Our active gels consist of three major components: 1) microtubules, 2) kinesin motors and 3) non-adsorbing depleting polymers. Kinesin motors are bound into synthetic clusters with tetrameric streptavidin. The depleting polymer bundles microtubules while the kinesin clusters simultaneously bind to and move along the neighboring filaments. For aligned microtubules with opposite polarity, kinesin clusters generate interfilament sliding that drives the flow of the background fluid.
At finite concentrations (≥0.5 mg/mL), active microtubules bundles form a self-rearranging 3D isotropic netowork, whose dynamics comprise repeating cycles of bundles extending, buckling, fracturing, and annealing. Such dynamics effectively drives the flow of the background fluid that is mostly composed of an aqueous buffer (~99.9%). In bulk suspensions, microtubule-based active networks generate turbulent-like flows that exhibit neither long-range order nor net material transport.
When active gels are confined in a toroid, they self-organize into a coherent flowing state. The gel drives the background fluids, whose motion is visualized with tracers.
The confocal microscopy reveals motion of microtubules (red) and tracers (green). Flow rates of microtubules and tracers reveal no measurable differences.
Coherent flows develop not only in toroidal but also cylindrical confinements. However not all confinements sustain the coherent states. Decreasing confinement heights suppresses coherent flows, inducing a transition to turbulent states.
Flowing direction can be controlled by decorating the toroid outer surface with notches.
Varying the toroidal radii does not alter flow rates, implying that the coherent flow is independent of channel curvature.
Varying circumferences does not alter flow rates, implying that coherent flow is independent of channel length.
The flow persists even in a 1.1-meter-long flexible tube, proving the feasibility of transporting materials on macroscopic scales internally.
Coherent flows develop in two (doublet) and three (triplet) connected toroids. Fluids in the doublet can develop a CCW and CW circulations (top left). In three adjoining toroids, fluids can flow in the outer edge while leaving center quiescent (bottom left) or develop two CCW and one CW circulations (bottom right).
In a thin toroid (right, h = 100 μm), microtubules move irregularly. Increasing confinement height to h = 400 μm triggers a self-organized transition from turbulent to coherent flows (left).
To extract 3D bundle structure, we use SOAX. The profile of microtubule bundles are represented as magenta snakes whose intersections are marked with green dots.
We have demonstrated that nonequilibrium transitions in active matter can be used for the robust assembly of self-organized machines that produce useful work by powering large-scale fluid flow. These machines are fueled by the collective motion of the constituent nanometer-sized motors and are thus evocative of living organisms. However, the goals of active matter research are not only to recreate materials found in nature but additionally are to discover and elucidate the full range of dynamical organization that is possible in living and nonliving systems. Toward this end, we are looking forward to collaborating with theorists, designing experiments for uncovering the underlying mechanisms responsible for such a transition from turbulent to coherent flowing states.
For more details, see Wu, et al. Science 355, eaal1979 (2017).