Circular and spiral-like crack patterns of desiccated bacterial deposits*

Desiccation cracks are ubiquitous and play a key role in a plethora of applications such as coating and printing. Here, we report desiccation crack patterns of drying suspensions of Escherichia coli (E. coli) with different swimming behaviors. Drops of bacterial suspensions with a volume of 2.5 µL and a volume fraction of 10-20% are deposited onto glass substrates for drying. We observe circular cracks in the consolidating film of wild-type E. coli with run-and-tumbling motions, whereas spiral-like cracks are observed in the film of mutant E. coli with tumbling motions. Using fracture mechanics, we show that the circular cracks arise from the locally ordered structure driven by the collective motion of wild-type bacteria and the tensile nature of the radial drying stress. In comparison, the spiral-like cracks are induced by film delamination due to the strong bending moment, which propagates along the azimuthal direction with increasing drying time. Our results elucidate the role of bacteria swimming behavior in desiccation cracks and further enhance the understanding of the rich mechanical instabilities in consolidating bacteria films.