The interplay between activity and elasticity in model active composites*

Soft materials immersed in an active fluid can self-organize to produce novel and striking patterns. Yet understanding these “active composites” is challenging because their behavior emerges from processes acting across multiple time and length scales. We propose a framework for modeling such systems, in which the active fluid is treated as a fluctuating field that is correlated in both space and time. Inspired by experiments on passive actin networks embedded in an active microtubule fluid, we use theory and computer simulations to study elastic networks driven by such a spatiotemporally correlated noise. At low activity, the resulting correlated network motion can be understood within the framework of linear elasticity theory. As activity is increased, the finite extensibility of bonds gives rise to non-Gaussian strain fluctuations and percolating networks of correlated strain, resulting in striking system-spanning motions. These features reflect how a tension between active forces that drive motility-induced phase separation and the constraints of network connectivity give rise to a new class of emergent behaviors. Our results demonstrate that the interplay of elasticity and active driving forces can give rise to new kinds of collective motions whose spatiotemporal dynamics can be controlled by tuning the length and time correlations of active fluids.