Transition from turbulent to coherent flows in confined three-dimensional active fluids

Abstract

Transport of fluid through a pipe is essential for the operation of macroscale machines and microfluidic devices. Conventional fluids only flow in response to external pressure. We demonstrate that an active isotropic fluid, comprised of microtubules and molecular motors, autonomously flows through meter-long three-dimensional channels. We establish control over the magnitude, velocity profile and direction of the self-organized flows, and correlate these to the structure of the extensile microtubule bundles. The inherently three-dimensional transition from bulk-turbulent to confined-coherent flows occurs concomitantly with a transition in the bundle orientational order near the surface, and is controlled by a scale-invariant criterion related to the channel profile. The non-equilibrium transition of confined isotropic active fluids can be used to engineer self-organized soft machines.One Sentence Summary: An isotropic fluid composed of nano-sized motors organizes into an autonomous machine that pumps fluid through long channels. Recent studies have revealed emergence of diverse complex patterns in synthetic systems of active matter (21-24). The next step is to elucidate conditions that transform chaotic dynamics of these systems into coherent long-ranged motion that can be used to harvest energy and thus power various micromachines (25-29).Here, we study 3D active fluids and demonstrate an essential difference with their conventional counterparts. The Navier-Stokes equations dictate that a conventional fluid comprised of inanimate constituents will flow only in response to an externally imposed body force, or stress and pressure gradients (29). This is no longer true for active fluids. Indeed, in living organisms, the entire cellular interior can assume large-scale coherent flows in absence of any externally imposed stresses, a phenomenon known as cytoplasmic streaming (30-32). Despite recent advances using living bacterial suspensions (13,14,33,34), creating tunable synthetic active 3 fluids that exhibit autonomous long-ranged flows on length scales large compared to constituent units remains a challenge. We use a 3D microtubule-based isotropic active fluid whose bulk turbulent flows are driven by continuous injection of energy through the linear motion of the constituent kinesin motors (24, 35). We find that confinement robustly transforms locallyturbulent dynamics of such active fluids into globally-coherent flows that persist on meter scales.Our experiments demonstrate that non-equilibrium transitions of synthetic active materials can be used to engineer self-organized machines in which nanometer sized molecular motors collectively propel fluid on macroscopic scales. Microtubule-based active isotropic fluids:The active fluid we study is comprised of microtubule filaments, kinesin motor clusters and depleting polymer (Fig. 1A) (24, 35). Kinesin motors are bound into synthetic clusters with tetrameric streptavidin (36, 37). The depleting polymer induces microtubule bundling (38), while the kinesin c...

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Cytoskeletal and Actin‐Based Polymerization Motors and Their Role in Minimal Cell Design

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Controlling Organization and Forces in Active Matter Through Optically-Defined Boundaries

“…To perform these tasks, cells spatiotemporally coordinate the interactions of force-generating, "active" molecules that create and manipulate non-equilibrium structures and force fields that span up to millimeter length scales [1][2][3]. Experimental active matter systems of biological or synthetic molecules are capable of spontaneously organizing into structures [4,5] and generating global flows [6][7][8][9]. However, these experimental systems lack the spatiotemporal control found in cells, limiting their utility for studying non-equilibrium phenomena and bioinspired engineering.…”

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