Editorial: Active and externally driven granular matter

Active and externally driven granular matter are examples of heterogeneous systems that can spatiotemporally respond in a multitude of non-trivial ways. They exhibit both remarkable self-organization and collective behaviors. Their governing parameters are often the packing fraction and interparticle interactions. However, mixtures of particles and their geometry and/or chiral nature can provide additional ways to affect the manyparticle behavior. Some of these aspects are explored in this Research Topic, with an emphasis on the overall emergent behavior and system-wide structural and dynamical features. The article by Lopez-Castaño et al. is an experimental study, reporting on the occurrence of vortex flow in binary mixtures of self-propelled disks. The authors find that this collective chiral mode is not only related to the spin of individual disks, but also depends on the local particle structure, illustrating how many-body effects conspire to result in a system-wide collective behavior. The article by Zhao et al. reports simulations on rachet-like particles driven by an imposed shear or extensional flow. The results suggest that, apart from chiral active fluids, chirally-shaped passive particle systems also exhibit a non-zero, odd viscosity, which indicates the mixed properties of conventional complex fluids and chiral viscous fluids. Gao et al. present theoretical work ultimately aimed at addressing furrow formation in cell populations. The authors build on prior active granular fluid models for cell constriction and consider cell size fluctuations. The study demonstrates that shape changes result from coordinated intercellular dynamics, where mechanical feedback plays an important role. The work illustrates how generation of tissue architecture can be driven by intricate collective effects governed by mechanical forces. Megías and Santos develop a theory, which they test with computer simulations, on granular gases in order to develop protocols that allow a system with a higher initial temperature to relax to a steady state quicker than another system with a lower initial temperature, a phenomenon known as the Mpemba effect. A key aspect of the work is the addition of stochastic forces and torques at the particle level which effectively inject energy into the particle translational and rotational degrees of freedom. Saitoh and Kawasaki perform simulations on dense suspensions driven by shear, and study, among other things, the properties of dynamical heterogeneities in the system. The authors find scaling OPEN ACCESS