Contact angle adhesion thermodynamics

Combination of Eq. 7 or Eq. 8 with the Young-Dupre equation, Eq. 3, suggests that the mechanical work of separation (and perhaps also the mechanical adhesive interface strength) should be proportional to (I -fcos6l) in any series of tests where other factors are kept constant, and in which the contact angle is finite. This has indeed often been found to be the case, as documented in an extensive review by Mittal [31], from which a few results are shown in Fig. 5. Other important studies have also shown a direct relationship between practical and thermodynamic adhesion, but a discussion of these will be deferred until later. It would appear that a useful criterion for maximizing practical adhesion would be the maximization of the thermodynamic work of adhesion, but this turns out to be a serious over-simplification. There are numerous instances in which practical adhesion is found not to correlate with the work of adhesion at ail, and sometimes to correlate inversely with it. There are various explanations for such discrepancies, as discussed below. 

These comments should not be interpreted to mean that measures of wettability are useless at predicting adhesion. They do seem clearly to indicate that contact angles and critical surface tensions reported for wood are not necessarily thermodynamic quantities or well-defined material parameters. Because most contact angles are dynamic values, they should be interpreted with caution and considered as relative measures of adhesion, for which the absolute scale is yet unknown. Further, we need to keep in mind that although wetting is necessary for adhesion, it may not be the limiting factor in many real situations. 

Consider a non-reactive system consisting of a binary liquid alloy A-B and an oxide substrate such as AI1O3 at constant temperature. A simple statistical thermodynamic model has been developed (Li et al. 1989) to predict the contact angle and the work of adhesion isotherms, 0(XB) and Wa(XB), from the known values of contact angles... 

I. B. Ivanov, B.V. Toshev and B.P.R. Radoev. On the Thermodynamics of Contact Angles. Line Tension and Wetting Phenomena in Wetting, Spreading and Adhesion, J.F. Padday. Ed., Academic Press (1978) p, 37. 

Equation (645) shows that contact angle is a thermodynamic quantity, which can be related to the work of adhesion and interfacial free energy terms. When 6 values are small, the work of adhesion is high and considerable energy must be spent to separate the solid from the liquid. If 0 = 0°, then W L = 2yv if 0 = 90°, then W L = yLV, and if 0 = 180°, then W1L = 0, which means that no work needs to be done to separate a completely spherical mercury drop from a solid surface (or a water drop from a superhydrophobic polymer surface), and indeed these drops roll down very easily even with a 1° inclination angle of the flat substrate. 

Assuming that no surface electrification is involved, the above group of equations are the basic thermodynamic relations for describing the equilibrium contact angle and wetting phenomena. In so far as details of molecular structure of the substances and surfaces play an important part, these purely thermodynamic equations would not be expected to suffice to permit us to describe the wetting, spreading, and adhesion of liquids on solids. 

In future studies, we propose to add to the contact angle measurements (which probe only 5-10 A of the layer) XPS and FTIR spectroscopy analysis, in order to understand the kinetics of the reaction in the interior of the pulsed plasma polymer thin film. Once quantitative elucidation of the reactivity of the pulsed plasma polymer thin film has been fuUy accomplished, adhesion strength measurements will be performed and correlations between adhesion parameters and thermodynamic parameters wiU be explored. This wiU be the subject of a further paper. 

Previous work done in this laboratory (3) measured the thermodynamic work of adhesion, W, of epoxy resin (Dow Chemical DER-331), of water, and of methylene iodide on single filaments of E glass coated with dilute (0.2 to 2.5 percent by weight) aqueous solutions of silanes. was calculated from contact angle (0) measurements using the formula... 

In summary, this chapter presents the basic thermodynamic principles and the work of adhesion that quantitatively characterize surfaces of materials. Laboratory techniques for surface characterization have been described, which allow an understanding of the chemical and physical properties of material surfaces. Empirical equations have been described for calculating surface tension (energy) of solid polymeric surfaces using contact angle and other parameters. 

Adhesion is defined thermodynamically by the change in free energy when two materials come into contact. The work of adhesion in the contact angle experiment has been defined [44] by Eq. (2). 

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