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

Dynamic contact mechanics experiments have been performed on small polydime thylsiloxane (PDMS) lenses and several substrates in both ambient air and i n dry nitrogen. The experimental results are analyzed with the Johnson- Ken dall -Roberts theory. While the theory adequately describes the approach da ta, it is unable to account for the large hysteresis observed upon retracti on. Adhesion hysteresis is shown to scale with the roughness of the substra te, the hydrophilicty of the substrate, the time of contact, and the ambien t humidity. The experimental results also demonstrate that this method is s ensitive to changes in the surface energy of the substrate, The cumulative adhesion hysteresis is quantified and is shown to be largest for rough, hyd rophilic substrates in relatively high humidity and smallest for smooth sub strates in dry nitrogen. The origin of the hysteresis is analyzed by consid ering favorable interfacial bonding resulting from water-mediated bonding b etween the substrate and oxygen atoms in the PDMS backbone or other polar s pecies on the polymer surface. Capillary forces are also postulated to cont ribute to the cumulative adhesion hysteresis.