Dynamics of drying

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 progresses, the emerging tensile and shear stresses inside the deposition are relieved by the formation over time of a characteristic pattern of consecutive cracks. Each set of cracks has a specific morphology, which is linked to the previous stress distribution. The photo shows two morphologies. The tangential spiral cracks at the ring’s edge develop first in random locations as bits of silica debris delaminate from the substrate. The straight, radially oriented cracks develop next as a response to the capillary pressure and shear stress operating between the droplet coating and substrate. (Submitted by Robert Mulka; R. Mulka et al., Colloids Surf., A 623, 126730, 2021, doi:10.1016/j.colsurfa.2021.126730 .)