The law of hydrodynamics governing the way internally driven systems such as biological cells and bacteria behave could explain their complex structure and their inherent properties. Hydrodynamics is used here to understand the physical mechanism responsible for changes in the long-range order of groups of particles. The present work concerns ordered groups of elongated self-propelled particles, studying the breakdown of long-range order due to fluctuations that render them unstable and give rise to complex structures.

The authors coined the term self-propelled nematics to refer to internally driven elongated particles that spontaneously align head to tail, like tinned sardines. These are characterised by an ordered state that is stationary on average. This means that there is a long-range order, whereas the locally preferred direction may vary throughout the medium due to local strains or disturbances.

It is found here that a uniform nematic state can be disturbed by density fluctuations associated with an upward current of active particles. Since the density in turn controls the onset of nematic order, this phenomenon is self-regulating and universal.

It is also found that instability could be triggered by a local distortion of particles’ orientation. Such a distortion results in local currents that in turn amplify the distortion, leading to instability deep inside the nematic state.

Ultimately, this work may help us gain a deeper understanding of pattern formation and dynamics in a variety of internally driven systems, from epithelial cells and soil bacteria such as Myxococcus xanthus, to colloidal self-propelled nanorods.