Stigmergic furrowing by an active suspension in a soft substrate

“Active fluids”, such as suspensions or colonies of motile bacterial cells, are known to self-organise to create complex patterns. It is of considerable interest to understand how such patterns may be modified by mechanical interactions of an active fluid with its environment. It has been observed that motile bacteria form furrows when confined to move as a monolayer across a soft semi-solid substrate [3]. We propose a particle-based model of active rods moving in a soft, elastically bound substrate. The substrate becomes fluid like after being displaced beyond a critical distance. Simulations show the emergence of permanent furrow networks with a distinctive fractal-like structure. We explain the physical origins of this behaviour through a coarse-grained model of stigmergy – the phenomenon of self-organisation through interactions between agents mediated by their environment.