Cohesion, along with adhesion (attraction between unlike
molecules), helps explain phenomena surface tension .
Adhesion is the tendency of dissimilar particles
or surfaces to cling to one another
Chemical adhesion :
Two materials may form a compound at the join.
The strongest joins are where atoms of the two materials
swap or share electrons (known as ionic bonding or covalent
Ionic bond : Ionic bonding is a type of chemical bond that
involves the electrostatic attraction between oppositely
Covalent bond : A covalent bond is a chemical bond that
involves the sharing of electron pairs between atoms.
A weaker bond is formed if a Hydrogen atom in one
molecule is attracted to an atom of nitrogen, oxygen,
or fluorine in another molecule, a phenomenon
called hydrogen bonding.
Chemical adhesion occurs when the surface atoms of two
separate surfaces form ionic, covalent, or hydrogen bonds.
is the action or property of molecules sticking
Mercury in a glass flask is a good example of the
effects of the ratio between cohesive and adhesive
forces. Because of its high cohesion and low adhesion
to the glass.
The cohesive forces between liquid molecules are
responsible for the phenomenon known as surface tension.
The molecules at the surface do not have other like
molecules on all sides of them and consequently they
cohere more strongly to those directly associated with them
on the surface.
surface tension examples
1 : walking on water
2 : floating of needle
enable the adhesive bonds between the adhesive and the
surface, the adhesive must first wet the surface; in other
words, it must be applied in the liquid form (as a solution,
dispersion, or hot-melt).
A measure for the wet ability of a surface is the angle of
contact that forms between a drop of liquid and a smooth,
relationship between surface energy, wet
ability, and contact angle
Atoms in the center of a material are surrounded by other
atoms, while atoms located on the surface of a material are
only interacting with the atoms closer toward the center.
a surface with high surface energy reacts with atoms
(molecules) from the surrounding in an attempt to form a
surface with lower energy level.
This phenomena explains why metal surfaces become
oxidized over time, because the metal oxide has a lower
surface energy than the clean metal surface.
the reason is simply that the clean metal surface has a
higher surface energy than the water, and therefore water
spreads easily in the surface.
When we try to bond a resin to a tooth surface, the tooth
surface has often a rather low surface energy. The low
surface energy has been achieved over time as a result of
nature's strive to decrease energy.
If we place a resin on such a surface, the resin may not
spread if the surface energy of the resin is higher than the
tooth surface. To avoid problems with spreading we etch the
By doing so we remove the passivating layer and
make the surface much more active. At the same
time it also becomes rough, something that will
enhance micro-mechanical retention.
After the surface has been etched, rinsed and dried,
its surface energy is higher and the resin will spread
easily in an attempt to coat the surface and
decrease the surface energy.
This will in turn mean that when the resin is placed,
the surface energy of the resin might be higher than
the surface energy of the saliva coated tooth
surface and the resin will not spread.
Under such conditions the saliva coated surface
needs to be reetched to remove the saliva film
before the resin is placed.
Intimate wetting will optimize the chance for
chemical interaction (chemical bond formation) and
To achieve such a condition, the surface energy of the
adhesive should be low and the surface energy of the
adherent should be high. The latter require that the bonding
procedure is conducted in as clean environment as possible