Re: superheated water

[John Denker <jsd@MONMOUTH.COM>]

At 09:48 AM 2/4/00 -0500, Michael Edmiston wrote a very nice note, of which
the focus was:
> But my main point remains... water being heated in a microwave
> does not set up convection currents because it is heating from the top down.
> This is what I see in three different microwave ovens I have tried.
> Microwaves do not penetrate as far into water and into food as many people
> think they do.

Right.

1) It is also worth keeping in mind that given the typical oven
construction (metal walls) there is guaranteed to be a node at the bottom.

For this reason, I installed a 3cm high spacer (i.e. a flat-bottomed glass
casserole dish turned upside down) on top of the turntable.  It resides
there permanently.

I recommend this for routine cooking, and I especially recommend it if
you're doing experiments.

=========

2) In the course of today's experiments, I stumbled across a spectacular
demonstration of superheating.

We have previously discussed the role of air-filled cracks and scratches as
sites for inhomogeneous nucleation.  It was pointed out that one way to
turn off this effect is to boil some water in your chosen vessel (to drive
off the air), let it cool in situ (to dissolve the air out of the cracks)
and then re-boil it to demonstrate superheating.

Here's another approach.

Warning:  This is _too_ spectacular.
  Plan A) Don't try it.
  Plan B) If you must try it, use the sort of blast protection, fire
protection, and remote manipulation you would use for pyrotechnic synthesis.

You can use _oil_ to turn off the effect of the cracks and scratches.  Put
a small amount of salad oil in a tall flask.  Swirl it around.  Then add a
smaller amount of water.  Clamp everything in place.  Heat it gently with a
fireproof electric heater (not with a flame).  I was able to get the oil to
250 F while there were still macroscopic amounts of water in there.  When
the water boiled, it made loud pop, pop-pop noises and launched geysers of
hot oil several inches above the surface.

Yikes!

Many variations are possible.  Be careful.