Drying silica-nanofluid droplets

Exsiccation of suspension droplets generates characteristic ring-like stains. The homogeneity of such coffee rings is disturbed by subsequent cracking. Placing silica droplets on copper and stainless steel substrates, the ensuing drying and cracking chronology is investigated. The formation of the coffee ring and its cracking are visualised through microscopic imaging and videotaping. Based on the cracking chronology, three crack generations are identified. The evolution of the 1st crack generation is studied in detail. Two fracture types – logarithmic spiral and straight radially oriented – characterise these initial cracks. While the first type is due to deposition delamination, the straight cracks are induced by the interplay of capillary pressure and shear stress between coating and substrate. Most noteworthy is our finding that the cracking speed of the straight cracks is one order of magnitude larger than that of the spiral cracks. The pattern of the 1st generation cracks defines the final crack network. Our findings provide a comprehensive understanding of the crack pattern formation following the desiccation of silica droplets. This study offers details on nanofluid exsiccation on metal substrates, thereby opening routes for depositing nanoparticles on heated and non-heated surfaces for various engineering applications.