Generalized Spreading Coefficients and Thin Film Forces

5 GENERALIZED SPREADING COEFFICIENTS AND THIN FILM FORCES 

Analogous arguments to those concerning generalized entry coefficients can be applied to spreading (see, e.g., reference [28]). Thus we can define a generalized spreading coefficient 

Combining Equations 3.25 and 3.26 yields for the generalized spreading pressure 

The classic spreading coefficients can be represented by simply substituting appropriate values for the limits of the integral in Equation 3.27. 

Another difference between oil spreading on the surface of an aqueous phase and antifoam oil drop entry concerns the nature of any capillary pressure present. Consider then the case of an oil drop already emerged onto the surface of a foaming solution. If the drop is small, the air-oil and oil-foaming liquid surfaces will be hemispherical forms a (convex) lens. In this case, the capillary pressure in the drop will oppose film thinning. A stable film could therefore in principle exist in equilibrium with such a drop even if the disjoining pressure in the film is negative. 

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