We examine a minimal model for an active colloidal fluid in the form of self-propelled Brownian spheres that interact purely through excluded volume with no aligning interaction. Using simulations and analytic modeling, we quantify the phase diagram and separation kinetics. We show that this nonequilibrium active system undergoes an analog of an equilibrium continuous phase transition, with a binodal curve beneath which the system separates into dense and dilute phases whose concentrations depend only on activity. The dense phase is a unique material that we call an active solid, which exhibits the structural signatures of a crystalline solid near the crystal-hexatic transition point, and anomalous dynamics including superdiffusive motion on intermediate time scales.

Viruses are nanoscale entities containing a nucleic acid genome encased in a protein shell called a capsid and in some cases are surrounded by a lipid bilayer membrane. This review summarizes the physics that govern the processes by which capsids assemble within their host cells and in vitro. We describe the thermodynamics and kinetics for the assembly of protein subunits into icosahedral capsid shells and how these are modified in cases in which the capsid assembles around a nucleic acid or on a lipid bilayer. We present experimental and theoretical techniques used to characterize capsid assembly, and we highlight aspects of virus assembly that are likely to receive significant attention in the near future.

Confinement of active particles dramatically alters their spatial distribution and mechanical properties.

Watching defects flow and grow: Orientational topological defects in liquid crystals, known as disclinations, have been visualized in polymeric materials or through mesoscale simulations of the local orientation of the molecules. Duclos et al. report the experimental visualization of the structure and dynamics of disclination loops in active, three-dimensional nematics using light-sheet microscopy to watch the motion of nematic molecules driven by the motion of microtubule bundles (see the Perspective by Bartolo). This setup makes it possible to directly watch the nucleation, deformation, recombination, and collapse of spatially extended topological defects in three dimensions. Science , this issue p. 1120 ; see also p. 1075