The physics of peeling paint

The complex mechanical properties of colloidal coatings are hard to measure because they vary spatially and temporally. Paint, for instance, starts as a fluid that, as the solvent evaporates, dries into a brittle solid that can crack and peel away from a substrate. To better understand the stresses that drive the fracture process, researchers led by Yale University’s Eric Dufresne have now adapted a technique from cell biology known as traction force microscopy. In the technique’s biological application, researchers observe a cell crawling across a rubber substrate and monitor the deformations within the rubber. Knowing the rubber’s mechanical properties, the researchers convert the displacement field into a stress field and deduce which parts of the cell exert force on the substrate. Dufresne and colleagues replaced the cell with a drying film of a silica-particle suspension, which they applied to a soft layer of silicone rubber that would deform as the film dried and cracked. To map those deformations and convert them to a three-dimensional stress field, the team monitored the motion of tiny, fluorescent tracers mixed into the rubber. In the plot of stress as a function of distance from the crack front, the normal stress (solid dots) shoots up rapidly just ahead of the crack front—with much greater magnitude than does the in-plane stress (open dots) and, reassuringly, with a scaling that roughly agrees with that predicted by classic fracture theory (red). (Y. Xu et al., Proc. Natl. Acad. Sci. USA 107, 14964, 2010 http://dx.doi.org/10.1073/pnas.1005537107 .)