There is now growing evidence of the impact of geometric confinement on the patterns of motion of biological materials. For example, experiments on cell monolayers and bacterial suspensions have shown that the interplay between the activity of the cells and the confinement size can stabilise otherwise chaotic flows of cells into coherent motion.
However, many such cellular systems are not composed of conserved number of particles. Indeed, in most natural conditions, cells in a monolayer or bacterial cells in a biofilm regularly proliferate and form a progressive front that infiltrates the surrounding environment. No current continuum theory in cell mechanics or in active matter can explain the mechanism of active matter invasion within confined spaces.