to as “top view”, induces persistent edge current mostly observed on bacteria swimming near the lateral boundaries. Such edge current can alter the geometric constraints ruling the self-organization of bacterial vortices by suppressing the anti-rotational mode. The extended geometric rule, which is in excellent agreement with experiment, brings new understanding of chiral active matter in order to organize larger scale flow.

![**Chiral edge current.** (a) Color map of the orientation of collective motion in a chiral bacterial vortex ($R = \SI{50}{\micro\meter}$). The edge, defined as the area within from the lateral boundary, is separated from the rest of the microwell (the bulk) by a dashed white line. Scale bar, . (b) Normalized azimuthal velocity $v_{\theta}$ in microwells of various sizes. The blue circles indicate the edge current, and the green squares indicate the motion in bulk. $v_{\theta}$ is averaged over 10 s and plotted with error bars representing standard deviation.[]{data-label="fig2"}](figure2_8.jpg)

A bacterial suspension of *Escherichia coli* was confined in microwells with the depth of , made of poly-dimethyl siloxane (PDMS) rendered hydrophilic with a polyethylene glycol coating, and sealed with an oil/water interface stabilized with a surfactant (Fig. \[fig1\](a))[@izri]. Top/bottom hydrodynamic asymmetry (later referred to as “asymmetric conditions”) consists in 