The (Surface) Tension is Unbearable

A couple weeks ago I wrote about bubbles, in the stock market and in cider. My cider has been bottled, but I’m still waiting for a taste, so to pass the time here is another post on bubbles, notably on the role of surface tension in the way they behave.

I first noticed surface tension as a kid, playing in the creek behind our house and observing the water bugs. The little dents on the surface of the water seem so much clearer in my memory than I would ever notice now. Maybe it was the wonder of childhood. Maybe it was because my eyes were much closer to the water.

I’m still fascinated by the organization of the bubbles in the photo I posted with the earlier link. They nested nearly perfectly, like billiard balls in a rack. Actually, they did nest perfectly in sections, but those sections had breaks in them, like little continents with fault lines between them. Billiard balls on a flat surface would nest perfectly, but not if the table had a convex shape. The roundness of the glass jug forced the little continental divides.

Like billiard balls in an hourglass …

It is amazing too that, like billiard balls, the little bubbles in the cider are nearly identical in size and shape. The shape, a sphere, is because of the surface tension of the liquid surrounding the gas. A sphere minimizes the surface area for a given volume of gas. (LaPlace’s Law.)

I wonder how my cider is doing.

But what about the size of the bubbles? Why are they all the same size, allowing the perfect nesting? It turns out that people have researched this, asking an even more fundamental question: How is it even possible for those microscopic yeast to produce bubbles? Luckily, someone has researched this very question using nanotechnology and Light Microscopy and shown that CO2 bubbles form inside yeast cells.*

Also, the S&P 500 has fallen 7 percent since its peak a few weeks ago. There. Now I can classify this as an economics blog.

Back to surface tension, the coolness that happens where a liquid’s molecules’ gravitational pull does not balance out at the molecular twilight zone between liquid and gas. (Okay, it is not exactly “between”, but I wanted the Zone reference.)

Did you know there are all kinds of surface tension? One kind is where there is a single surface, like the sheet of water on which the water bugs walk. The bubbles inside a liquid also have a single surface. Bubbles floating in the air, though, have two surfaces, one on the inside of the bubble, and another on the outside. Speaking of which, here is a video of my airlock from a batch of Zombie Hunter EPA that should be ready before Halloween. Did I mention that the properties of liquids affect their surface tension and whether or not they form bubbles? The sanitizer inside my airlock was no match for the vigorous Zombie Hunter fermentation. I don’t fear the foam, but the surface tension carried the liquid outside the airlock, risking oxidation. I had to swap for a blow-off tube until the yeast stopped freaking out.

My airlock bubbleth over…

I also ended up rigging a blow-off tube to my new cider batch, since it seemed likely that the krausen could bubble over, a prediction that turned out to be correct. As it turned out, my blow-off tube went down and back up, leaving some of the foamy liquid krausen at the bottom of the tube. The surface tension properties of this liquid favored big lumbering bubbles.

Bubbles in the blow-off tube

One thing you will notice if you take a video of an airlock in slow motion—as perfectly normal people do (Jim’s Law)—is that the air on the high pressure side will, for a moment, be lower than the level of the liquid, with no physical obstruction keeping the gas from rising. The only thing holding it back is the surface tension on the liquid.

A bubble in the airlock

I’ve been reading up on some mead recipes. It seems this ancient beverage takes a take a year from must to mead. Man, I would never pass the marshmallow test.


* Swart CW, Dithebe K, Pohl CH, et al. Gas bubble formation in the cytoplasm of a fermenting yeast. FEMS Yeast Res. 2012;12(7):867-869. doi:10.1111/j.1567-1364.12004.x

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