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February 2006

Surface Energy

One of the most frequent tasks accomplished by our ramé-hart goniometers is to measure surface energy. With our software-driven models (200 and up), surface energy can be quantified using a number of different tools.

But first, let us review what surface energy really is. Two hundred years ago the famous British Scientist 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.

In addition to his studies in surface energy, Young was also quite an expert on light and vision systems. His works recognize the wave nature of light and he is the first to understand that light travels as transverse waves.

Thomas Young (Library of Congress)

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.

Both DROPimage Standard and DROPimage Advanced include a number of surface energy tools. One is the work of adhesion tool. In order to use this tool, you must have at least one contact angle measurements but more are better.

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 is included with both versions of DROPimage and measures the standard linear plot.

The new One Liquid Surface Energy tool in DROPimage is based on the work of Good and Girifalco who describe the work of cohesion as being dependant on the geometric mean of the surface tension of the two phases.

The (Two Liquid) Surface Energy tool in DROPimage is based on the Fowke's 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 passed about 10 years ago; 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 prodigous 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 -- and then the contact angle of a drop of liquid -- typically water -- is measured. This type of experiment is accomplished using the DROPimage Solid-Liquid-Liquid Surface Energy tool and requires either a ramé-hart Environmental Fixture or Environmental Chamber. Since n-Hexadecane, for example, has a rather high melting point, the Environmental Chamber with Proportional Temperature Controller provides an ideal work environment for this type of study.

If you are interested in receiving a more technical paper which includes expanded information on the above concepts complete with formulas and a bibliography, please contact us.


We appreciate your comments, your continued business and look forward to working with you in the near future. 



Carl Clegg
Director of Sales
Phone 973-448-0305
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