From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.physics
Subject: Re: A Physics Student poses some interesting questions....
Date: 5 Jan 1996 07:14:54 GMT

In article <30ecb510.6470933@nntp.interaccess.com>,
rooster@interaccess.com (TS) says:

>>>1.)How can adhesion be used to explain the sticking of objects to a 'smooth'
>>>surface such as someone's forehead?  Are inter-molecular forces 
>>>involved/Friction?
>>Molecules get close, start to get electrostatic attraction between
>>molecules - good example, two very smooth pieces of glass. 
>
>This is a 1st semester fluid dynamics question.  It has nothing to do
>with electrostatic attraction.  When two very smooth flat surfaces (ie
>glass) are pressed together you no longer have atmospheric pressure on
>the area in contact, so the net atmospheric force is to hold the
>surfaces together.  A small amount of water between the surfaces seals
>the 'vacuum' between the plates and will make it very difficult to
>separate them (aside from sliding them apart).  This is NOT the same
>effect when you take a piece of wet paper and slap it against your arm
>or whatever.  In this case you don't have flat, smooth surfaces and
>the cohesive/adhesive forces of the fluid dominate.  Try wetting your
>kitchen counter (assuming it is very smooth and flat) and pressing a
>small, flat piece of glass onto it ... then try getting it off!  It's
>very difficult without sliding it.

I am surprised that they teach this theory in fluid dynamics. Adhesive
forces can amount to several thousand pounds per square inch. Two
flat plates pressed together with a film of water inbetween would
have enormous adhesive strength, far beyond the 14.7 psi of the
atmosphere. The tensile strength of water is variously quoted, but
it is no less than 3,000 psi. You can measure values of this
magnitude by bringing two pistons against a column of thoroughly
degassed water and then measuring the force required to pull them
apart.

Jerry (Ico)

From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.physics
Subject: Re: A Physics Student poses some interesting questions....
Date: 6 Jan 1996 07:05:23 GMT

In article <30edffcf.669645@nntp.interaccess.com>,
rooster@interaccess.com (TS) says:

>glhurst@onr.com (Gerald L. Hurst) wrote:
>
>>I am surprised that they teach this theory in fluid dynamics. Adhesive
>
>It was stated in class and then demonstrated in lab.

That is very sad because the theory is wrong. When you say it was
"demonstrated" in the lab do you mean to say that they quantitatively
demonstrated that two glass (or metal) plates with a film of water
between them required exactly 14.7 psi to pull them apart?  Did your
teacher actually suggest that there was no non-vacuum adhesion
between the water and the plates?  Do you not believe that water
has a tensile strength of several thousand psi and that its adhesive
strength is routinely also in this range? 

Note that the water simply assures good physical contact. Two very
flat glass or metal plates are quite capable of adhering strongly
without water and, for that matter, without surrounding air.

Jerry

From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.physics
Subject: Re: Can you explain this ? Mystery in the Microwave!
Date: 5 Jan 1996 07:23:28 GMT

In article <4cidis$cck@canopus.cc.umanitoba.ca>, lange@mbnet.mb.ca (John
Lange) says:

>Hauke Reddmann (fc3a501@rzaixsrv1.uni-hamburg.de) wrote:
>: No mystery at all. Phase transitions (as liquid water -> vapor)
>: use to need nucleation sites. The microwaves heat the water
>: very equally, and it's getting superheated. Now imagine a dust
>: corn flying in, or the shock of your steps...A nucleation site
>: is born, and WOOSH!, everything goes into steam and boils over.
>: Put a plastic spoon in next time.
>
>Seems impossible that you could have something so smooth and clean.
>
>Plus, the water was on a rotating platform.
>
>But I guess that explains it then.. Thanks.
>
>(I'll not do the dishes from now on)

You might note that the superheating and consequent bumping
of water is much greater if you try to reboil water that
has been recently degassed by boiling. You can bet on 
a miniexplosion if you first heat a cup of water, let it boil
for a while then cool it a few degrees and reheat it to
boiling.

When chemists distill liquids in the lab, they learn to dread
long processes. After distilling most any liquid mixture for an 
hour or so, all the nucleation sites and microbubbles have been
used up and the ebulition becomes a bumpy nightmare unless
fresh boiling chips are added.

Jerry (Ico)

From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.physics
Subject: Re: A Physics Student poses some interesting questions....
Date: 8 Jan 1996 02:09:37 GMT

In article <4cplbe$mf0@pipe11.nyc.pipeline.com>, egreen@nyc.pipeline.com
(Edward Green) says:

>'glhurst@onr.com (Gerald L. Hurst)' wrote: 
> 
>>The tensile strength of water is variously quoted, but 
>>it is no less than 3,000 psi. You can measure values of this  
>>magnitude by bringing two pistons against a column of thoroughly 
>>degassed water and then measuring the force required to pull them 
>>apart. 
> 
>How does one reconcile this with water splashed against a wall breaking
>into thousands of droplets quite easily?  Or with the positive vapor
>pressure of water at room temperature?  The water in the column should
>boil.   
> 
>I suppose this is a non-equilibrium situation then,  wanting nucleation.  I
>also suppose you will say in the first case the water is parting by shear, 
>which is not available to the liquid trapped in the cylinder.  Is that the
>idea? 
> 
>How long can water be maintained under tension?   Is  the tensile strength
>of water in the CRC handbook? 
> 
>[Beware,  I may ask a follow up question on xylem and phloem.] 
 
You got it. Tall trees get water transported to their leaves by
drawing it up in tension far higher than it could be pushed
by atmospheric pressure. This transport relies on the tensile
strength of water.

How long can a column of pure water support tension higher than 
the vapor pressure? Indefinitely.

