Preventing clustering of active particles in microchannels

The trajectories of microswimmers moving in narrow channels of widths comparable to their sizes are significantly altered when they encounter another microswimmer moving in the opposite direction. The consequence of these encounters is a delay in the progress of both swimmers, which can be conceptualized as an instantaneous effective backward displacement. Similarly, the modeling of tumble events in bacteria, which occur over a finite time, can be represented as an instantaneous effective displacement in addition to a change in direction. Such effective displacements can be incorporated directly into a kinetic theory for the partial densities of swimmers moving in the channel. The linear analysis of the resulting equation yields the critical density at which clusters emerge. The methodology is then applied to the case of soil bacteria moving in long channels of cross-section 1.8 μm × 1.8 μm. The tracking of the swimmers permits the straightforward acquisition of the effective displacements, which in turn allows the critical density (ρcrit ≃ 0.10 bact/μm) to be predicted prior to cluster formation. The advantage of this proposed approach is that it does not necessitate the determination of an effective density-dependent speed, which is a requisite of the standard motility-induced phase separation theory. Microswimmers navigating narrow channels face delays when encountering others moving in opposite directions. These delays can cause microswimmers to accumulate and form clusters that block the entire channel. Here, the authors use a kinetic theory incorporating effective displacements obtained from experiments with soil bacteria, to predict the critical densities for cluster formation.