Cracks—fractures caused by material stress that exceeds the strength of the material—are ubiquitous in everyday life. They’re also observed in the drying of silica suspension droplets, as seen in the microscopic image. Robert Mulka of Wrocław University of Science and Technology in Poland; Matthias Buschmann at the Institute of Air Handling and Refrigeration in Dresden, Germany; and their colleagues wanted to study cracking and the process of exsiccation, or drying out, in more detail. To do so, they dosed droplets of water-based suspensions with silica nanoparticles on a stainless steel substrate. A microscope with an attached camera then recorded the drying process in a laboratory environment with a constant temperature and relative humidity. This image, captured at the very end of the drying process, shows the characteristic “coffee ring” drying pattern.
A complex interplay of Marangoni convection and capillary flow inside the droplet transports the silica nanoparticles outwardly. As exsiccation...
