Gravitropically Stabilized Self-Assembly of Active Microcrystallites and Spinning Free Janus Particles

Active colloidal microcrystallites capable of generating flow patterns around or through their porous network are introduced, which in combination with “free microspheres,” create self-assembled active clusters with multiple moving parts. Fluid flow draws microspheres within a microcrystallite's local environment toward—and aggregate at—the edge of the microcrystallite, where the previously translational movement transitions to continuous spinning. These experiments show that the spinning frequency decreases with an increase in diameter and that when the center of mass of a spinning particle is shifted off-center—here Janus spheres—a time-varying angular frequency is observed. Weight-anisotropy also leads to a particularly intriguing phenomenon, which manifests as the spontaneous realignment of the rotational axis to a preferential direction; this effect is attributed to a gravitropic self-correcting mechanism. Thus, the dynamics of the self-assembled active structure remains stable over long time periods, despite being subjected to significant noise, for example, Brownian forces.