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Original publication: doi.org/10.1038/s41598-022-15577-w

When particles are forced through a sufficiently narrow geometric constriction, their flow becomes unsteady due to the development of temporary clogs, e.g. arches, which strongly perturb their free motion. Such behavior has been studied in great detail, e.g. for granular matter driven through funnels or colloidal particles flowing through geometric constraints. Independent of the specific system, empirically, one finds an intermittent particle current which is governed by exponentially distributed burst sizes and a power-law dependence of the clogging times. In addition to particles which are driven through constrictions by external forces or fluid flows, clogging is also observed in self-propelling systems, e.g. sheep herds, pedestrian crowds or active grains. Despite considerable differences compared to externally driven systems, their intermittent clogging behaviour is very similar. However, the effect of cohesion, which is often dominating the behavior of group-forming living systems, has not been investigated so far.

In our study, we experimentally and numerically investigate a system of active colloidal particles (APs) which are propelled through a two-dimensional (2D) funnel and whose mutual interactions can be precisely controlled via an optical feedback-loop. In our specific case, we considered APs whose interactions are dominated by cohesion (implemented via a tunable cohesion strength β) and alignment being motivated by social rules governing the behaviors in many living collective systems. Similar to other studies without cohesive interactions, we find an exponential burst distribution and an algebraic decay in the clogging time distribution. Remarkably, we find that cohesion has, however, only a very weak influence on the particle flow, up to a certain threshold. This threshold is the transition point from an arch-dominated clogging regime at low cohesion strength (small β) to a cohesion dominated clogging regime at high cohesion strength ( β > 1).