Contact Angles and Hysteresis on Soft Surfaces

Wetting experiments were performed to determine how contact angles and their hysteresis change with the mechanical properties of soft substrates. Drops of water and ethylene glycol were used on elastomeric surfaces. Elastomers (natural and butadiene rubber) were crosslinked with peroxide to give a wide range of moduli. The chemical nature of these surfaces was characterized using attenuated total reflecting infrared spectroscopy and X-ray photoelectron spectroscopy. Roughness was determined by atomic force microscopy. After crosslinking residue was removed from the surfaces, they were similar in chemical nature and roughness. Nevertheless, contact angle hysteresis changed quite dramatically with bulk tensile modulus E of the substrates, due to deformation at the contact line. For example, water drops on soft, uncrosslinked butadiene rubber pulled up a ridge that was visible to the unaided eye, resulting in contact angle hysteresis of 69°. Crosslinking butadiene rubber with 2% peroxide increased E by almost 50-fold. Consequently, its contact angle hysteresis fell to 22°. For variation in the contact angles due to deformation to occur, the height of the ridge pulled up by the surface tension of the liquid drop must be greater than the local roughness. For the surfaces in this study, this occurred where E was less than 5 MPa.