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# PLATEAU’S BUBBLES

SOAP BUBBLES make for mathematical wonders and excellent physics. One of the principal proponents of this was 19th-century Belgian physicist Joseph Plateau, whose name is associated with these ephemeral objects. He did other cool things as well.

Joseph Antoine Ferdinand Plateau, 1801-1883, as shown in an early Daguerreotype dated 1843.

One of his early studies concerned perception of a moving image, of what we now call movies. A result of this was Plateau’s phenakistoscope, an animation device that exploits the persistence of an image to trick the eye into seeing motion. The word comes from the Greek, φενακιζειν, phenakizein, meaning “to trick,” “to deceive.”

Mount this disc on a piece of cardboard, spin it on a pencil point while viewing only one portion—a handmade phenakistoscope. For a particularly impressive one from Wikipedia Commons, see http://goo.gl/yCUKK.

Plateau also studied our perception of sunlight and of colors. In particular, he identified the phenomenon of our mind retaining a color image in its complementary hues. A classic example of this is to stare at the dot in this image below for 20 seconds, then close your eyes and identify what you see.

The physics of soap bubbles also attracted Plateau’s attention. For instance, it’s clear that a single soap bubble is spherical simply because a sphere has the least surface area for that given volume. Nature chooses the most efficient enclosure.

This got Plateau thinking about something eventually called the Double Bubble Conjecture (now properly a theorem, as its full mathematical proof appeared in 2002). It states that, for any two separate volumes, a double bubble is the shape of minimal surface area enclosing them. Said another way, any other shape enclosing the same volumes has more surface area.

It turns out this theory has practical application in areas as varied as optics and electrical engineering. However, our interest here is purely geometrical, in describing the part separating the two volumes.

If the two bubbles are of equal volume, then their interface is a circular disc. But what if one bubble is larger than the other? Image from the University of California, Berkeley; see http://goo.gl/2NkGW.

In 1873 (and remember the year), Plateau published his findings in Statique Expérimentale et Théorique des Liquides Soumis aux Seueles Forces Moléculaires. In it, he offered a neat geometrical construction by which the interface of any two dissimilar bubbles can be determined.

You’ll want a pair of compasses and a 30-60-90 triangle, the sort of thing left over from high-school geometry. (One assumes it’s still taught these days….) Otherwise, freehand will do.

From any starting point C, draw line f, then at 60 degrees from it, line g. Repeat the process as shown below with line h at 60 degrees from line g.

Next, on line f, mark the radius of the larger bubble from point C. Call this center A. Do the same for the radius of the smaller bubble on line g. Call this center B.

Now, as is well known to students of Euclid (see www.wp.me/p2ETap-Fg), two points determine a line. So draw this line j determined by A and B, and label as point K its intersection with line h.

Then use your compasses to draw the two bubbles as shown, one centered at A, the other at B.

Last, their interface is the arc of the circle with center K.

Clean up the construction details, and you have the two bubbles and their interface. This construction is so elegant that it brings tears to my eyes.

The two bubbles and their interface, per Plateau’s construction.

I conclude by citing that, traceable to his solar investigations in 1829, Joseph Plateau went totally blind in 1843. ds

© Dennis Simanaitis, SimanaitisSays.com, 2013

### 3 comments on “PLATEAU’S BUBBLES”

1. Bill Rabel
May 26, 2013

The color illusion one experiences after staring at the teal, black and tan flag is one I remember from Scientific American many years ago.

I suspect a related phenomenon: Many people tell of the “green flash” that appears right as the setting sun disappears over the Pacific Ocean. I suspect that they are staring at a single spot on the glowing sun, and when it disappears, they see the complimentary color, green. This is further supported by the fact that no one seems to be able to photograph the green flash.

2. simanaitissays
May 29, 2013

I finally got around to reading my “Science” magazine for 10 May 2013. Wouldn’t you know, it contains a research article titled “Multiscale Modeling of Membrane Rearrangement, Drainage, and Rupture in Evolving Foams.” This takes Plateau’s work into the world of dynamics.
Its summary in the same issue is titled “A Fresh Start for Foam Physics.” It can be seen at http://goo.gl/Yvvcp.

3. Guillermo Valle (@guillefix)
November 12, 2018

You can think of it as a construction that allows the tension forces at the intersection of the interfaces to cancel out (because they are three interfaces an they meet at 120 degrees of each other)!

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This entry was posted on May 26, 2013 by in Sci-Tech and tagged , , .