Supercritical

I don’t know how it came up, but while we were at the beach Hero ended up explaining sublimation to my little sister S.  Hero and I both took a semester of thermodynamics in college, but there was plenty we couldn’t remember, so we ended up looking stuff up on wikipedia.

For those who were sensible enough to not take thermo, sublimation is when a substance goes straight from solid to gas.  The “smoke” pouring off dry ice is one example of this; freezer burn (where some of the ice in the frozen food evaporates) is another.  The reverse of sublimation is deposition.

(My brain likes the symmetry of the connotations and secondary meanings of those two words.  Sublimation means purification or refining, which feeds into the psychological definition of diverting energy from one impulse to another, usually more socially acceptable, use.  Deposition literally means to make or lay down a deposit, or to depose a ruler, but the first definition that springs to mine is the legal one about giving a testimony.  So the opposite of being refined and ennobled is something complicated and legal. Lovely.)

Anyway, as we were staring at a phase diagram and discussing the various phase shifts, I noticed an area I didn’t know much about: the supercritical region.

Carbon dioxide phase diagram

The supercritical region – the extremes of pressure and temperature – is where weird things happen.

Well, reasonably weird at any rate.  Supercritical fluids are essentially both gas and liquid at the same time.  That is weird.  But apparently it’s useful, because it dissolves things like a liquid, but evaporates like a gas, so it gets used to extract caffeine from coffee beans, among other things.  It gets pumped in, dissolves some of the caffeine, and then evaporates without a trace.  Or, since it also acts like a liquid but has no surface tension to drag on delicate tissues, it gets used for drying delicate biological specimens or aerogel structures: supercritical carbon dioxide is pumped in, replaces the water that was wetting the item, and then evaporates when the pressure lifts, leaving the specimen dry and perfectly undistorted.

The latter process is called supercritical drying.  The phase shift from liquid water to gas creates all sorts of surface tensions that damage or distort the item being dried, which can be a real problem.  Supercritical fluid allows the water to go around that shift, leaving it unharmed.

It made me think of Yeshua and the grace He represents.  We need to be perfect if we are going to enter God’s rest, because God is perfect.  But the normal transition from imperfect to perfect creates huge amounts of tension – we can’t do it and stay sane.  But Yeshua became God and man (like the supercritical fluid being both liquid and gas) to allow us to get around that barrier.

Neat, isn’t it?

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