Polar flocking of active clusters

Biological active matter, such as populations of cells and animals, often change between different flocking regimes. One example is shoaling, milling and schooling transitions in fish. Synthetic active matter consist of self-propelled inanimate units and can emulate the flocking behavior of biological active matter. Here we report on a system of electro-propelled rolling granular beads with tuneable interactions. Many active matter regimes is realized in the same experiment as a function of the electric field: including active crystals and clusters with long range polar order, a stripe phase of clusters, and polar liquid flocks. Remarkably, the crystal to liquid transition occurs at a different velocity threshold than the local to global polar order transition. The stripe phase is reminiscent of those seen in quasi two-dimensional matter with competing interactions. The experimental system offers a physical model for flocking transitions in biological active matter, and can also open new routes for controlling self-assembly in soft matter technologies