An Equation of State for Active Matter

Bird flocks, mammal herds, cellular tissue, and artificial self-propelled colloids are all examples of “active” matter—an assembly of self-driven units that take energy from the environment to produce motion [1]. Active matter is, in many ways, analogous to a gas of atoms or molecules, except that the particles are far from equilibrium, and therefore cannot be described with standard statistical physics. A thermodynamic equation of state, such as the relation between pressure and density in an ideal gas, would, however, be useful to characterize these systems. Such an equation could, in the long term, lead to a “theory” for certain forms of living matter and the principles that underlie their dynamics. Taking an important step towards this goal, Alexandre Solon at Diderot University in Paris and his colleagues have derived an equation of state for a “gas” of self-propelled colloidal spheres. The equation relates the pressure the spheres exert on a wall to the spheres’ properties, and the predictions of this equations are potentially testable in, for example, solutions of motile colloids [2].