The Spreading Coefficient

Young s equation is usuaUy found to be a very useful and adequate means of describing wetting equihbria in most circumstances. However, it is sometimes found useful to define another term that indicates from a thermodynamic point of view whether a given liquid-sohd system will be wetting (0 = 0°) or nonwetting (0 0°). Such a term is the spreading coefficient, S. 

For a spontaneous process (such as spreading) to occur, the free energy of the process must be negative. In terms of surface free energies, then, one can write the relationship 

FIGURE 17.8. An interesting phenomenon sometimes encountered in spreading is the autophobic effect. When the liquid is first applied to the surface, it has a small contact angle however, as adsorption occurs at the solid-liquid interface, the orientation of the adsorbed molecules makes the interface more hydrophobic, leading to a larger contact angle. 

At first sight, the autophobic phenomenon mentioned above would seem to be an exception to Equation (17.9). However, if a relationship like that in (17.8) is invoked, then 

An alternative to the relationship in Equation (17.10) is the so-called equilibrium spreading coefficient 

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If a mass of some substance were placed on a liquid surface so that initially it is present in a layer of appreciable thickness, as illustrated in Fig. IV-1, then two possibilities exist as to what may happen. These are best treated in terms of what is called the spreading coefficient. 

According to Eq. FV-9 (with = 1), the spreading coefficient for a liquid of lower surface tension to spread on one of higher surface tension is always negative. Demonstrate whether this statement is true. 

Derive the expression (in terms of the appropriate works of adhesion and cohesion) for the spreading coefficient for a substance C at the interface between two liquids A and B. 

For hquid systems these surface energies expressed in mj/m are numerically equivalent to the surface tensions in mN/m(= dyn/cm). If the adhesive is phase 1 and the release coating is phase 2, then the spreading coefficient, S, of 1 on 2 is as given in equation 2. 

The Spreading process is governed by the spreading coefficient S defined as in equation 4 (30) where c is the surface tension of the foaming medium, C the surface tension of the defoamer, and C. the interfacial tension between them. 

Complete wetting, i.e. spontaneous spreading should always be sought to maximize adhesion. This condition occurs when, with reference to Fig. 4, it is not possible to satisfy the horizontal force balance, i.e. ys > Vl + Ysl- The thermodynamic driving force for the spreading process is the spreading coefficient. 

As long as the liquid actually wets the rough surface, a less contentious approach linking the roughness factor to the extent of contact would seem to be via the spreading coefficient as shown in Eq. 20 and summarised in Table 1. If air is trapped within pits by the liquid, a composite surface is produced. 

When the fiat substrate is not wet (5 < 0), the wetting of a rough surface requires that the spreading coefficient... 

Spreading Coefficient. The spreading coefficient is defined as the difference of the surface tension of the foaming medium cry, the surface tension of the defoamer aj, and the interfacial tension of both materials a /. 

It can be readily seen that the spreading coefficient S becomes increasingly positive as the surface tension of the defoamer becomes smaller. This indicates the thermodynamic tendency of defoaming. 

However, the value of yab needs to be considered as the equilibrium value, and therefore if one considers the system at nonequilibrium, then the spreading coefficients would be different. For example, the instantaneous spreading of benzene is observed to give a value of Sa/b as 8.9 dyn/cm, and therefore benzene spreads on water. On the other hand, as the water becomes saturated with time, the value of water decreases, and benzene drops tend to form lenses. The short-chain hydrocarbons such as hexane and hexene also have positive initial spreading coefficients, and spread to give thicker films. Longer-chain alkanes, on the other hand, do not spread on water (e.g., the Sa/b for C16(hexadecane)/water is -1.3 dyn/cm at 25°C. 

In the following table are given a few values, chiefly from the data of Harkins, of the spreading coefficients for a number of liquids which spread,upon water and for liquids which do not spread but form lenses, in this latter case the value of cr [o + o- z is evidently negative. 

Further investigation has shown that the spreading coefficient varies with the temperature, a point which we shall refer to later (p. 87). 

Harkins WD, Feldman AJ (1922) Films - spreading of liquids and the spreading coefficient. J Am Chem Soc 44 2665-2685... 

Combine your results to calculate the spreading coefficient, Sb/a- WUl the epoxy wet the PVC Assuming that complete wetting can occur (and that a sufficiently thin layer of adhesive is applied), how will the adhesive joint fail when stressed i.e., will the epoxy or the PVC/epoxy interface fail first ... 

II.7 The following data were obtained for the work of adhesion between the listed liquids and carbon black. Use these data, together with the surface tensions of pure liquids from Appendix 4 (or from additional sources as necessary), to calculate the spreading coefficients for the various liquids on carbon black. 

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