Effects of bulk viscoelasticity and surface wetting on the contact and adhesive properties of a soft material

Because of the wide practical applications of soft materials, the surface contact and adhesive behaviors need to be comprehensively understood. Here, we systematically investigated the effect of bulk viscoelasticity and surface wetting on the dynamic behavior of contact and adhesion of soft polydimethylsiloxane (PDMS) surfaces. Experimental results showed that during the indentation process, the relation between loads and contact radius were close to the Johnson-Kendall-Roberts (JKR) mode, but the load-penetration curves presented a roughly linear dependence with increasing preloads transferring from the JKR to Hertz mode; the detaching process exhibited a pronounced "stick-split" behavior, where the spherical probe first stuck to the soft PDMS without decreasing the contact area, and then the crack of the contact edge opened and split, providing high adhesive forces far exceeding the JKR prediction. The introduction of liquids between the probe and soft PDMS strongly decreased the contact area, reduced the stick time, and weakened the dry adhesive strength depending on the surface tension. The internal friction induced by the segmental motion of long-chain molecules and the interfacial resistance of liquid were suggested to be responsible for these phenomena.