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Within a solid mass, molecules are surrounded by identical molecules with like energy measurements. At the surface of the solid, however, molecules are likely to be in contact with air and sometimes liquids or other masses of differing energy measurements. As a result, surface energy measurements are always different from those found within the mass of a solid.
All three phases — solid, liquid, and air — have measurable surface energies and surface tensions. Molecules have both what are called "adhesive drives" and "cohesive drives" that govern the ways in which they interact with the molecules around them. Adhesive drives seek to bind with other types of molecules, while cohesive drives seek to bind with like molecules. If cohesive drives are more prominent, liquid molecules will only bead up on a surface as they resist other molecules; the reverse is true if adhesive drives are more prominent.
Surface energy, surface tension, and surface density tension measurements analyze wetting behavior between liquids and solids when scientists are testing for the thermodynamic properties of solids. When liquid molecules on a solid surface bead up in cohesion, scientists can measure the contact angle of those molecules on the solid surface. This contact angle measurement is taken with an instrument called a goniometer, which determines to what degree cohesion or adhesion are in the ascendancy. Two other forces can influence surface energy in this case. If the surface is rugged or known as hydrophobic, this causes liquids to bead at higher angles; conversely, when a surface is hydrophilic, a liquid drop may spread out to cover as much of the surface as the liquid molecules can reach.
Surface energy measurements are usually taken at extremely high temperatures, when solids react by slight movements under the stress of heat but volume is nearly constant. Liquid surface energy measurements are taken by using what is called a "liquid membrane stretch" of the surface area. One method, called the Dynamic Wilhelmy Method, involves immersing a solid in a liquid that has been previously measured for surface tension, and then measuring the wetting forces as the solid is released from the liquid. Another method, called the Powder Contact Angle Method, is used when scientists need to know the absorption levels and surface energies of porous materials and powders.
Practical applications of these surface energy measurements and test procedures benefit the development of industrial and consumer products. Metals coated with polymers rely on scientists' knowledge of adhesion and cohesion for their strength and durability. Materials' surface energy measurements are taken for oxidation and chemical bonding applications. In lithography, inks must be applied in a manner that allows image areas to absorb inks and non-image areas to remain free of inks; surface energy research has refined these processes.
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