Surface tension & wettingin dental materials

Introduction :

Cohesion, along with adhesion (attraction between unlike molecules), helps explain phenomena surface tension .

Adhesion :

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.

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The strongest joins are where atoms of the two materials swap or share electrons (known as ionic bonding or covalent bonding, respectively).

Ionic bond : Ionic bonding is a type of chemical bond that involves the electrostatic attraction between oppositely charged ions.

Covalent bond : A covalent bond is a chemical bond that involves the sharing of electron pairs between atoms.

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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.

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Cohesion

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is the action or property of molecules sticking together.

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.

Surface tension

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.

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Cohesion & surface tention

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surface tension examples

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1 : walking on water

2 : floating of needle

Wetting

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, plain surface.

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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.

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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.

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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 tooth surface.

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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.

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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 mechanical interlocking.

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

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Thank you

dr.mohsen karimi