Surface and interfacial free energies

Contact angles between solids and liquids of known surface tension can provide an indirect measurement of solid surface free energy, allowing a comparison between different surfaces. Just as liquids have surface tensions or surface energy, so do solid surfaces by virtue of the fact that they are surfaces. Surface tension of solids goes unnoticed because they are usually too rigid to be visibly distorted by the interatomic, rather than intermolecular, forces 

Low energy surfaces HiO (water)t CFRP (heavily sanded) Carbon fibre  

The problem with high-energy adherend surfaces is that atmospheric contaminants are readily adsorbed on them, so reducing the surface free energy of attraction for the adhesive. Kinloch(2) suggests that the polar nature of structural adhesives will lead to displacement of the less polar, often hydrocarbon, contaminants. Polar water molecules, on the other hand, seem less likely to be readily displaced and, in view of their very small size in relation to an adhesive macromolecule, are able to adsorb in vast numbers on the surface. 

Zisman(18) measured the critical surface tension, 7c of a number of different metal and oxide surfaces which were exposed to atmospheres with controlled humidity. It was found that yc for all substrates was lowered to —45 mJ m at 0.6% r.h. and to —37 mJ m at 95% r.h. Gledhill et a/.(19) extended this work to the wettability of mild steel substrates of different surface rugosities and deduced the value of -ysv as a function of relative humidity (Fig. 3.5). Further, the enhanced wetting of gritblasted steel at low humidities was reflected directly in higher joint strengths. Hence 

The huge discrepancy between the magnitude of the attractive forces available for adhesion and measured joint strengths is attributed to insufficient interfacial contact, air voids, cracks, defects and stress concentrations. 

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Parks, G. A. (1984), "Surface and Interfacial Free Energies of Quartz", J. of Geophysical Research 89, 3997-4008. 

In the limits of established contact mechanics models, including those developed by Johnson-Kendall-Roberts (JKR) [5] or by Derjaguin, Muller, and Toporov (DMT) [6], the measured forces are a function of the chemical identity of the contacting surfaces (via the work of adhesion W12 that depends on the surface and interfacial free energies involved). In addition, we need to consider the nature of the medium, the radius of the AFM tip, and also temperature and loading rate. 

Definitions of surface and interfacial free energy, and surface and interfacial tension... 

Interfacial regions actually have a greater depth than one molecular layer in each phase, but since nearly all of the interfacial characteristics are confined to the first layer, these regions are referred to as monolayers. Although surface and interfacial tensions are discussed, these terms may be replaced by surface and interfacial free energies without affecting the argument or equations. 

It is recognized that one requirement for the establishment of strong adhesive joints is that intimate molecular contact occur at the interface. This means that the adhesive must be able to spread over the surface of the substrate, displacing air and contaminants that may be present. The surface and interfacial free energies are of prime importance in determining whether good interfacial contact can occur and in providing a measure of the thermodynamic work of adhesion. " The interactions that can occur at the interface and that are discussed in the different mechanisms of adhesion determine how well the adhesive will adhere to the substrate. 

It was shown in Chapter 1 that the work of adhesion between two phases, x and y, in an inert medium, can be expressed in terms of the surface and interfacial free energies shown in Eq. (3). ... 

SURFACE AND INTERFACIAL FREE ENERGIES 29 2.4.1.3 Additivity of components... 

The thermodynamic work of adhesion, W, required to separate a unit area of a solid and a liquid phase forming an interface across which secondary forces are acting may be related to the surface and interfacial free energies by the Dupre equation. The reversible work of adhesion, Wa, in an inert medium may be expressed by ... 

The interfacial adhesion between nanocomposites, and between nanocomposites and virgin materials, is determined by the surface and interfacial free energies of each respective material. Modification of the surface to create greater surface area, in addition to imparting free radical surface effects to create new surface bonds with introduced chemical functionalities, are primary approaches for maximizing interfacial adhesions. Surface roughening by mechanical or chemical means has been successful for low volume processes. Acid-etch and lasers, for example, have been used as distinct methods for promoting adhesion of nanocomposite materials to enamel. 

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