Effects of contact time, humidity, and surface roughness on the adhesion hysteresis of polydimethylsiloxane

Dynamic contact mechanics experiments have been performed on small polydimethylsiloxane (PDMS) lenses and several substrates in both ambient air and in dry nitrogen. The experimental results are analyzed with the Johnson-Kendall-Roberts theory. While the theory adequately describes the approach data, it is unable to account for the large hysteresis observed upon retraction. Adhesion hysteresis is shown to scale with the roughness of the substrate, the hydrophilicty of the substrate, the time of contact, and the ambient humidity. The experimental results also demonstrate that this method is sensitive to changes in the surface energy of the substrate. The cumulative adhesion hysteresis is quantified and is shown to be largest for rough, hydrophilic substrates in relatively high humidity and smallest for smooth substrates in dry nitrogen. The origin of the hysteresis is analyzed by considering favorable interfacial bonding resulting from water-mediated bonding between the substrate and oxygen atoms in the PDMS backbone or other polar species on the polymer surface. Capillary forces are also postulated to contribute to the cumulative adhesion hysteresis.