At 6:41 am ((PST)) Sun Jan 13, 2008, in Digest 2374 Stefan Trethan wrote:
>[snip]
>I also see the wrinkling when put in water, but i believe this is
>different to the thermal expansion/contraction. Paper tends to get a
>lot larger when taking up water.
This is because it is made of fibres of cellulose, a long-chain
carbohydrate molecule, which absorb the water and swell up
in all directions - but only the X & Y directions are particularly
noticeable. Water also loosens the bonds between fibres
leading to permanent effects, such as the wrinkling of a
previously smoothly calendered sheet due to the disturbance
of the fibres being retained on drying.
>[snip] It is different to your fingers getting all
>wrinkly by the way,
Not very different, except that skin has a high content of
structural protein rather than carbohydrate. Proteins
generally have quite a high affinity for water.
>this is because the salty water inside you is
>drawn out into the less salty water on the outside,
Not true, body membranes are quite retentive of electrolytes:
the presence of salt in sweat is only because the glands
actively pump out the ions (and the water follows) not
because the salt leaks from your body.
Otherwise bathing would be a very risky activity rather than
a pleasure, a pastime and a sport.
As a further example of this, consider death by drowning.
(That's not a recommendation of a course of action, btw ;-).
If a child drowns in fresh water and is resuscitated there is
still a possibility of death through electrolyte imbalance
from the amount of water which has passed through the
thin lung membrane, diluting the child's small blood.volume.
If a person drowns in sea water and is resuscitated there
is still a risk of secondary drowning caused by salty
residues accumulating water out of the bloodstream.
In both these cases the water chases the electrolytes,
the "salt" doesn't move: look up "osmosis".
>not because of your skin expanding
>(although it may do that, too).
Nope, that's exactly what it is.
The skin is fairly waterproof, so that we don't leak away,
and relatively dry on the outside since most of us live in a
low RH atmosphere. (There are comfort and health problems
with living in a high humidity environment.)
So the outer layer, high in the protein keratin, has a lower
hydration level than if it were in equilibrium contact with water.
Given prolonged contact with water, there isn't enough real
estate on your body to accommodate the skin's expansion in
area but since your skin is anchored in place it has to wrinkle
rather than sliding off one end or the other.
>I'm told if you bathe in very salty water (e.g. dead sea)
>this effect does not occur.
>I am not certain if this is a myth or true.
The effect can be true and your explanation a myth:
it is the high osmotic pressure of the salt water that
counteracts the hydrophilic nature of the skin protein
restricting any water absorbtion to a level below which
wrinkling is evident.
Regards, LenW
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