Study Describes Nemantic Defects on Swimming Bacteria

Active systems and active matter are rapidly evolving scientific areas that attract interest because of a rich variety of phenomena inherent to nonequilibrium systems. Most of these examples come from nature – schools of fish, flocks of birds, bacteria colonies, but there are also systems of artificial origin. Modeling of such systems has its own challenge: due to the enormous number of objects, one has to develop simplified models that are able to capture the variety of phenomena observed experimentally. 

Prof. I. Aranson (Huck Chair Professor of Biomedical Engineering, Chemistry and Mathematics of Pennsylvania State University and Adjunct professor at Northwestern ) and his ESAM Ph. D. student M. Genkin developed a computational 2D model that simulates bacteria flow in a liquid crystal.  The related paper has been printed in Phys. Rev. X. The other two authors are A. Sokolov and O.D. Lavrentovich. The new phenomena predicted by this model describes bacteria accumulation in +1/2 and depletion in -1/2 topological defects.

The explanation comes from the geometry of the topological defects. The +1/2 defect is the region where bacteria trajectories intersect while the -1/2 defect is where almost no trajectories are around. The former sucks out bacteria from the surrounding space, while the latter expels them. However, the effect is not that obvious: bacteria do not like to cluster and therefore can turn around/swim in other directions to prevent grouping. Bacterium would prefer to swim in areas with the least number of neighboring bacteria. However, they are not able to do it in very clever way, thus clustering and depletion still take place.

Liquid crystals force bacteria to arrange and move along parallel straight lines, like in a perfectly arranged military march. Too much bacteria can destroy the perfect order. As a result, bacteria trajectories resemble city roads that twist, turn and intersect each other. Similar to cars in city traffic, one would expect bacteria accumulation on loaded intersections and depletion in rural areas, even though every driver tries to avoid traffic.

The phenomena was experimentally observed by A. Sokolov and I. Aranson in Argonne National Lab, thanks to fluorescent bacteria that shed light on this aspect.

Applications include bacteria control and transport. The defects loaded with bacteria can be guided by external fields, surface-pattering or temperature gradients that are created on demand.

A trajectory map and fluorescent image of swimming bacteria