Sessile Droplets at a Solid/Elastomer Interface

Abstract

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 = γSR − (γSL + γLR), where γSR, γSL, and γLR 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° and (ii) the radius R of the wet area scales like R ∝ H2/h0, where H is the thickness at the center and h0 = |S|/E is a characteristic length in the range of 100 Å. From the shape analysis we extract h0 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 = −2γLR and compares well with an independent measurement of γLR.