Pairing Order Transition in Chiral Bacterial Vortex {#supplemental-information-for-edge-current-and-pairing-order-transition-in-chiral-bacterial-vortex .unnumbered}
====================================================================

Materials and Methods
=====================

Device microfabrication
-----------------------

The device used in this experiment is a flow cell that contains an array of microwells of various radii $R$ ($\SI{20}{\micro\meter} \leq R \leq \SI{150}{\micro\meter}$) and three types of shape: single circular microwells, pairs and triplets of overlapping identical circular microwells. Their depth is in all the experiments[@beppu]. The flow cell is made of a cover glass slide (Matsunami, S1127, ) and a cover slip (Matsunami, C218181, ) separated by a double-sided adhesive tape (NICHI-BAN, NW-10, ).

The microwells are fabricated using standard soft lithography techniques. Briefly, PDMS polymer and curing agent (Dow Corning, 98-0898, Sylgard184) at 90-to-10 mass ratio was spin-coated at 1000 rpm for 30 seconds to reach a thickness of , on a mold made of a photoresist (SU-8 3025, Microchem) pattern (thickness ) cured and developed through conventional photo-lithography on a silicon wafer (Matsuzaki, Ltd., $\phi$2-inch wafer). After a curing at for an hour, the PDMS film is cut around a single array of microwells and peeled off to be bonded on its unpatterned surface to the glass cover slide that has been exposed to air plasma beforehand (, 