Scalar Active Matter: Field Theories, Phase Ordering and Fluid Dynamics

Michael E. Cates

School of Physics and Astronomy

University of Edinburgh

The phase ordering (demixing) of simple fluids with attractive interactions can be described by dynamical phi-4 scalar field theories whose diffusive dynamics may or may not be coupled to a momentum-conserving solvent ('Model B' and 'Model H' respectively in the Hohenberg-Halperin nomenclature). Active fluids containing spherical self-propelled particles can also undergo phase ordering into dense and dilute isotropic phases (although unlike simple fluids, no attractions are required in this case). The phase ordering dynamics can again be described by phi-4 type theories, but these now contain terms that violate time reversal symmetry and which explicitly prevent construction of a free energy functional. I will describe what we have discovered so far about the effect of these symmetry-breaking terms on the phase ordering of scalar active matter as embodied in 'Active Model B' and 'Active Model H'.