Active polymerized membranes

The emergence of order from disorder is the common counter-intuitive property of complex systems. In mixtures of sterically interacting colloids and nonadsorbing polymers, for instance, colloidal crystals may self-assemble to maximize the polymers’ entropy. In contrast to such equilibrium examples, a basic description of nonequilibrium self-organization remains a challenge. I will present experiments where material organization is governed by stresses that are exerted by mechanically active microscopic components. Active chaotic flows drive the assembly and large-scale dynamics of polymerized membranes in an aqueous environment. Initially, homogeneously and isotropically distributed actin filament bundles condense into a thin layer where they connect to form a porous elastic membrane. The polymerized membranes then exhibit out-of-plane bending fluctuations that exceed thermal motions by orders of magnitude. Active bending endows the fluctuating membranes with in-plane deformation soft modes that coarsen into large, millimeter-scale, strain fluctuations. For membranes that are a few millimeters in width, system-size displacement oscillations appear that are coupled to unidirectional flow waves. Active stress is thus an emerging paradigm for the assembly and dynamics of matter. I will discuss future extensions of this principle.