can exhibit unusually large hysteresis, an additional factor Wetting experiments were performed to determine how contact may influence the contact angles. The vertical component angles and their hysteresis change with the mechanical properties of the liquid surface tension can deform the underlying of soft substrates. Drops of water and ethylene glycol were used surface ( 2 ) , by pulling up a ridge, as depicted in Fig. 2. on elastomeric surfaces. Elastomers (natural and butadiene rub-This ridge can accentuate changes in u a and u r . For examber) were crosslinked with peroxide to give a wide range of moduli.
ple, natural rubber ( NR ) has shown u a 0 u r twice that of
The chemical nature of these surfaces was characterized using polyethylene ( PE ) ( 3 ) , even though they have similar attenuated total reflecting infrared spectroscopy and X-ray photosurface tensions ( 4, 5 ) .
electron spectroscopy. Roughness was determined by atomic force
Wetting of soft surfaces has been the aim of several theomicroscopy. After crosslinking residue was removed from the surfaces, they were similar in chemical nature and roughness. Never-retical studies (6-11), but to the best of our knowledge, theless, contact angle hysteresis changed quite dramatically with little experimental work has been done to relate contact angle bulk tensile modulus E of the substrates, due to deformation at hysteresis to the mechanical properties of soft, elastic subthe contact line. For example, water drops on soft, uncrosslinked strates. Therefore, we have performed some wetting expeributadiene rubber pulled up a ridge that was visible to the unaided ments using soft rubber surfaces in hopes of answering the eye, resulting in contact angle hysteresis of 69ะ. Crosslinking butafollowing questions. How soft must a surface be for deformadiene rubber with 2% peroxide increased E by almost 50-fold. tion at the contact line to significantly affect the observed Consequently, its contact angle hysteresis fell to 22ะ. For variation contact angles? And, if deformation does occur, how do the in the contact angles due to deformation to occur, the height of contact angles change with the mechanical properties of the the ridge pulled up by the surface tension of the liquid drop must substrate? be greater than the local roughness. For the surfaces in this study, this occurred where E was less than 5 MPa.