Session MD - Bio-Fluid Dynamics X.
MIXED session, Tuesday afternoon, November 23
Fifth Avenue Room, Westin Seattle
Concentrated self-stacked domains of swimming, rod-shaped, peritrichously flagellated bacteria (Bacillus subtilis) move faster than individual organisms can swim. The domains are phalanxes of cells, aligned, and all moving in the same direction. PIV measurements show that they maintain coherence over hundreds of micrometers (> 10^4 cells) and durations of one or more seconds. Steric repulsion generates ordered, but non-polar arrays. What mechanism then creates quorum polarity, defined as essentially codirectional, posterior alignment of all the cells' flagella? We discovered that, confronted by an obstacle, these bacteria often stop, then swim smoothly ``backwards" at standard speed. Evidently there is no preferred polarity: the flagella flip, the bundle reconstitutes. We infer that by an analogous process steric constraints and collectively generated flow cause quorum polarity, unanimity of swimming direction. This phenomenon is significant both for microbial ecology and for generating a, perhaps useful, reproducible polar liquid crystal with self-propelled domains. The speed of the zooming domains, and the coherence times and lengths are considered elsewhere.