Curved crack patterns by drying bacteria suspensions

Particulate suspensions undergoing drying are prone crack as the tensile stress builds up and exceeds a critical value. In contrast to conventional straight desiccation cracks in dried colloidal films, two new types of curved cracks are observed by drying sessile drops of bacteria Escherichia coli (E. coli) in water on glass slides. We use wild-type E. coli with run-and-tumbling motions and mutant E. coli of tumblers in our experiments. After deposition onto the substrate, the first drying front occurs at the drop periphery and moves continuously towards the drop center followed by a secondary drying front moving in the same direction. Spiral-like cracks are finally observed for dried E. coli tumblers, whereas dried wild-type E. coli display circular crack patterns. These results suggest the crucial role of bacterial mobility in the self-assembly of bacteria during drying, which alters the direction of the tensile stress development and consequently leads to cracks of different morphologies. The spacing and its dominant controlling parameters of the observed cracks are also discussed. Our findings shed light on the drying dynamics of bacterial suspensions and can potentially be used to design functional coatings and novel self-assembly structures of active particles.