Marangoni swimmer pushing a particle raft in confined channels

Synthetic self-propelling swimmers are a crucial model system for active matter. Marangoni swimmers are a class of such swimmers and propel at an interface exploiting the symmetry-breaking resulting from the surfactant convection induced by the Marangoni flow. Single swimmer and multi-swimmer dynamics have been well explored in contrast with interactions of such swimmers with passive particles deposited on the interface. Here, we study the interaction of camphor-agarose swimmers with a raft of passive particles in an annular channel using experiments, scaling analysis and theoretical modeling. Depending on the amount of deposited particles, we report a steady swimming behavior (at low concentration) and an oscillatory behavior (at high concentration). We argue that the dominant cause of velocity reduction in the presence of increasing passive particles is not the local packing fraction or chemical gradient of camphor. Rather, it is an effective drag from the particle raft which causes the reduction in the swimming velocity. Our study highlights the potential of these swimmers for cargo transport at the interface under confinement.