  Elastomers swollen with non-polar fluids show potential as anti-adhesive
materials. We study the effect of oil fraction and contact time on the adhesion
between swollen spherical probes of PDMS (polydimethylsiloxane) and flat glass
surfaces. The PDMS probes are swollen with pre-determined amount of 10 cSt
silicone oil to span the range where the PDMS is fluid free (via solvent
extraction) up to the limit where it is oil saturated. Probe tack measurements
show that adhesion decreases rapidly with an increase in oil fraction. The
decrease in adhesion is attributed to excess oil present at the PDMS-air
interface. Contact angle measurements and optical microscopy images support
this observation. Adhesion also increases with contact time for a given oil
fraction. The increase in adhesion with contact time can be interpreted through
different competing mechanisms that depend on the oil fraction where the
dominant mechanism changes from extracted to fully swollen PDMS. For partially
swollen PDMS, we observe that adhesion initially increases because of
viscoelastic relaxation and at long times increases because of contact aging.
In contrast, adhesion between fully swollen PDMS and glass barely increases
over time and is mainly due to capillary forces. While the relaxation of PDMS
in contact is well-described by a visco-poroelastic model, we do not see
evidence that poroelastic relaxation of the PDMS contributes to an increase of
adhesion with glass whether it is partially or fully swollen.
