(a,b) Snapshot of inward-growing bacterial colonies (Bacillus subtilis) on different surfaces. Initial bacterial distribution is defined by non-contact lithographic techniques with blue light exposure. Inward-growing domains are formed by merging colonies originating from annulus-shaped initial distribution. (a) On an agarose surface, growing colonies easily form multi-layer structures and provide narrow monolayer inward-growing domains. However, (b) on a low friction polycarbonate (PC) surface, bacteria form monolayer colonies and provide large-scale inward-growing domains. Scale bar 500 μm. (c) Magnified fluorescence image superimposed with director field of the inward-growing domain indicating the radial alignment of bacteria. Δr represents the width of the monolayer region. Scale bar 50 μm. (d) Radial order parameter (SR) and (e) radial velocity profile (vr) as a function of distance across the monolayer colony. Velocity profile was extracted by using PIV algorithms around the center of the annulus shape (Figure 5—video 2). (f) The experimentally measured inner and outer and critical radius. d, e, f are averaged over four different colonies, starting from the same initial annulus-shaped distribution.