title: Wetting and phase separation in soft adhesion.
abstract: In the classic theory of solid adhesion, surface energy drives deformation to increase contact area whereas bulk elasticity opposes it. Recently, solid surface stress has been shown also to play an important role in opposing deformation of soft materials. This suggests that the contact line in soft adhesion should mimic that of a liquid droplet, with a contact angle determined by surface tensions. Consistent with this hypothesis, we observe a contact angle of a soft silicone substrate on rigid silica spheres that depends on the surface functionalization but not the sphere size. However, to satisfy this wetting condition without a divergent elastic stress, the gel phase separates from its solvent near the contact line. This creates a four-phase contact zone with two additional contact lines hidden below the surface of the substrate. Whereas the geometries of these contact lines are independent of the size of the sphere, the volume of the phase-separated region is not, but rather depends on the indentation volume. These results indicate that theories of adhesion of soft gels need to account for both the compressibility of the gel network and a nonzero surface stress between the gel and its solvent.
authors.affiliation: Department of Mechanical Engineering and Materials Science, Yale University, New Haven, CT 06511;
authors.forename: Katharine E
authors.initials: KE
authors.lastname: Jensen
authors.affiliation: Department of Mechanical Engineering and Materials Science, Yale University, New Haven, CT 06511;
authors.forename: Raphael
authors.initials: R
authors.lastname: Sarfati
authors.affiliation: Mathematical Institute, University of Oxford, Oxford, OX1 3LB, United Kingdom;
authors.forename: Robert W
authors.initials: RW
authors.lastname: Style
authors.affiliation: Department of Mechanical Engineering and Materials Science, Yale University, New Haven, CT 06511;
authors.forename: Rostislav
authors.initials: R
authors.lastname: Boltyanskiy
authors.affiliation: Department of Chemical Engineering, Lehigh University, Bethlehem, PA 18015.
authors.forename: Aditi
authors.initials: A
authors.lastname: Chakrabarti
authors.affiliation: Department of Chemical Engineering, Lehigh University, Bethlehem, PA 18015.
authors.forename: Manoj K
authors.initials: MK
authors.lastname: Chaudhury
authors.affiliation: Department of Mechanical Engineering and Materials Science, Yale University, New Haven, CT 06511; eric.dufresne@yale.edu.
authors.forename: Eric R
authors.initials: ER
authors.lastname: Dufresne
keywords: adhesion; phase separation; soft matter; surface tension; wetting