  The hydrodynamic flow field around a catalytically active colloid is probed
using particle tracking velocimetry both in the freely swimming state and when
kept stationary with an external force. Our measurements provide information
about the fluid velocity in the vicinity of the surface of the colloid, and
confirm a mechanism for propulsion that was proposed recently. In addition to
offering a unified understanding of the nonequilibrium interfacial transport
processes at stake, our results open the way to a thorough description of the
hydrodynamic interactions between such active particles and understanding their
collective dynamics.
