"Leon" wrote in message
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"Agki Strodon" wrote in message
news:aKqEc.2199

But that's a different physical phenomenon.

Is it? The water like glue displaces the air that is between the 2
surfaces. If there is a spot in a glued up joint that has an air space,
the
joint is week at that point


I don't want to be seen as a pedantic jerk by you chaps even though my
students often called me one but:

Glass sheets are held together by water between them because the water has
driven out the air and occupies the space between the two sheets. The
sheets themselves are then forced together (and against the water lamina) by
the air pressure that pushes them together at about 14 lb/sq in. The only
mediating pressure is the very slight air pressure on the thin lamina of
water between the sheets of glass that forces the water to push outward
against the sheets by pushing inward on the water. Surface tension has not
much to do with it unless there is an interactive attractive force in the
glass (or whatever) that pulls the water molecules toward the sheet but this
would be very slight in almost all cases of materials. Air pressure is the
force both holding the sheets together and, to an extremely small degree,
pushing them apart by pushing on the water lamina. If the glass sheets are
uniformly flat and measure, say 10"x10", each sheet is 100 sq in. The air
pressure produces 14 lb/sq in on each sheet so the total is [14 lb/sq in x
100 sq in/sheet x 2 sheets] - [the pressure on the water from the sides], or
2800 lb. (I ignored the very very slight pressure on the water lamina
because it close to zero). 2800 pounds is the amount of force needed to
separate the sheets if there's no other factor. We don't need that much
because when we slide the sheets over each other we reduce the amount of air
pressure push against them at the places where they are together. Finally,
by sliding the sheets enough, the holding pressure is reduced to an easy
force to apply.

Glues work differently and don't depend on air pressure to hold things
together. There are essentially two aspects, the adhesion of the glue to
the surfaces and the internal bonding strength between the molecules that
make up the glue itself. The solvent for the glue (water, alcohol,
whatever) is just a carrier. The glue dries by losing its carrier and (in
some cases) the chemical nature of the glue itself changes when that
happens. When it dries, it bonds to the things being glued and internally
inside the dab of glue between them. In the old days, both these bonds were
relatively weak and either the glue's internal bonds broke or the bond with
the material broke. Some glues produce bonds to materials that are stronger
than the material itself is internally AND the internal glue intermolecular
strength is also stronger.

Spaces in glued up joints produce weakness because theres no glue in the
spaces to form a bond.

Agkistrodon