Hmmm. I don't think you have this quite right. According to your
description, why wouldn't the water just be forced out of the gap?
The sheets are held together because of the surface tension of the laminar
fluid layer. Pulling the sheets apart would seriously increase the free
surface area of the fluid which surface tension seeks to minimize. I think
it is pretty clear that this has nothing to do with the bonding in glue
joints, since, after all, the glue in short order ceases to be a fluid.
Cohesion (-> surface tension) is important, but adhesion is the key.
Roughing the surfaces, as most glue manufacturers recommend, exploits this
aspect. However, glue films should be thin, just not too thin. Have you
ever been impressed by the strength of a cured gob of glue?
However, it is far to easy to over-generalize when speaking of glue.
On 6/30/04 1:37 PM, in article
4TCEc.20861$ firstname.lastname@example.org, "Agki Strodon"
In the course of learning to work wood, I suppose it would do me good
to learn about adhesives. Any recommended tomes on the subject? I
don't need "all" the science, but I would like to see coverage similar
to that presented in Flexner or Jewitt's finishing books.
You don't really need a book if you read Chapter 9 of the Forest Products
Lab Handbook (link below). It outlines the properties of a properly
prepared surface and discusses, in detail, the fact that glue bonding is
almost entirely a mechanical process rather than a chemical one. It sounds
like several people in this thread may benefit from reading Chapter 9. You
can download it - FOR FREE (the best part)- right here:
On Wed, 30 Jun 2004 21:28:48 -0400, "Howard Ruttan"
Thanks for the link - and the excellent discussion as presented by all
parties. This is the wreck at its best.
Happy 4th of July all of you USAmericans!
Going "golfing" of all things today.....ugh.
Depends on the glue and the wood. Actually, I was thinking of non-porous
materials when I wrote the quoted text--forgetting the context of the
thread. Most wood glues work through 'wetting'-- the glue penetrates
several cells deeply in the porous wood, forming mechanical (interlocking)
(and also chemical) bonds. Thus smooth well mating surfaces are optimal as
clamping then very effectively helps force the glue into the wood. A
uniform thin layer is desirable. Machine and tool marks obviously diminish
this and create distinct pockets.
With very dense, oily wood, porosity is minimal. The story is different.
Epoxies do not depend on deep penetration and require different preparation.
Slightly roughing the surface is sometimes desirable; see for example,
I think I will say no more having already fallen into the generalization
trap. Mr Ruttan's suggested reference,
<http://www.fpl.fs.fed.us/documnts/FPLGTR/fplgtr113/fplgtr113.htm is a good
On 6/30/04 10:57 PM, in article Xns9518E983227A9MacCool@184.108.40.206, "Mac
I'm not getting this right, I don't think but if you are asking why water
between the two sheets of glass doesn't just run out, here's why:
Three reasons are most important.
First, the intermolecular attraction between the positive hydrogen atoms in
the polar water molecules and the negative oxygen atoms in the polar silicon
dioxide molecules of the glass is greater than that between the water
molecules themselves. This results in the "wetting" of the glass and the
disappearance of the water's surface. The interaction keeps the water in
place on the glass.
Second, there is air pressure at the edges of the glass pushing against the
very thin water surface that occurs between the pieces of glass. Since the
pressure is the same all around the resultant is a force pushing the water
towards the center of the glass. It doesn't go there because of the
considerations of "wetting" (as discussed above) pulling the water and
forcing its dispersal over all the glass surfaces.
Third, there is also a thin surface of water at the edges of the water
lamina forming a concave meniscus. This is the only place where surface
tension exists in the system because it's the only water surface, there
being no water surface inside the lamina. This is also a result of the
first reason above.
This phenomenon works with oils to. Or no fluids. and not using glass
but any very flat surfaces. I would suspect if you reduced the air
pressure in a chamber and measured the pll away forces of the two
plates, nothing would change. Gosh it is even noticable at HD seperating
sheets of plywood. What are Johansson blocks?
Agki Strodon wrote:
There is NO surface tension inside the fluid layer. There cannot be because
there is no surface inside the water layer. The water wets the glass and
the interactions occur between water and glass molecules. Surface tension
occurs only at the edges of the lamina because surface tension is defined as
the intermolecular attraction between the molecules of the fluid at the
boundary of the fluid. Surface tension works against wetting and the
superiority of the SiO2 - H2O interaction over the H2O - H20 interaction
accounts for the wetting of the glass. If you smear paraffin on the glass
and then put water on it, the water will bead up and retain surfaces (and
surface tension) because the water-water attractive forces are greater than
the water-paraffin attractive forces.
To the force of air pressure holding the plates together, we can add the
resultant force of all them damned SiO2 - H20 attractions. I think, though,
that it's rather small compared to the air pressure. We could set up an
apparaturs to measure it.
Quite so! Now for the biggest question in physics - one that Feynman and
the physics faculty at Cal Tech could not answer- why does a stick of
spaghetti almost always break into three or more pieces when you bend it by
I'm sure he knew that it was the harmonic vibrations of the
first 'snap' which multiplied in the pieces and facilitated
further breaks. </swag>
California's 4 Seasons: Fire, Flood, Drought, & Earthquake
http://www.diversify.com NoteSHADES(tm) glare guards
Hmmm, interesting. My guess partially agrees with Lar. I'd say the harmonic
from the initial break travels down the remaining stick until it reaches the
second flex point. When the harmonic and the flex point meet, the spaghetti
Now, just how *do* brass screws permanently disappear in sawdust? Why is it
That, sir, is due to the scavenger elf (with his brass magnet) who
is hiding under all that sawdust and who absconds with them.
"Not always right, but never uncertain." --Heinlein
http://www.diversify.com Wondrous Website Design
If the silicone molecules inter-attract with the silicon dioxide molecules
more strongly than they intra-attract amongst themselves, no. The plates
would stick through wetting of the glass. They may be easy to separate and
that is a way of finding the strength of the interactions.
As to whether anyone has done it, I dunno.
I must have missed the gist of the original post but here's some more
If the glass plates are dead smooth, as in optical flats, and put together
with nothing in between, they will be forever joined in a short while. No
air space at all, there is a vacuum,, and the glass, an amorphous liquid,
yep liquid, will grow together by molecular migration. This also applies to
steel gage blocks as Johansen or Hoke blocks, but it takes a little longer
for them to become a unit. Been there, done that.... This has no parallel
to woodworking joint gaps, though...
As for wood, cleanly cut, flat, preferably planed and not sanded, wood bonds
better than rough wood any day.
No, a (slightly) roughened surface holds better than a smooth one because
there is more surface area for the glue to bond to.
Doug Miller (alphageek-at-milmac-dot-com)
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