Formation of circular crack pattern in deposition self-assembled by drying nanoparticle suspension

Curved cracks widely exist in nanoparticle (NP) deposition produced by drying colloidal suspension. Circular cracks, for example, initiate and propagate along a circular trajectory. One feasible theoretical explanation of a circular crack is the Xia-Hutchinson model, in which a preexisting track (flaw loop) in the film is necessary for initiating and propagating the crack on the circular path. Here, we report the first experimental evidence of dried deposition to support this model. Our results indicate that cracks along the circular trajectory can surprisingly "pass" across a 180 μm air gap. Moreover, two arc-path cracks originate in different areas and propagate to meet, forming a circular trajectory. These unexpected crack initiation and propagation indicate that the crack propagates alone the "preformed" track, experimentally confirming the hypothesis proposed by the Xia-Hutchinson model. The transition of the circular crack to a radial one indicates that the deposition microstructure is the dominant factor for the crack formation.