Why Do Champagne Bubbles Come From One Spot?

APPLIED FIZZICS Field Notes, Edition #1

As some of you know, I am a former physics professor. I left academe in the 1990s and soon thereafter started this company.

I love being an entrepreneur and building a company, but one thing I dearly miss is teaching.

Then it hit me: Applied Fizzics has given me another classroom. Instead of teaching physics to forty students, I now have the opportunity to share a few interesting ideas with thousands of people who, I hope, enjoy asking "Why?" as much as I do.

So I thought I'd start a series of essays that I'm loosely calling Applied Fizzics Field Notes. They'll usually be about something I find interesting—sometimes related to bubbles, pressure, carbonation, or engineering, and sometimes not. If I can leave you knowing one thing you didn't know five minutes ago, I'll consider these notes a success.

For the first edition of Field Notes, let's talk about the thing that Applied Fizzics is all about: bubbles.

The next time you pour a glass of Champagne or beer, I want you to notice something.

The bubbles don't seem to form everywhere equally.

Instead, they seem to rise in tiny streams from just a few spots in the glass. You can see a few bubble streams in the image below.

Why?

After all, the carbon dioxide is dissolved throughout the liquid. Shouldn't bubbles form everywhere at once?

The answer lies in something physicists call nucleation.

Creating a bubble isn't as easy as it sounds. The very first microscopic bubble has to overcome the surface tension of the liquid. That's surprisingly difficult. Carbon dioxide molecules are constantly trying to escape from solution and form a bubble, but without a suitable starting point, most of them simply remain dissolved.

Said another way, it is virtually impossible for the dissolved CO₂ in solution to spontaneously "tear a hole" in the body of liquid and form a bubble, as the force of surface tension is just too great. It's kind of like trying to lift a very heavy weight on the floor that you can't get your fingers under--there's just no way to get started.

You may have noticed this when blowing up a balloon. It's hardest to get the balloon started. Once it's partially inflated, each additional breath becomes easier. That's because when the balloon is very small, its surface area-to-volume ratio is large, and the elasticity of the rubber membrane (its "surface tension," if you will) dominates over the force of your breath.

That's where tiny imperfections come in.

A microscopic scratch in the glass. A speck of dust. Even a tiny cellulose fiber left behind from drying the glass with a towel can trap an invisible pocket of gas when the glass is filled. Once that little pocket exists, carbon dioxide molecules begin diffusing into it, making it larger until it finally breaks free and floats to the surface.

But when the bubble breaks free, it leaves a bit of itself behind, which becomes a little pocket of gas for the next bubble to form on.

Then the process immediately starts over.

The same tiny imperfection produces another bubble.

Then another.

Then another.

That's why those beautiful bubble streams always seem to originate from exactly the same location.

The next time you pour a beer or a glass of Champagne, take a close look near the bottom of the glass. You'll discover that almost every bubble you see is coming from a fixed point on the glass, from a single microscopic scratch or piece of debris that's completely invisible to the naked eye.

Once you notice it, you'll never be able to unsee it.

Physics Corner (a deeper dive for fellow science geeks...)

Want to go a little deeper? Here's how I would explain it to my physics students. If you've already had enough physics for one day, feel free to skip ahead to today's quiz at the end of this email.

Surface tension is often "explained" with a bit of exasperated hand waving as the thing that lets water striders skitter across a pond or that allows a razor blade to float on water, some other similar non-explanation. But surface tension is a little more complicated than that, and since surface tension is the most important factor in determining whether bubbles form, it deserves a better explanation. So let's dig in.

The first thing to understand is that water is a polar molecule. A water molecule consists of one oxygen atom bonded to two hydrogen atoms, at an angle of about 105 degrees between the two hydrogens. So water is a "bent" molecule, not linear or symmetric. Because of the way the electron clouds are pulled asymmetrically in the bent geometry of the molecule, the oxygen side of the molecule is slightly negative compared to the more positive hydrogen side.

The polarity of water molecules makes them attract each other, with the slightly positive hydrogen end of one molecule attracting the slightly negative oxygen end of a neighboring molecule. These temporary intermolecular hydrogen bonds are responsible for water's unusually high surface tension.

In the body of the liquid, these attractions tend to cancel each other out, because these attractive forces come from all around the molecule--up, down, left, and right--equally. But for molecules at the surface of the liquid, say in a glass of water, there is water on one side of the surface, and air on the other, so the forces are not symmetric. At the surface, water molecules have many neighboring water molecules on one side (the water side) and none on the other (the air side). As a result, the attractive forces are no longer completely balanced, which makes the pull asymmetric toward the liquid side.

This tells us something very important about bubble formation--or lack thereof. And droplet formation too.

In a falling droplet of water, a roughly spherical shape results, because the surface of the water is everywhere being pulled inward. It is very much as if the surface of the water has an elastic skin over it, like a balloon. And the smaller the droplet, the more firmly it is held together, just like a balloon.

It's the same with bubbles. The microscopic "seed" of a bubble has a surprisingly difficult job getting started. Because of surface tension, an extremely tiny bubble has a very high internal pressure. The smaller the bubble, the higher its internal pressure. In fact, cutting the bubble's radius in half doubles the pressure required to keep it from collapsing; basically, Laplace's law in plain English. Unless there's a microscopic cavity, scratch, or fiber to shelter that initial bubble, it collapses almost immediately.

In principle, bubbles can form spontaneously in the middle of a perfectly clean liquid (called homogeneous nucleation), but the degree of gas supersaturation required is so extreme that, for everyday beverages like Champagne or beer, it essentially doesn't happen. The degree of supersaturation required for spontaneous nucleation would have to be many times higher than occurs in ordinary Champagne or beer; some sources say 10-20 times higher, although the exact number depends on temperature, pressure, and the liquid involved. Instead, nearly every bubble you see begins life at a microscopic nucleation site, on top of a microscopic bubble that didn't get fully wetted when the glass was filled.

Today's Quiz:

I'm going to end every Field Notes with a quiz or a question. Here's one I asked my students for years:

Suppose you had a perfectly clean, perfectly smooth glass with no microscopic scratches, dust particles, or fibers. Would bubbles:

   A) Form normally

   B) Form much more slowly

   C) Not form at all until the liquid was disturbed

What do you think? Leave your answer in the comments below. In Edition #2, we'll explore the answer—and why it's probably not what you'd expect.

Until next time, stay curious.

Cheers,