Spontaneous motion in hierarchically assembled active matter

Active materials are hierarchically assembled, starting from extensile microtubule bundles, to form emulsions with unexpected collective biomimetic properties such as autonomous motility. Self-driven active matter Autonomous motion is a characteristic of living organisms; by consuming energy, cells and their components can generate motion without the need for externally applied force. This paper reports the creation of polymer gels, liquid crystals and emulsions that mimic this behaviour using biological molecules as building blocks. The authors assemble microtubules into hierarchical bundles and then into percolating networks. In the presence of ATP as the chemical energy source and the molecular motor protein kinesin, spatiotemporally chaotic flows are generated by creating hydrodynamic instabilities and enhanced fluid transport. When confined to the surface of emulsion droplets, the microtubule networks form two-dimensional active liquid crystals that impart autonomous motility to the emulsion droplets. This work raises the exciting possibility that chaotic behaviour of this type could be engineered to be tunable and controllable. With remarkable precision and reproducibility, cells orchestrate the cooperative action of thousands of nanometre-sized molecular motors to carry out mechanical tasks at much larger length scales, such as cell motility, division and replication 1 . Besides their biological importance, such inherently non-equilibrium processes suggest approaches for developing biomimetic active materials from microscopic components that consume energy to generate continuous motion 2 , 3 , 4 . Being actively driven, these materials are not constrained by the laws of equilibrium statistical mechanics and can thus exhibit sought-after properties such as autonomous motility, internally generated flows and self-organized beating 5 , 6 , 7 . Here, starting from extensile microtubule bundles, we hierarchically assemble far-from-equilibrium analogues of conventional polymer gels, liquid crystals and emulsions. At high enough concentration, the microtubules form a percolating active network characterized by internally driven chaotic flows, hydrodynamic instabilities, enhanced transport and fluid mixing. When confined to emulsion droplets, three-dimensional networks spontaneously adsorb onto the droplet surfaces to produce highly active two-dimensional nematic liquid crystals whose streaming flows are controlled by internally generated fractures and self