One of the most reliable anthropomorphic rules of thumb in physics is that any system “wants” to minimize its energy, and will move in the direction of the lowest-energy state available. Of course, inanimate physical objects don’t really have desires, so this is really just a statistical-likelihood kind of thing, like the tendency for entropy to always increase. You rarely go wrong, though, using the “want” formulation, at least on a conceptual level: if you’re confronted with some system and want to know what it will do in the future, ask yourself whether there’s any way for it to reduce its energy, and if there is, odds are good that that’s what will happen.
This has all sorts of implications for everyday objects. One of the more aesthetically elegant, if ephemeral, of these is the shape of objects like soap bubbles and water droplets. These are defined by surface tension, which I’ve written about before. Whenever you have a boundary between a liquid and air, or between two different liquids, there’s an energy associated with that boundary and it will shape itself in such a way as to minimize that energy. For a single bubble or droplet, that nearly always means a spherical surface– the energy due to surface tension increases with the surface area, so the system naturally “wants” to move toward the shape with the lowest possible surface area for a given volume.
Except, sometimes it doesn’t. That’s one of the big results from the lab of Prof. Eli Sloutskin at Bar-Ilan University in Israel (which, credit where due, I discovered thanks to this blog post from Doug Natelson). They study the behavior of droplets of oil (a particular kind of alkane with some “surfactant” added) floating in water, and find that at the right temperature, they can get droplets with all sorts of weird shapes, with angles and facets. The image above shows a droplet right around the thickness of a human hair that’s taken on the shape of an icosahedron– the bright spots in the image are the corners of one of the triangular facets making up that shape.
These are not, it’s important to note, droplets that have simply frozen solid. The temperature here is above the melting point of the oil, and the drops are still liquid. They demonstrate this with a clever use of “optical tweezers” (part of last year’s Nobel Prize in Physics), showing that they can drag their faceted droplets around by the boundary, but that the inner parts are still fluid. (There’s a nice video demonstration of this behavior in their paper in the Proceedings of the National Academy of Sciences, which is free to read and has more cool pictures.)
As they change the temperature of the water holding the droplets, they see all sorts of weird behavior, both stable droplets with facets and more transient effects– angular shapes that sprout “tails” from the corners, and droplets that spontaneously thin out in the center and split in three (the triangular droplet in the second image is doing this kind of thing). These are effects that actually increase the surface area of the droplets, exactly the opposite of the sort of thing we normally see with soap bubbles and water droplets.
So, what’s going on here? Has physics been wrong all along about how droplets of stuff really behave? Not at all– the “minimize the energy” principle is still sound, it’s just that the particular system they’re studying has interesting quirks. The energy due to interactions between water, oil, and surfactant molecules at a microscopic scale depends on the temperature of everything, and works out so that the minimum-energy state for a droplet in a particular range of temperatures involves the boundary layer– which is only one molecule thick– “freezing” into a solid lattice, with the molecules arranged in a hexagonal pattern. You can’t cover a sphere with only hexagons, though– this is why the classic soccer ball pattern has a dozen black pentagons interspersed among white hexagons– so you get “defects” where the surface changes from one orientation to another. The hexagonal geometry of the frozen molecules in the boundary requires 12 defects to make a closed surface; these become the corners of the icosahedron. Meanwhile, the surfactant is, as the name suggests, mostly confined to the surface layer, so the oil in the interior remains liquid. Thus, icosahedral liquid droplets with solid facets.
There’s even a range of temperatures in this system for which the usual dependence of energy on surface area reverses. That’s what drives the formation of “tails” and spontaneous splitting of droplets and the rest– the droplets are still trying to achieve the lowest energy possible, it’s just that under these particular conditions, that means making more surface, not less.
I mention these experiments not just because they’re beautifully counter-intuitive and photogenic– though they are both– but because there’s nothing particularly exotic going on here, physics-wise. The central principle of energy minimization is the same as it ever was, and the microscopic interactions between molecules that give rise to the surprising behavior are just boring old electromagnetism (specifically, van der Waals forces). This system that behaves in a way so spectacularly counter to our intuition about how the surfaces of droplets ought to behave doesn’t involve new particles or interactions, just boring old Standard Model physics.
That’s important because the persistent failure to find “new physics” in the form of particles and interactions not covered by the Standard Model of particle physics is regularly used to drive a crisis narrative. Physics is facing some sort of existential danger, according to this, because there hasn’t been any new physics discovered in mumble years (the exact number depends on who you ask). Without new physics, we’ll collapse into irrelevance– or even worse, become boring.
The physics of faceted droplets, to my mind, is a beautiful counterexample to the idea that we’re facing some sort of “end of physics.” Nothing in the oil-and-water system is remotely exotic or “new,” and yet these well-understood elements of “old physics” combine in unexpected ways to produce a nearly endless variety of phenomena. Including seemingly simple systems that run directly against our intuition of how such things ought to behave.
So, contrary to the crisis narrative that gets a bit too much attention, I don’t think we’re in any danger of physics becoming boring. Even if particle physicists never manage a definitive discovery of “new physics,” we’re not close to running out of interesting phenomena emerging from the physics we already have.