Sunday, July 08, 2018

Physics in the kitchen: Frying tofu

I was going to title this post "On the emergence of spatial and temporal coherence in frying tofu", or "Frying tofu:  Time crystal?", but decided that simplicity has virtues.

I was doing some cooking yesterday, and I was frying some firm tofu in a large, deep skillet in my kitchen.  I'd cut the stuff into roughly 2cm by 2cm by 1 cm blocks, separated by a few mm from each other but mostly covering the whole cooking surface, and was frying them in a little oil (enough to coat the bottom of the skillet) when I noticed something striking, thanks to the oil reflecting the overhead light.  The bubbles forming in the oil under/around the tofu were appearing and popping in what looked to my eye like very regular intervals, at around 5 Hz.  Moreover (and this was the striking bit), the bubbles across a large part of the whole skillet seemed to be reasonably well synchronized.  This went on long enough (a couple of minutes, until I needed to flip the food) that I really should have gone to grab my camera, but I missed my chance to immortalize this on youtube because (a) I was cooking, and (b) I was trying to figure out if this was some optical illusion.

From the physics perspective, here was a driven nonequilibrium system (heated from below by a gas flame and conduction through the pan) that spontaneously picked out a frequency for temporal oscillations, and apparently synchronized the phase across the pan well.  Clearly I should have filmed this and called it a classical time crystal.   Would've been a cheap and tasty paper.  (I kid, I kid.)

What I think happened is this.  The bubbles in this case were produced by the moisture inside the tofu boiling into steam (due to the local temperature and heat flux) and escaping from the bottom (hottest) surface of the tofu into the oil to make bubbles.  There has to be some rate of steam formation set by the latent heat of vaporization for water, the heat flux (and thus thermal conductivity of the pan, oil, and tofu), and the local temperature (again involving the thermal conductivity and specific heat of the tofu).  The surface tension of the oil, its density, and the steam pressure figure into the bubble growth and how big the bubbles get before they pop.  I'm sure someone far more obsessive than I am could do serious dimensional analysis about this.  The bubbles then couple to each other via the surrounding fluid, and synched up because of that coupling (maybe like this example with flames).   This kind of self-organization happens all the time - here is a nice talk about this stuff.  This kind of synchronization is an example of universal, emergent physics.


Anonymous said...

How'd the tofu turn out?

Douglas Natelson said...

Pretty good. Crispy. Nice garlicky sauce. The key is to start w extra firm tofu, and get as much water out of it as you can (e.g., using paper towels) before frying.

Anonymous said...

Can you do a post on Berry curvature and its significance in solid state physics.

Michael van Dyk said...

Time crystal frying tofu? What's next, quantum-entangled hard-boiled eggs?!

Pizza Perusing Physicist said...

To add to what Anon at 10:12 AM said:

If you want to do a post on the significance of Berry Phases in Solid State Physics, might I recommend doing a specific post on what is, in my opinion, one of the most underappreciated applications of topology to condensed matter physics: The Modern Theory of Polarization, as pioneered by, among others, David Vanderbilt, Rafael Resta and Dominic King-Smith. Seriously, I am constantly astonished by how little popular press this gets.

Douglas Natelson said...

Anon, PPP, thanks for the suggestions, and let me think about how to write about that. And I need to read about the theory of polarization as well - I am only tangentially familiar with it, consistent with PPP’s comment about its comparatively modest profile outside of practitioners.