Encoding Structure, Mechanics, and Dynamics of a Soft Solid with an Active Fluid

The principle of free energy minimization allows one to design macroscale properties of equilibrium assemblages to microscopic interactions. In comparison, our understanding of how large-scale structure and dynamics occur away from equilibrium is less complete. We describe a hierarchical self-organizing process of remarkable complexity. Starting with an initially uniform mixture of microtubule-based active fluid and passive actin filaments, we observe the emergence of complex structures and dynamical patterns on length scales ranging from nanometers to millimeters. Active fluids sculpt the structure, shape, mechanics, and dynamics of the actin network. Eventually, one observes the formation of macroscopically large actin-based thermalized membranes whose out-of-plane bending rigidity and in-plane oscillatory patterns are driven by the enveloping actin fluid. Taken together, these experiments demonstrate a need for developing a theoretical understanding of out-of-equilibrium self-organizing processes.