When attracted by a stimulus (light), microswimmers can build up very densely at a constriction and thus cause jamming. The microalga Chlamydomonas Reinhardtii is used here as a model system to study this phenomenon. Its negative phototaxis makes the algae swim away from a light source and go through a microfabricated bottleneck-shaped constriction. Successive jamming events interspersed with bursts of algae are observed. A power-law decrease describes well the distribution of time lapses of blockages. Moreover, the evacuation time is found to increase when increasing the swimming velocity. These results are reminiscent of crowd dynamics and, in particular, what has been called the faster is slower effect in the dedicated literature. It also raises the question of the presence of tangential solid friction between motile cells densely packed. Furthermore, we demonstrate the existence of a transition from a jammed phase described by a power-law decrease to an uninterrupted phase described by an exponential decay, we characterize such a transition as a function of door size and swimming velocity. Interestingly, the exponential flowing regime shows what might be referred to as “a faster is faster” regime.

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• Living matter & active matter
• Packing & jamming problems