapes of the "Platonic Solids". See Cenco #32239K on page 338 of the 94 catalogue. The classic 1902 book by C. V. Boys is also available from Cenco on page 338. It is entitled "Soap Bubbles and the Forces that Mould Them".

I have a toy called I beleive The Magic Bubble Wand that can make enormous bubbles several meters across. It is available in most toy stores and always delights.
Date: Wed, 16 Nov 1994 08:15:58 EST
From: Gurbax Singh (UM Eastern Shore) GSINGH@UMES.UMD.EDU
Subject: Re: Journal - Soap bubbles

There is a good discussion of the surface tension, at the molecular level, in `PHENOMENAL PHYSICS BY CLIFFORD E. SWARTZ', pp 350 - 356, in the 1981 edition. Swartz attempts to answer questions similar to those raised in the MCTP-email discussions. The discussion of the surface tension in the text is at a typical `Freshman' level.
Gurbax Singh
Date: Fri, 18 Nov 1994 17:04:55 -0500
From: Jay Zimmerman - Towson State University E7M2ZIM@TOE.TOWSON.EDU
Subject: Soap Bubbles Again

Dear Tom,

I have looked again at my books from Frank Morgan. They will probably not be too helpful, except for the bibliography. Frank has several articles and notes that seem to be more helpful. I suggest you e-mail him at fmorgan@williams.edu for his notes and more references. He also has a reference "Soap Films and problems without unique solutions" in Am. Sci. 74 (1986), 232-236.

Sincerely, Jay Zimmerman