Tunable corrugated patterns in an active nematic sheet

Abstract

Significance To what extent can we engineer matter that shapes itself? To investigate this question we study an aqueous solution containing molecular motors that walk on protein filaments. When the filaments are long and attract each other, bundles of filaments are parallelly oriented. We show that such a nematic solution in the presence of multimers of motors has an unexpected behavior: It forms a fluid film that autonomously wrinkles. The observed wrinkles have a well-defined wavelength that decreases with increasing motor concentration. The wrinkles either are stable or break into a chaotic flowing state at high motor concentration, providing insights into how to engineer static or dynamic materials with this class of active matter. Active matter locally converts chemical energy into mechanical work and, for this reason, it provides new mechanisms of pattern formation. In particular, active nematic fluids made of protein motors and filaments are far-from-equilibrium systems that may exhibit spontaneous motion, leading to actively driven spatiotemporally chaotic states in 2 and 3 dimensions and coherent flows in 3 dimensions (3D). Although these dynamic flows reveal a characteristic length scale resulting from the interplay between active forcing and passive restoring forces, the observation of static and large-scale spatial patterns in active nematic fluids has remained elusive. In this work, we demonstrate that a 3D solution of kinesin motors and microtubule filaments spontaneously forms a 2D free-standing nematic active sheet that actively buckles out of plane into a centimeter-sized periodic corrugated sheet that is stable for several days at low activity. Importantly, the nematic orientational field does not display topological defects in the corrugated state and the wavelength and stability of the corrugations are controlled by the motor concentration, in agreement with a hydrodynamic theory. At higher activities these patterns are transient and chaotic flows are observed at longer times. Our results underline the importance of both passive and active forces in shaping active matter and demonstrate that a spontaneously flowing active fluid can be sculpted into a static material through an active mechanism.