Re: superheated water

["John S. Denker" <jsd@MONMOUTH.COM>]

At 05:58 PM 2/3/00 -0500, Michael Edmiston wrote in part:
> the water heats first at the top; just the
> opposite of water on a stove.

and suggested the explanation:

> Most microwave ovens bring the waves in from the top.

To this I might add another contributing explanation:  The bottom of the
oven is a metal plate.  I'll bet it's always a node in the electric field,
no matter how/where the microwaves are brought in.

Because of this obvious node, I have for years put a glass spacer in the
bottom of my microwave ovens, so that the food sits several CM above the
floor.

---------------

Later, at 09:48 AM 2/4/00 -0500, Michael Edmiston wrote a very nice note
that said in part:
> The fact that an existing gas/liquid interface helps initiate boiling is
> well known.  This is how a boiling chip works.  It is also how cracks and
> fissures work.  It is also well known that once these channels, cracks,
> fissures are full of liquid, the gas/liquid interface is gone and can no
> longer help initiate boiling.

This suggests yet another contribution to the story of why superheating
might be more observable in microwave ovens than on the stovetop:  the M.O.
has a timer.  That means there are many more ways for a sample of water to
be strongly heated (to drive off dissolved gasses), left to cool (and
absorb gasses out of the cracks) and then be boiled.

---------------

A third point:  The story about gas in the cracks depends on the assumption
that the water _wets_ the cracky substance.  If I understand the physics
here (and I'm not 100% sure of that) one can imagine a "super duper"
boiling chip made of a substance so hydrophobic that it would work even in
the absence of gas in the cracks.