Foams can be great examples of mechanical metamaterials.
Consider my shaving cream. You might imagine that the (mostly water) material would just pool as a homogeneous liquid, since water molecules have a
strong attraction for one another. However, my shaving cream contains
surfactant molecules. These little beasties have a
hydrophilic/polar end and a
hydrophobic/nonpolar end. The surfactant molecules can lower the overall energy of the fluid+air system by lowering the energy cost of the liquid/surfactant/air interface compared with the liquid/air interface. There is a balancing act between air pressure, surface tension/energy, and gravity that has to be played, but under the right circumstances you end up with formation of a dense foam comprising many many tiny bubbles. On the macroscale (much larger than the size of individual bubbles), the foam can look like a very squishy but somewhat mechanically integral solid - it can resist shear, at least a bit, and maintain its own shape against gravity. For a recent review about this, try
this paper (apologies for the paywall) or a
taste of this in a post from last year.
What brought this to mind was my annual annoyance yesterday in preparing
what has become a regular side dish at our family Thanksgiving. That recipe begins with rinsing, soaking, and then boiling split peas in preparation for making a puree. Every year, without fail, I try to keep a close eye on the split peas as they cook, because they tend to foam up. A lot. Interestingly, this happens regardless of how carefully I rinse them before soaking, and the foaming (a dense white foam of few-micron-scale bubbles) begins well before the liquid starts to boil. I have now learned two things about this. First,
pea protein, which leaches out of the split peas, is apparently a well-known foam-inducing surfactant, as explained in
this paper (which taught me that there is a journal called
Food Hydrocolloids). Second, next time I need to use a bigger pot and try adding a
few drops of oil to see if that suppresses the foam formation.
1 comment:
Never expected that I would leave this sort of comment here :) There are two things you could do to mitigate the problem you mention: add one or two tablespoons of olive oil, which would also add to the flavor profile, and skim the foam formed with a slotted spoon during the first few minutes of boiling.
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