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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. 

Abstract Artificial self‐propelled colloidal particles have recently served as effective building blocks for investigating many dynamic behaviors exhibited by nonequilibrium systems. However, most studies have relied upon excluded volume interactions between the active particles. Experimental systems in which the mobile entities interact over long distances in a well‐defined and controllable manner are valuable so that new modes of multiparticle dynamics can be studied systematically in the laboratory. Here, a system of self‐propelled microscale Janus particles is engineered to have contactless particle–particle interactions that lead to long‐range attraction, short‐range repulsion, and mutual alignment between adjacent swimmers. The unique modes of motion that arise can be tuned by modulating the system's parameters. 

Abstract Self‐propelled colloids are primed to become scaled up, nano‐ and microscale inorganic analogues of molecular motors and machines. In order to advance toward the ambitious goal of employing such active particles to form genuine man‐made small scale machinery, a significantly diversified library of particle types, capable of a wide range of motive behaviors, must be available. Here, it is shown that the dynamics of photoactivated, self‐phoretic particles can be engineered by targeted design of metal–semiconductor heterojunctions. This effect is demonstrated with three different microswimmers consisting of an elongated semiconducting tail made from anatase titanium dioxide; all three of which would otherwise be identical absent vapor‐deposited coatings of gold at different locations on the tails. The specific location of the heterojunction determines the swimming behavior for each type. Although here only one shape and material combination is focused upon, engineering active particles with site‐specific metal–semiconductor heterojunctions is a general technique for achieving desired kinematic behavior in active colloidal matter.