My Favorite Thing About Soapy Surfaces, I

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I’m Matt, also still a grad student at Cornell. I wanted to start of with a post about a problem which has motivated (sometimes very indirectly) much of my interest in PDEs, geometry and category theory (!)

Let’s talk about elastic membranes sitting in Euclidean 3-space \mathbb{E}^3. Any smooth surface in \mathbb{E}^3 inherits a complex structure which is given by the cross-product with the unit normal, so we might as well assume that we are always working with a conformal parametrization f : D \rightarrow \mathbb{E}^3 with D \subset \mathbb{C}.

To understand minimal surfaces geometrically, it is helpful to first think about curves. Imagine holding a rubber band along a curve and pinning it at its endpoints. If we let go, each point will get pulled on by its neighbors and move off some distance in the normal direction. The force felt by each point is proportional to the curvature. In other words, for curves in \mathbb{E}^3 the curvature vector \kappa_p is exactly the force felt by p due to the tugging of its neighboring points.

Now what if we had a whole surface made of elastic instead of just a curve? Each normal plane at p intersects the surface in some curve — the curvature of this curve tells us how much p is being tugged on by its neighbors in this particular direction. So to find out the total influence of neighboring points on p, we just need to add up all these curvatures. It isn’t too hard to convince yourself that the total force felt by p is the mean curvature vector H_p = \frac{1}{2}(f_{xx} + f_{yy}) N_p. So for the surface to be sitting still like a nice soap film and not wiggling all over the place, the Laplacian f_{xx} + f_{yy} had better be zero everywhere; this property characterizes minimal surfaces.

We can say all that in a nice invariant way by utilizing the * operator (precomposition by a 90 degree rotation): the Laplacian is given (more-or-less) by the operator d*d, so f defines a minimal surface iff d*df = 0. Can we interpret this condition more geometrically? You bet!

Let’s start with a minimal surface S and a closed, contractible curve \gamma \subset S. \gamma cuts out a disc-shaped subset X \subset S. Now meditate on this: how does S - X tug on X? You should be able to convince yourself that the force felt by X is \int_\gamma \eta ds where \eta is the unit tangent vector field which is perpendicular to \gamma. So in order for X to stay put, it had better be true that \int_\gamma \eta ds = 0 for all contractible loops \gamma. In other words, *df is a closed form (d*df = 0).

Now if df is closed and *df is closed, then cos(\theta) df + sin(\theta) *df certainly is as well: we could replace \eta with the unit tangent vector at angle \theta from \gamma and all calculations in the previous paragraph would still be true. So we could start with a conformally parametrized minimal surface S = f(D), replace df with df_\theta =  \cos(\theta) df + \sin(\theta) *df and then integrate df_\theta to get a new surface S_\theta. Everything that applied to the old surface applies to these new ones as well — in particular, they are all minimal!

This is weird: minimal surfaces don’t come alone, but in S^1 families! What is even stranger is that each surface in a family has the same Gaussian curvature (K) and the same mean curvature function (H =  0) as the others. For a generic surface in \mathbb{E}^3, K and H are enough to determine the surface up to Euclidean motion, so there is something truly exceptional going on here.

Well, that was an awful lot for a first post and I didn’t even get to the really neat and weird stuff yet. I think I’ll break here — there are ten thousand other incredible properties of minimal surfaces that I could go in to, but the internet might run out of space first. But before you leave, check out the S^1 family associated to the catenoid up close and personal: http://www.foundalis.com/mat/helicoid.htm

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3 Responses to “My Favorite Thing About Soapy Surfaces, I”

  1. Greg Muller Says:

    I was watching the movie you linked to, and I noticed that the 45 degree mixing of the helicoid and the catenoid is also pretty cute. Is there any chance it is also embeddable? I can’t imagine that it is, but I couldn’t figure out the problem.

  2. mnoonan Says:

    I think that the helicoid is the only other surface in the catenoid family that is embedded. As is the usual state of affairs in differential geometry, the local problem is somewhat reasonable to solve and the global problem is essentially impossible so I’ll have to go off and stroke my beard for awhile before I can prove that the catenoid and helicoid are the only cases…

  3. Surfaces and Spinors « The Everything Seminar Says:

    […] this blog, I wrote a post about how minimal surfaces don’t come alone, but actually live in one-parameter families. The critical observation is this: if is a conformal parametrization of a minimal surface then […]

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