Two Types of Wetting The Spreading Parameter

If the parameter S is positive, the liquid spreads completely in order to lower its surface energy 0e =0). The final outcome is a film of nanoscopic thickness resulting from competition between molecular and capillary force (see chapter 4). 

The first method consists in tallying up the capillary forces acting on the line of contact (also called triple line) and equating the sum to zero. When normalized to a unit length, these forces are the interface tensions between the three phases (S/L/G). By projecting the equilibrium forces onto the solid plane, one obtains Young s relation (which he derived in 1805)  

It is evident that 6e can be defined only if the spreading parameter is negative. Oe increases when the liquid is non-wetting. 

The projection of the capillary forces onto the vertical axis is balanced out by a force of reaction exerted by the solid. If the solid is hard, no distortion is observable. If, on the other hand, it is soft (e.g., rubber or a coat of paint), it does distort. That is the reason why a water drop left on a fresh coat of paint leaves behind a circular mark. 

The second method relies on calculating the work done by moving the line of contact by a distance dx  

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