Thomas R. Powers, Annemarie J. Van Parys, and Kenneth S. Breuer, Division of Engineering, Brown University

In bacterial chemotaxis, cells such as E. coli drift up chemical gradients by means of a directed random
walk. Near the beginning of each step of a walk, the rotating helical flagella which propel the cell form a
bundle. Using slender-body theory, we show that the counter-rotation of the cell body necessary for torque
balance is sufficient to wrap the flagella into a bundle, even in the absence of the swirling flows
produced by each individual flagellum. Using macroscopic experiments, we also study the viscous flows set
up by two rotating helices and their role in bundling.