Youngs Relation No Longer Holds

Young s relation is valid only in the immediate vicinity of the line since the shape of the drop will be determined by the elastic deformation of the elastomer. Together with the spreading parameter S and Young s modulus E of the rigid substrate, we can introduce a characteristic length ho, which we call the elastic length, defined by 

While the capillary length (introduced in chapter 1) describes the competition between gravity and capillarity, the length ho describes the competition between, the deformation energy of the soft matrix and the surface energy. Young s relation would be valid only over dimensions srnaller than ho, where other types of forces (such as van der Waals) modify the profile anyway. 

We will see shortly that analyzing the shape of a liquid drop trapped at the solid/elastomer interface enables us to determine the value of 5. 

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