Soft adhesive failure under shear

Many mishaps in the marine, transport, and energy sectors have resulted from the adhesion of ice to surfaces, prompting the need for anti-icing and de-icing strategies. Using coatings of soft elastomers such as polydimethylsiloxane (PDMS) has been a simple approach at the forefront of research used to mitigate or annul icing problems. Researchers have used a variety of methods to quantify ice/PDMS adhesion, however there is not yet broad consensus as to the true adhesive strength of an ice/PDMS interface. In an attempt to identify universal values of adhesive strength, we conduct lap shear experiments on PDMS coatings against ice or glass substrates while monitoring the adhesive failure with a fast camera. Interestingly, our results reveal that the crack at the PDMS/substrate interface is often present and growing before the peak forces set in, contradicting the prevailing analysis and potentially explaining some of the variation reported in the literature. To better understand the effects of our discovery, a scaling model was developed to verify the basic underlying physics of the process. The model revealed the importance of an intrinsic length scale caused by viscoelastic losses - samples which are long in comparison to the intrinsic length show little deviation from the standard treatment, however, samples which have lengths comparable or smaller than the intrinsic length show large deviations. The experiments and model were validated with Finite Element Analysis (FEA) which suggest the viscoelastic losses are not incurred in the bulk, but are localized to the interface.