Another Pleasant Experiment

I clearly remember the first time I read the introduction to The Physics of Foams by Denis Weaire and Stefan Hutzler. The book begins with what they call “a pleasant experiment”, pouring a glass of beer, watching the bubbles rise, merge and form a foam. They describe the process so vividly, blending science with wonder and everyday life.

"Cheers!" they wrote. "The rest, so to speak, is geometry."

What a fantastic way to start a scientific book!

A foam in a glass of beer is not just a physical phenomenon; under certain circumstances and with the proper company, it can be an ephemeral, poetic moment, much like other moments we wish we could hold into but cannot.

But not all foams are the same. Take the foam of a cappuccino, for example, it might linger a bit longer than a lager’s foam. It’s perhaps more resilient, creamy and stable. Maybe. It depends on the formulation and processing method, I guess.

Today, I’d like to replicate Weaire and Hutzler’s experiment, but in my own way. Instead of a glass of beer, I’ll make a cappuccino. Not just because it’s early in the morning and I’d rather not start my day with alcohol, but because, as a researcher working with foams, I feel a special connection to the creamy, long-lasting foam of a cappuccino.

The Pleasant Experiment: Making a Cappuccino

It is Tuesday, 8 a.m. I start by brewing my coffee in an Italian coffee pot. As the dark, rich liquid fills the pot, the aroma wraps around me like a warm hug. And then something strange happens. I feel… something. I assume it’s endorphins, triggered by the scent of coffee, but for all I know, it could be something else entirely. Some deep, ancestral memory of mornings past or simply a response from years of caffeination. Maybe one day I’ll figure it out.

I pour the coffee into my favorite cup and reach for my French press, my absolute favorite tool for making espumita at home. I pour in some warm milk and start pressing the plunger. The fine mesh of this French press traps air and forces it into the milk, creating a velvety foam.

Now, I should mention that I have tested different French presses, and this one is simply the best. Why? Because the mesh at the top of the plunger is finer than most, meaning it creates smaller bubbles, leading to a more uniform and stable foam. A larger mesh would let bigger air pockets form, creating an uneven foam structure that collapses more easily. And by moving the mesh to the very top of the plunger, I’m maximising the amount of air incorporated into the milk, making the foam even denser.

Finally, I sprinkle some cocoa powder on top, completing the experience. And now, I take my cappuccino back to bed, breaking every productivity rule I know.

I pause for a moment before taking a sip, just admiring the foam. Smooth and creamy, holding its form in some areas, collapsing in others. And then I have a thought:

"The first thing that comes to my mind when looking at this foam will tell me if I am, above all, emotionally intense, a nerd, or just ridiculous".

The thought arrives. And now I know.

And it’s fine.

But why is this foam so stable? And how does it differ from beer foam? Let’s think about it together.

The Science of Cappuccino Foam

Liquid foams are structures made of gas bubbles dispersed in a liquid matrix. And what is a matrix, you ask? It’s just a fancy way of saying ‘the stuff holding everything together’, like milk in cappuccino foam, or like that well-run orchestra, where every musician plays their part at their right timing.

But what makes cappuccino foam so special? Let’s break it down:

1. Surface Tension: The Bubble’s Armor

To create a stable foam, it is desirable to lower the liquid surface tension. But what is surface tension? Consider the surface of a liquid like an invisible and very thin blanket on top of the liquid that is created by the molecules at the surface of the liquid being pulled inward by the attraction of the molecules below them. This inward pull tries to minimise the surface area, making it difficult for air bubbles to form and stay there (bubble formulation increases the surface area).

A better way to visualise this is to think about blowing bubbles in water vs. in water with soap. If you blow through a straw into water, large bubbles will form and escape quickly. This is because water’s high surface tension pulls the molecules tightly together, making it hard for air to stay trapped. You shall not pass.

However, if you add soap, the surface tension decreases, making it easier for air to mix into the liquid. This happens because the soap molecules rush to the surface, inserting themselves between the tightly packed water molecules. This weakens the surface tension, making the blanket of the liquid easier to penetrate.

Milk behaves similarly, but instead of soap, milk proteins (mainly casein and whey) act as soap molecules, reducing surface tension and making foam formation easier. Besides, this creates a flexible layer around each bubble, reducing the pull of surface tension and making it easier for bubbles to remain stable.

2. Drainage: The Slow Collapse of Foam

Once a foam is formed, gravity immediately starts pulling the liquid downward through the thin films between the bubbles. This is called drainage, and it weakens the foam over time.

However, in cappuccino foam, milk proteins and fats make the liquid slightly more viscous, which slows down the drainage. This is why cappuccino foam feels creamy—it holds onto its liquid content much longer than beer foam.

3. Coalescence: The Bubble Merger

Bubbles love merging. And who doesn’t, with the right bubble? If two bubbles meet and their thin film breaks, they combine into a larger bubble—a process called coalescence. This is one of the main reasons why foams collapse.

But in cappuccino foam, proteins in the milk form a flexible but strong layer around each bubble, preventing them from merging too easily. This keeps the foam structure stable for a longer time.

4. Coarsening: When Bigger Bubbles Take Over

At the same time, while the liquid leaves due to gravity, the air inside is moving too. The air slowly diffuses between bubbles, reshaping the foam from the inside out. It’s a dynamic structure, one that seems stable but is constantly changing.

Over time, gas diffuses from smaller bubbles to larger ones, causing the foam’s structure to change. This is called coarsening—the slow evolution of a foam toward instability.

In cappuccino foam, the protein-stabilised bubbles resist this process better than beer foam does. That’s why beer foam, which lacks strong stabilizers, starts looking uneven and patchy much faster than cappuccino foam.

5. Geometry: The Hidden Order in the Foam

Foams may seem random, but their bubbles organize themselves in predictable ways.

• When first formed, bubbles are round.

  
  

• As liquid drains, they get squeezed together, becoming polyhedral, like tiny geometric sculptures.

  
  

A Philosophical Difference

We can look foams in different ways. Liquid foams can fleet. They rise, form beautiful bubbles, and then collapses into nothing. It’s poetic, a symbol of impermanence and change.

But they also hold their ground, resist collapse, defying gravity for as long as possible. It’s a quiet symbol of resilience.

As I sip my cappuccino, I think of Weaire and Hutzler’s pleasant experiment. Their beer foam was poetry in motion, a fleeting masterpiece of physics. My cappuccino foam, while different, is no less fascinating. And if this cappuccino foam tells me anything, it’s that above all, I am bubbly, but with some structure.