In situ observation of stereoscopic shapes of cavities in soft adhesives

By using a new visualization technique, we succeeded in the in situ observation of the stereoscopic shape of cavities formed in a thin film of soft adhesives in the debonding process of the probe-tack test. We found that the cavity shape and the interfacial fracture behavior are strongly influenced by the cross-linking density of the soft adhesives: for soft adhesives, the cavity is nearly spherical, and the fracture slows down basically due to the blunting of the cavity edge, while for hard adhesives the cavity becomes pancake-like, and the fracture is due to the coalescence of cavities at the adhesive-probe interface.