A crowd at the entrance to a narrow corridor behaves differently than in a spacious hall: some people get pressed against the walls, others are pushed forward. Something similar happens when an elongated particle ends up in a fluid teeming with self-propelled dumbbells. Previously, it was unclear why such a particle moves faster along its long axis than across it, and how the crowd density affects its speed.

In a computer model, scientists placed an ellipse in a two-dimensional suspension of active dumbbells that repel each other upon contact. Each dumbbell consists of two beads connected by a spring and swims forward under its own force. When there are few dumbbells, they only occasionally bump into the ellipse, which barely moves. But as the crowd thickens, the ellipse starts to accelerate: its speed and rotation increase more than tenfold.

However, the acceleration is not unlimited. Above a threshold density, the dumbbells form a dense ring around the ellipse that acts like a cage, restricting motion. The more active the dumbbells, the earlier this threshold is reached. So there is an optimal density: below it the crowd helps, above it hinders.