Spiral and croissant cracks in drying thin films

摘要

Drying mud or crazing in ceramics glaze leads to familiar hierarchical cracks network wherea new crack connects perpendicularly to older ones. We report unusual spirals and croissantscrack patterns in methylsiloxane drying thin films, varnish film or paint moderately adhering ona substrate. Although proportional to the thickness of the film, the lengthscale of the pattern issurprisingly orders of magnitude larger than this thickness. We show how the propagation is drivenby a cooperation between fracture and delamination although the lengthscale is only selected by aminimisation of the elastic energy in the film independently of the adhesion and fracture energies.We finally describe the range of physical parameters where this peculiar crack mode propagates.