Geometric effects induce anomalous size-dependent active transport in structured environments

The variations of transport efficiency in structured environments between distinct indi-viduals in actively self-propelled systems are both hard to study and poorly understood. Here, we study the transport of a nontumbling Escherichia coli strain, an active-matter archetype with intrinsic size variation but fairly uniform speed, through a periodic pillar ar-ray. We show that long-term transport switches from a trapping dominated state for shorter cells to a much more dispersive state for longer cells above a critical bacterial size set by the pillar array geometry. Using a combination of experiments and modeling, we show that this anomalous size dependence arises from an enhancement of the escape rate from trapping for longer cells caused by nearby pillars. Our results show that geometric effects can lead to size being a sensitive tuning knob for transport in structured environments, with implications in general for active matter systems and, in particular, for the morphological adaptation of bacteria to structured habitats, spatial structuring of communities, and for antibiofouling materials design.

Automated summary

The transport efficiency in structured environments for individuals in actively self-propelled systems is difficult to study and poorly understood. In this study, we investigated the transport of a non-tumbling Escherichia coli strain through a periodic pillar array, and found that the long-term transport switches from a trapping dominated state for shorter cells to a more dispersive state for longer cells. This anomalous size dependence is caused by an enhancement of the escape rate for longer cells caused by nearby pillars. Our results demonstrate that geometric effects can significantly influence transport in structured environments, which has implications for active matter systems and bacterial adaptation to structured habitats.