  Active colloids constitute a novel class of materials composed of
colloidal-scale particles locally converting chemical energy into motility,
mimicking micro-organisms. Evolving far from equilibrium, these systems display
structural organizations and dynamical properties distinct from thermalized
colloidal assemblies. Harvesting the potential of this new class of systems
requires the development of a conceptual framework to describe these
intrinsically nonequilibrium systems. We use sedimentation experiments to probe
the nonequilibrium equation of state of a bidimensional assembly of active
Janus microspheres, and conduct computer simulations of a model of
self-propelled hard disks. Self-propulsion profoundly affects the equation of
state, but these changes can be rationalized using equilibrium concepts. We
show that active colloids behave, in the dilute limit, as an ideal gas with an
activity-dependent effective temperature. At finite density, increasing the
activity is similar to increasing adhesion between equilibrium particles. We
quantify this effective adhesion and obtain a unique scaling law relating
activity and effective adhesion in both experiments and simulations. Our
results provide a new and efficient way to understand the emergence of novel
phases of matter in active colloidal suspensions.
