Surface instability of soft films with coupled tension-shear interactions

Abstract

The surface roughening instability in a soft thin film, induced by interactions with a contactor is investigated accounting for both tension and shear effects in the interaction potential. A key finding of this study is that, in the presence of coupled tension-shear interactions, the instability mode will depend on whether the contactor is brought towards the film (contact mode) or the contactor is retracted from a fully sticking configuration (peeling mode). In the contact mode, the wavelength of the roughening is about three times the film thickness, in agreement with previous works, while in the peeling mode it is about four times the film thickness. Although the analysis indicates a rich and complex dependence on the parameters of the interaction, the wavelength of the instability is essentially independent of the interactions for physically reasonable values of interaction parameters. The results are in agreement with available experiments. The results of the present study are useful in providing insights into the physics of friction and cavitation at soft interfaces.