A Marangoni swimmer pushing a particle raft under 1D confinement

Active matter systems, due to their spontaneous self-propulsion ability, hold potential for future applications in healthcare and environmental sustainability. Marangoni swimmers, a type of synthetic active matter, are a common model system for understanding the underlying physics. Existing studies of the interactions of active matter with passive particles have mostly focused on the modification of the behavior of the passive particles. In contrast, we analyse here experimentally the impact on the self-propulsion of camphor-infused agarose disks (active) of their interactions with floating hollow glass microspheres (passive) within an annular channel. Two distinct regimes are observed: a steady regime with unidirectional motion of the swimmer at low packing fractions (ϕ_ini≲ 0.45) and an oscillatory regime with to-and-fro motion at higher packing fractions (ϕ_ini≳ 0.45). In the former, the swimmer pushes nearly the entire particle raft together with it, like towing a cargo, causing a decrease in swimmer speed with increasing packing fraction due to the additional drag from the particle raft. A simplified force-balance model is finally proposed that captures the experimental trend in swimmer speed reasonably well.