Self-propelling particles create virtual funnels to penetrate narrow constrictions

Bacterial populations represent a natural mix of active motile cells and passive non-motile cells competing within mechanically complex environments. Self-propulsion confers an evolutionary advantage on motile bacteria, enabling them to access constricted spaces that are otherwise inaccessible to nonmotile bacteria. The dynamics of active and passive subpopulations under confinement critically influence how physical constrictions organize the particle types spatiotemporally. To investigate the effects of confining geometries on motile/non-motile bacterial systems, we employed a microfluidic device featuring narrow microchannels. We observed that self-propulsion enabled motile E. coli to penetrate the microchannels. Non-motile E. coli cells remained excluded under all conditions. Quantitative analysis revealed that the motile cells frequently reoriented to enter the channels, unlike their nonmotile counterparts. The reorientation was flagellar-thrust dependent and helped the cells penetrate the narrow openings even when approaching the channels obliquely. This effect occurred within a few hundred nanometers from the channel mouth, creating a virtual funnel that eventually guided the cell body into these narrow openings. Naturally, the funneling was absent in the case of non-motile cells. A diffusion model suggested that the funneling effect increased the flux of motile cells into confined regions by several orders of magnitude compared to nonmotile cells. Further, the increased viscous drag within these narrow channels did not significantly influence the relative flux, although it did decrease the absolute motile cell flux. We leveraged these insights to develop a separation assay that successfully isolated purely motile cells from mixtures of both motile and non-motile bacteria. Given that active and passive matter often coexist, these findings hold practical implications for the mechanical patterning of their assembly and spatial distribution.