Imagine a column of deaerated water under tension. Now assume
that a few random collisions produce a few molecules in one 
spot with enough energy to give a local vapor pressure 10 or
even a hundred (unlikely) times the normal vapor pressure. Will
the bubblet continue to grow because of the tension? No, because
the surface tension of water, which is a two dimensional 
reflection of its tensile strength, will generate a few thousand
psi surface pressure and collapse the bubble.

You can break bodies of water in two if you are allowed to exert
a moderate force through a substantiial distance - a form of
leverage. In this case all you have to do is put in enough energy
to account for the increased surface energy. This is easy up to 
a point, but in order to get really fine subdivision, the surface
and thus the energy must increase until you reach the molecular 
level where the cost is about 540 cal/g.

It doesn't take a lot of energy to break a column of water; all
it takes is a lot of force through a very short distance. In a
piston or in xylem there are no free degrees of motion so you
need a lot of force to buy a little energy.

If you think about it, water molecules would like to whiz around
like little independant bullets at about 1000 mph, but intermolecular 
forces bind these particles into a tightly packed mass. It is no
surprise that they stick together so firmly.

Would you like to have a bash at calculating the tensile strength
of water? Try this approach: 

Imagine a small bubble of gas in water; the pressure in this
little sphere is given by 

P = S*pi*2*R/(pi*R^2) = 2*S/R (circumference*tension/area of 
equatoreal plane)

where S is the surface tension in dynes/cm (about 75) and R is
the radius in cm.

Water molecules are of the order of, say, 10^-8 cm, so let's
size our arbitrary bubble at 10^-7 cm, a roughly molecular
level hollow sphere.

The calculated pressure is then of the order

2*80/(10^-7) dynes/cm2 = 1.5*10^9 dynes/cm2 which translates
into about 2*10^4 psi.

We don't really expect 2-digit accuracy from such a spit and 
hope calculation, but it does give an indication of the order
of the tension it would take to allow expansion of the 
nanobubbles (I made that up) and is thus a rough estimate of 
the potential tensile strength of water.

If you run the same calculations for larger bubbles, say in
the microscopic or colloidal range of 10^-4 cm you can kiss
tensile strength goodbye and rely only on simple atmospheric
pressure to hold the column together as in the case of a
simple suction pump.

Tests run with wet, flat, polished plates have little 
trouble with bubbles of any significant magnitude.

I warn any readers that the material in this post consists
of my own ruminations - it is not based on any published
material I can remember at the moment, but some of it may 
be right anyway :)

Jerry (Ico)

From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.chem
Subject: Re: CO2 in coke
Date: 19 Feb 1996 20:10:30 GMT

In article <wpenrose.265.001C22AF@interaccess.com>,
wpenrose@interaccess.com (William R. Penrose) says:

>When you add ice, some CO2 bubbles are nucleated by irregularities on the ice 
>surface.  (If you really want to demonstrate this, add a spoon of sugar or 
>salt to a glass of coke sometime.)  Molecule-sized bubbles add more surface 
>area, so CO2 passes into them and they grow at a rapid rate.

Bill, I think on further consideration you might decide that
"Molecule-sized" bubbles would not help much as nucleation sites,
at least in the case of Coke fizz. Larger bubblets are more likely
to get the job done.

Jerry (Ico)

From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.physics
Subject: Re: A Physics Student poses some interesting questions....
Date: 8 Jan 1996 06:59:56 GMT

In article <4cq5nk$84r@pipe11.nyc.pipeline.com>, egreen@nyc.pipeline.com
(Edward Green) says:

>On the other hand,  I was taught that tall trees overcome this by actively
>maintaining a high *pressure* at their base.  I have seen this in
>textbooks;  it was the conventional wisdom  (circa 1981).   Now,  I am
>gratified that an industrial chemist seems to support my intuition,  but...
> have you checked or followed this literature at all?  I wonder what
>botanists believe today.  
 
It is my understanding that SOME plants do indeed generate high
root pressure, but by no means all. I believe they have run
experiments in which trees which are cut off above the roots
continue to suck up water to heights far above the 32 foot 
atmospheric limit. The last material I read in a biology book 
of uncertain age insisted that the sap in tall trees was under
tension and that the transport was driven by pull at the leaf
level. That book was purloined by my daughter-in-law, who's asleep
at the moment, but I'll post a reference tomorrow.

Jerry (Ico)

From: glhurst@onr.com (Gerald L. Hurst)
Newsgroups: sci.physics
Subject: Re: A Physics Student poses some interesting questions....
Date: 8 Jan 1996 20:41:26 GMT

In article <4cq5nk$84r@pipe11.nyc.pipeline.com>, egreen@nyc.pipeline.com
(Edward Green) says:

>On the other hand,  I was taught that tall trees overcome this by actively
>maintaining a high *pressure* at their base.  I have seen this in
>textbooks;  it was the conventional wisdom  (circa 1981).   Now,  I am
>gratified that an industrial chemist seems to support my intuition,  but...
> have you checked or followed this literature at all?  I wonder what
>botanists believe today.  

As I promised yesterday, I located a book which is very explicit
in describing the major role of tension as the mechanism of
sap transport in tall trees, including phenomena such as
vapor breaks induced by bubbles from freezing sap.

BIOLOGY, Third Edition, Neil A. Campbell (UCLA, Riverside),
The Benjamin Cummings Publishing Company, 1992, 706-708.

This is my daughters Biology text for her current course at 
the University of Texas.

Referenced in the above for additional reading is the following:

PLANT PHYSIOLOGY, L. Taiz and E. Zeiger, same publisher, 1991,
Chapters 3-7 "cover transport on a detailed but accessible level."

Jerry (Ico)