Sessile droplets at a solid/elastomer interface

The spreading parameter S is the fundamental parameter controlling the stability of thin liquid films intercalated between a hydrophobic solid and a soft elastomer (Young modulus E): S = gamma(SR)-(gamma(SL) + gamma(LR)), where gamma(SR), gamma(SL), and gamma(LR) are respectively the solid/elastomer, solid/liquid, and liquid/elastomer interfacial tensions. We describe here a simple experimental method to determine S. We monitor by interferometry the profile of liquid droplets squeezed at the solid/elastomer interface. S is negative and the droplets do not spread. We find that (i) an intercalated droplet is a very flat semiellipsoid with a macroscopic contact angle of 90 degrees and (ii) the radius R of the wet area scales like R proportional to H-2/h(0), where H is the thickness at the center and h(0) = \S\/E is a characteristic length in the range of 100 Angstrom. From the shape analysis we extract h(0) and as a consequence the spreading parameter S. This measurement was performed for different liquids sandwiched between a cross-linked silicone polymer and chemically modified glass slides. Liquid droplets trapped at the solid/elastomer interface give similar results except that we get ellipsoid instead of semi-ellipsoid. In this particular case the spreading parameter is simply given by S = 2 gamma(LR) and compares well with an independent measurement of gamma(LR).