Surface energy measurements analyze how molecules interact with each other on the surface of solids, liquids, and air. Adhesive and cohesive drives affect wetting behavior, and contact angle measurements are made with a goniometer. Surface energy measurements are used in the development of industrial and consumer products, such as polymer-coated metals and inks for lithography.
Within a solid mass, molecules are surrounded by identical molecules with similar energy measurements. On the surface of the solid, however, the molecules are likely to be in contact with air and sometimes with liquids or other masses of different 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 motions” that govern the ways they interact with the molecules around them. Adhesive drives try to bind to other types of molecules, while cohesive drives try to bind to similar molecules. If the cohesive units are more prominent, liquid molecules will stick to a surface only when they resist other molecules; the reverse is true if the adhesive units are more obvious.
Measurements of surface energy, surface tension and surface density tension analyze the wetting behavior between liquids and solids when scientists are testing the thermodynamic properties of solids. When liquid molecules on a solid surface build up in cohesion, scientists can measure the contact angle of those molecules on the solid surface. This contact angle measurement is done with an instrument called a goniometer, which determines how far the cohesion or adhesion is on the rise. Two other forces can affect the surface energy in this case. If the surface is rough or known to be hydrophobic, this causes liquids to bead at higher angles; conversely, when a surface is hydrophilic, a drop of liquid can extend to cover the entire surface that the molecules of the liquid can reach.
Surface energy measurements are usually made at extremely high temperatures, when solids react with slight motions under the heat stress but the volume is nearly constant. Liquid surface energy measurements are made using what is called “liquid membrane stretching” of surface area. One method, called the dynamic Wilhelmy method, involves immersing a solid in a liquid that has previously been 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. Polymer-coated metals rely on scientists’ knowledge of adhesion and cohesion for their strength and durability. Surface energy measurements of materials are performed for oxidation and chemical bonding applications. In lithography, inks must be applied in such a way that image areas absorb the inks and non-image areas remain ink-free; surface energy research has refined these processes.
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