Surface energy
From Wikipedia, the free encyclopedia
Surface energy quantifies the disruption of chemical bonds that occurs when a surface is created. In the physics of solids, surfaces must be intrinsically less energetically favourable than the bulk of a material; otherwise there would be a driving force for surfaces to be created, and surface is all there would be (see sublimation (physics)). Cutting a solid body into pieces disrupts its bonds, and therefore consumes energy.
If the cutting is done reversibly (see reversible), then conservation of energy means that the energy consumed by the cutting process will be equal to the energy inherent in the two new surfaces created. The unit surface energy of a material would therefore be half of its energy of cohesion, all other things being equal; in practice, this is true only for a surface freshly prepared in vacuum. Surfaces often change their form away from the simple "cleaved bond" model just implied above. They are found to be highly dynamic regions, which readily rearrange or react, so that energy is often reduced by such processes as passivation or adsorption.
As first described by Thomas Young in 1805 in the Philosophical Transactions of the Royal Society of London, it is the interaction between the forces of cohesion and the forces of adhesion which determines whether or not wetting, the spreading of a liquid over a surface, occurs. If complete wetting does not occur, then a bead of liquid will form, with a contact angle which is a function of the surface energies of the system.
Surface energy is most commonly quantified using a contact angle goniometer and a number of different methods.
Thomas Young described surface energy as the interaction between the forces of cohesion and the forces of adhesion which, in turn, dictate if wetting occurs. If wetting occurs, the drop will spread out flat. In most cases, however, the drop will bead to some extent and by measuring the contact angle formed where the drop makes contact with the solid the surface energies of the system can be measured.
Young also developed the well-regarded Young's Modulus which is used to measure the stiffness of a material as well as Young's Equation which defines the balances of forces caused by a wet drop on a dry surface. If the surface is hydrophobic then the contact angle of a drop of water will be larger. Hydrophilicity is indicated by smaller contact angles and higher surface energy. Water has high surface energy by nature; it's polar and forms hydrogen bonds.
In the case of "dry wetting", one can use the Young-Dupree equation which is expressed by the work of adhesion. This method accounts for the surface pressure of the liquid vapor which can be significant. Pierre-Gilles De Gennes, a Nobel Prize Laureate in Physics, describes wet and dry wetting and how the difference between the two relates to the whether vapor is saturated or not.
During the 1950's, Dr. William Zisman, the inventor of the original ramé-hart contact angle goniometer, developed a method whereby a series of contact angle measurements taken with different liquids on the same solid can be plotted to determine what is referred to as the Critical Surface Tension (not to be confused with regular "surface tension"). The Critical Surface Tension may or may not be equal to the surface energy. They will be equal if the interfacial tension is 0 at 0 angle. Zisman's Plot works best on low-energy polymer substrates. The Zisman's plot tool as well as a half dozen other surface energy tools are part of the DROPimage software currently available from ramé-hart for use with their contact angle goniometer tool.
The new One Liquid Surface Energy tool is based on the work of Good and Girifalco who describe the work of cohesion as being dependent on the geometric mean of the surface tension of the two phases.
The (Two Liquid) Surface Energy tool is based on the Fowkes' extended theory. Dr. Frederick M. Fowkes (b. 1915) was instrumental in popularizing concepts that were based on acid-base interactions. The two liquid method requires one polar liquid (such as water) and one apolar liquid such as methylene iodide. Dr. Fowkes died in 1990; he was a professor at Lehigh University.
The DROPimage Acid-Base Tool is based on the work of Dr. C. J. van Oss, et al, and requires three different liquids -- one apolar and two polar. van Oss has shown that the contribution due to acid-base interactions (this includes hydrogen bonding) can be expressed in terms of the product of their electron donor and electron acceptor components. The polar liquids typically include water and either formamide or glycerol while the apolar component may be methylene iodide or bromonaphthalene. Accuracy of this tool can be increased by prodigious sampling. The algorithm calculates the surface energies using Monte Carlo simulation. We're not aware of anyone who has developed a similar tool.
Since most liquids will spread on a high energy surface, it becomes difficult if not impossible to measure the contact angle as it approaches zero. In 1977, Dr. J. Shultz developed a method in which the solid is submerged in a liquid -- such as n-hexane, n-octane, n-decane, or n-hexadecane-Liquid Surface Energyntal Chamber. Since n-Hexadecane, for example, has a rather high melting point, the Environmental Chamber with Proportional Temperature Controller provides an ideal work environment
