Shrinkage star-shaped cracks: Explaining the transition from 90 degrees to 120 degrees

Contraction due to drying or cooling of materials yields various self-organized crack patterns. The junctions between the cracks are complex and form in some conditions, star-shaped cracks with mostly 90 degrees or 120 degrees intersection angles. Any physical explanation of the selection of the angle is lacking. Here, we report directional drying of colloids experiments in capillary tubes allowing to obtain a reversible transition between 90 degrees and 120 degrees. We show that the transition is governed by a linear elastic fracture mechanics energy minimization principle hence by only one dimensionless parameter: the ratio between the Griffith length (balance between the energy needed to create cracks and to deform the material elastically) and the cell size. We give a straightforward characterization technique to estimate Griffith's length by changing the cell geometry. As a bonus, we deduce from it the toughness of drying colloidal suspensions. We underline that the method may be applied to a broad area of materials, from suspensions (colloids, paints or mud) to engineering (ceramics, coatings) and geological materials (basalt, sediments). Copyright (C) EPLA, 2010