I always start my day with a cup of coffee. Sometimes it’s a regular coffee, and on good days I get to treat myself to a latte. The one thing I enjoy in a good latte is the foam. It’s surprising that the texture of foam makes a liquid taste so different. So, I recently turned my Foldscope towards asking – what make a good foam.

What makes a good foam?

As I write this post; I am standing next to my coffee machine trying to “froth” milk. And I always wonder how long is long enough? Usually I will inject hot steam into a cup of milk. The longer you do this; the more fine grained the foam gets. Foam is just a lot of bubbles in milk. Too large a bubble; and they will settle and pop out quickly, loosing the texture I was after at the first place. to get a better sense of what this foam looks like, what’s the smallest size scale of these bubbles?

Imaging milk froth under a Foldscope:

I mounted a drop of milk foam into a glass slide and inserted that in my Foldscope. The classic first image I get is called a “wet foam” where the thin films that make foam still have enough fluid. You can easily see fluid flow (milk fat droplets) flowing in this complex 3D network.

It’s fascinating to consider “foam” as a 3D complex microfluidics manifold. It’s incredibly hard to make 3D microfluidics – necessary for vascularization of tissue – foam provides a perfect platform to self-assemble 3D microfluidic networks.

What’s incredible is the fact that “fat droplets” in milk act as flow racers – so you can watch the fluid flow inside. It’s clear to me; that this is a “plug flow” with the boundary condition being a slip (water-air interface); so the effective resistance in this self-assembled microfluidic manifold is significantly different (smaller) as compared to a physical 3D microfluidic manifold.

Note to self: explore use fat droplets as flow tracers.

Here is another video of wet foam:

Next, we turn our attention to protein thin films!

Now, what happens when you let your coffee sit for a while. Say a few hours. Not only does the foam “dry” our by draining all the fluid due to gravity, but also the proteins in milk coagulate in a way to form a strange “protein thin film”. An indication of this is visible in these Foldscope images of dry foam.

It’s fascinating to see that the complexity of 3D structure is preserved a well

Now, this dried up material is mostly milk proteins, fat all dried up and condensed into thin films. If you see carefully; the actual facets of these polygons are actually thin films that are visible when they pop.

Now imagine, how complex is the structure of my latte when I take a sip. All this matters for the perfect texture. Next trip you sip your coffee – pull out your Foldscope and wonder (and wander) into the world of foam.

Cheers

Manu

Ps: this was the most yummy Foldscope for me. Proof attached.