  For monolayers of chemically active particles at a fluid interface,
collective dynamics are predicted to arise owing to activity-induced Marangoni
flow even if the particles are not self-propelled. Here we test this prediction
by employing a monolayer of spherically symmetric active TiO_2 particles
located at an oil-water interface with or without addition of a non-ionic
surfactant. Due to the spherical symmetry, an individual particle does not
self-propel. However, the gradients produced by the photochemical fuel
degradation give rise to long-ranged Marangoni flows. For the case in which
surfactant is added to the system, we indeed observe the emergence of
collective motion, with dynamics dependent on the particle coverage of the
monolayer. The experimental observations are discussed within the framework of
a simple theoretical mean field model.

Soft Condensed Matter;Chemical Physics