Alteration of self-assembled patterns by microorganisms in evaporating droplets
The science of self-organization comprises a diverse range of processes where a disordered system of components form ordered pattern or structure spontaneously without any external instruction . Plentiful examples of this phenomenon appear in nature at almost all scales . Over the past decades, self-assembly has become the apple of many researchers eye by offering breakthroughs for many applications in not only physics but also chemistry, biology and material sciences . Among several self-assembly methods, using evaporating droplets shines out as it provides ease and simplicity. Along with these advantages, it increases its popularity by providing the opportunity of obtaining a variety of patterns such as uniform depositions, central bumps, polygons and hexagons  and more famously (coffee) rings . Nonetheless, most of the studies resulting in these patterns have been carried out by using Brownian particles which uctuate randomly due to the collisions with the molecules of the surrounding uid, while only little is known when it comes to active particles suspended in evaporating droplets. The self-propelling nature of active particles  permits them to explore their environment differently from Brownian particles and opens new doors in this research line. Being in the quest of understanding what will happen in the presence of active particles such as the well-studied bacteria Esherichia coli (E.coli ), we investigate and explore the self-assembled patterns in evaporating droplets by using digital video microscopy. We demonstrate that the presence of E.coli bacteria tunes the self-assembled patterns. Moreover, we enrich the patterns by introducing salt. We show that the activity of these microorganisms has an in- uence on salt crystallization based on the characteristic dendritic crystals obtained with active and motile bacteria and unaltered, regular crystals obtained with nonmotile bacteria with inhibited activity. Our results suggest a simpler, faster and cheaper method in which common salt can be used as a biomarker to detect bacterial activity.