Cracking in drying silica-polymer films: Morphology transitions

We performed direct imaging of crack nucleation and propagation in drying aqueous silica-poly(vinyl alcohol) suspension coatings. The suspension films are dried on a transparent hot stage to acquire time-lapse video images, which enable us to measure the growth rate of nucleating and connecting cracks as a function of time. The direct imaging reveals a morphology transition from network- to star-like patterns as decreasing particle/polymer weight ratio: the propagating cracks contact with each other at higher weight ratios, whereas they stop propagating in a finite drying time and remain as distinct patterns at lower particle contents. The measured nucleation rate of the distinct crack exponentially decays with an increase in particle contents.