Kulim Kht Surface Free Energy

Determination of the surface energy of materials by using contact angle measurements 
What is surface free energy?

 Let's take a piece of a material and try to divide it into two parts. To accomplish this task, some energy needs to be spent in order to overcome interatomic forces holding the parts together. If the separation of the parts is carried out so gently that no deformation is induced in the bulk material, the energy spent can be associated with the excess energy of the two new interfaces formed. In theory, assuming that the above action can be reverted, the same amount of energy should be regained upon putting the two parts back together. In real systems, the splitting up a material into smaller parts always induces stresses and deformations in each of the parts formed, and therefore some energy is dissipated as heat and some is stored as elastic deformation. As a consequence, the accurate experimental determination of surface energy is only possible for isotropic liquids, in which bulk stresses quickly relax and the excess surface energy coincides with the surface tension. For solids, the true value of the surface energy cannot be measured. When applied to solids, the term "surface energy" acquires a totally different meaning and can be viewed as an "adhesive" parameter characterizing the affinity of the surface to other materials. The higher the surface energy of a solid, the more energy is gained upon bringing this surface into contact with other materials. Interfacial interactions play a key role in all multicomponent materials irrespectively of the number and type of their components or their actual structure. Recognition of the role of the main factors influencing interfacial adhesion and proper surface modification may lead to significant progress in many fields of research and development, as well as in related technologies [1].

Why measure surface free energy?  Ability to measure the surface energy of various materials is essential for ensuring compatibility between the given base material and the top coating one wishes to apply onto it or other materials one wishes to attach to it. The most straight-forward applications include matching a paint to a substrate or matching an adhesive formulation to the materials one expects to be glued together.  In many industrial applications, special tools are used for modifying the surface energy of various materials at will. Thus, to promote adhesion, the surface can be "activated" by plasma treatment or chemical etching, whereby its energy is increased, or on the contrary, it can be passivated by lubrication, silylation, or hydrogenation (see Figure 1). For instance, to ensure good adhesion of printing ink to polyethylene or plastic film, such as used in the production of packaging bags, the surface of the film is plasma-treated prior to printing. The same method can be used to enhance the adhesion of the polyethylene-polypropylene laminate applied to the surface of paperboard, which is an essential operation in the paperboard converting technology, or the adhesion and endurance of a teflon layer on the surface of kitchen items. Dewaxing a metallic surface is needed prior to lacquering to ensure good adhesion of lacquer to metal. Waxing a car does not only adds glance but also reduces the adhesion of dirt to the lacquer. And after teflon-based waxes have become available on the market, care needs to be taken that a paraffin-based wax not be applied over a teflon-based one, while doing it the other way around would work fine.