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Abstract 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. 

Abstract It is demonstrated how the strength of activation for photocatalytic, self‐propelled colloids can be enhanced with a constant, uniform magnetic field. When exposed to ultraviolet light and hydrogen peroxide, the titanium dioxide‐based colloids become actively propelled. Due to the iron oxide core, a uniform field oriented perpendicular to the surface where motion takes place causes the asymmetrically shaped particles to rotate, which consequently leads to an increase in activity. The field‐dependent dynamics of self‐propulsion is quantified, and a qualitative description of how this effect arises is proposed. Since the application of the field is easily reversible, modulating the field on‐and‐off serves as a de facto “switch” that controls particle behavior. 

Abstract The evaporation of liquid droplets containing colloids is an omnipresent natural phenomenon that has received much attention due to the fundamental effects it entails, as well as the multitudes of fields in which it can be applied. The deposition of particulates onto a solid surface during evaporation tends to form ring‐like stains, which are a hallmark of the “coffee‐ring effect.” A wide variety of systems has already been employed to suppress or enable this effect, however, little attention has been focused on particles in restricted geometries that are driven far from equilibrium. Here, we investigate how self‐propelled, “active” catalytic Janus microspheres affect the ring stains left behind during the drying of a geometrically confined suspension containing such particles. Self‐propulsion results indirectly from the decomposition of hydrogen peroxide (H₂O₂) on the catalytically active hemispherical shell, while the diametrically opposite face is inert; this is how the system is driven out of equilibrium. The magnitude of activity can be controlled by adjusting the volume concentration of aqueous H₂O₂within the suspension. This parameter strongly influences the ring‐shaped microstructures obtained, especially when the concentration is sufficiently high to produce oxygen bubbles that take over the motion as opposed to auto‐phoresis. 

Self-propelling, light-activated colloidal particles can be actuated in water alone. Here we study the effect of adding different amounts of a gold/palladium alloy to titanium dioxide-based, active colloids. We observe a correlation between alloy-thickness and the average speed of the particles, and we discover an intermediate thickness leads to the highest activity for this system. We argue that a non-continuous thin-film of the co-catalyst improves the efficiency of water-splitting at the surface of the particles, and in-turn, the performance of “fuel-free” self-propulsion.