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Adhesion is the surface attraction between the surfaces of
two bodies. The term is sometimes used to denote the
tendency of two adjacent surfaces, which may be of
different chemical compositions, to cling to each other,
whereas cohesion is used to refer to attraction between
portions of a single body. For example, if a sheet of glass
is lowered into water and withdrawn some water will cling
to the glass (adhesion) but the rest will be pulled back
into the main body of water (cohesion).
The force of attraction (adhesion) is attributed to
electromagnetic interactions produced by fluctuations in
the distribution of electrons in the molecules of the
facing surfaces. The distance between the molecules of the
facing surfaces is a determining factor in the amount of
force exerted. A surface that may appear smooth to the
naked eye actually may be too rough to hold its molecules
close enough to a facing surface to produce an
electromagnetic bond. Gauge blocks, pieces of metal used
for taking accurate measurements, have such smooth surfaces
that their facing surfaces can be made to stick to each
other by twisting them together.
In the human body, when tissues or organs that are usually
separated unite or grow together, the process is called
adhesion. This abnormal occurrence may come about after
inflammation or during healing after a surgical operation.
The forces of adhesion are also used to make some very
useful products. Adhesive is a substance used to bond two
or more surfaces together. Most adhesives form a bond by
filling in the minute pits and fissures normally present in
even very smooth surfaces. Adhesive bonds are economical,
distribute the stress at the bonding point, resist moisture
and corrosion, and eliminate the need for rivets and bolts.
The effectiveness of an adhesive depends on several
factors, including the resistance to slippage and
shrinkage, malleability, cohesive strength, and surface
tension, which determines how far the adhesive penetrates
the tiny depressions in the bonding surfaces. 

vary with the purpose for which they are intended. Such purposes now include the increasing use of adhesives during surgery. Natural adhesives have been replaced in many uses by synthetics; but animal glues, starches, gums, cellulose, bitumens, and natural rubber cements continue to be used in large volumes. Organic adhesives derived from animal proteins include glues made from collagen, a constituent of the connective tissues and bones of mammals and fish; blood albumen glue, used in the plywood industry industry; and glue made from casein, a protein contituent of milk, are employed in wood bonding and in paint. Vegetable adhesives include starches and dextrins derived from corn, wheat, potatoes, and rice used for bonding paper, wood, and textiles. Gums such as agar and algin when moistened provide adhesion for such products as stamps and envelopes. Synthetic adhesives, used either alone or as modifiers of natural adhesives, perform better and have a greater range of application than the natural products. Most of them form polymers, huge molecules incorporating large numbers of simple molecules to form strong chains and nets that link surfaces in a firm bond. Thermosetting adhesives, which are transformed into tough, heat resistant solids by the addition of a catalyst or the application of heat, are used in such structural functions as bonding metallic parts of aircraft and space vehicles. Thermoplastic resins, which can be softened by heating, are used for bonding wood, glass, rubber, metal, and paper products. Elastomeric adhesives, such as synthetic or natural rubber cements, are used for bonding flexible materials to rigid metals. As you can see basic areas of chemistry, like the forces of adhesion, can be put to use for making products needed in the real world. 


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