The directed transport of microparticles in microfluidic devices is vital for efficient bioassays and fabrication of complex microstructures. There remains, however, a need for methods to propel and steer microscopic cargo that do not require modifying these particles. Using theory and experiments, we show that catalytic surface reactions can be used to deliver microparticle cargo to specified regions in microchambers. Here reagents diffuse from a gel reservoir and react with the catalyst-coated surface. Fluid density gradients due to the spatially varying reagent concentration induce a convective flow, which carries the suspended particles until the reagents are consumed. Consequently, the cargo is deposited around a specific position on the surface. The velocity and final peak location of the cargo can be tuned independently. By increasing the local particle concentration, highly sensitive assays can be performed efficiently and rapidly. Moreover, the process can be repeated by introducing fresh reagent into the microchamber.

Self-propelled, active colloidal systems are of great current interest from both fundamental as well as practical standpoints, with potential applications in nanomachinery, nanoscale assembly, robotics, fluidics, and chemical/biochemical sensing. This perspective focuses on chemicallypowered catalytic nano and micromotors. We review the major advances to date in motor design, propulsion mechanisms and directional control, and inter-motor communication leading to collective behavior. We conclude by discussing the next steps in going forward: the fundamental questions that remain to be addressed and new design principles required for useful applications.

We report an autonomous oscillatory micromotor system in which active colloidal particles form clusters, the size of which changes periodically. The system consists of an aqueous suspension of silver orthophosphate microparticles under UV illumination, in the presence of varying concentrations of hydrogen peroxide. The colloid particles first attract each other to form clusters. After a short delay, these clusters abruptly disperse and oscillation begins, alternating between clustering and dispersion of particles. After a cluster oscillation initiates, the oscillatory wave propagates to nearby clusters and eventually all the clusters oscillate in phase-shifted synchrony. The oscillatory behavior is governed by an electrolytic self-diffusiophoretic mechanism which involves alternating electric fields generated by the competing reduction and oxidation of silver. The oscillation frequency is tuned by changing the concentration of hydrogen peroxide. The addition of inert silica particles to the system results in hierarchical sorting and packing of clusters. Densely packed Ag PO particles form a non-oscillating core with an oscillating shell composed largely of silica microparticles.

We report an autonomous oscillatory micromotor system in which active colloidal particles form clusters, the size of which changes periodically. The system consists of an aqueous suspension of silver orthophosphate microparticles under UV illumination, in the presence of varying concentrations of hydrogen peroxide. The colloid particles first attract each other to form clusters. After a short delay, these clusters abruptly disperse and oscillation begins, alternating between clustering and dispersion of particles. After a cluster oscillation initiates, the oscillatory wave propagates to nearby clusters and eventually all the clusters oscillate in phase‐shifted synchrony. The oscillatory behavior is governed by an electrolytic self‐diffusiophoretic mechanism which involves alternating electric fields generated by the competing reduction and oxidation of silver. The oscillation frequency is tuned by changing the concentration of hydrogen peroxide. The addition of inert silica particles to the system results in hierarchical sorting and packing of clusters. Densely packed Ag3PO4 particles form a non‐oscillating core with an oscillating shell composed largely of silica microparticles.

Self‐organization through communication is observed in many systems throughout nature, often resulting in a collective response to a change in the environment. Herein, we demonstrate a synthetic system that exhibits chemical communication between small active and larger inactive particles which drives the reversible assembly of the latter into colloidal crystals. Specifically, we report the autonomous hexagonal packing of inert silica particles due to the oscillatory behavior of neighboring silver phosphate micromotors under UV light in the presence of hydrogen peroxide. The colloidal crystals are formed under UV illumination and relax into a disordered state when the light is turned off. Furthermore, oscillatory waves generated by the active particles cause “autonomous annealing”, or elimination of defects between crystal boundaries of the silica colloidal crystals.

The front cover artwork is provided by the Sen group at Penn State (USA). The image shows how active Ag3PO4 particles herd inactive SiO2 particles into colloidal crystals that exhibit oscillatory motion. The crystals disperse then rejoin/reorganize, undergoing “autonomous annealing”. Read the full text of the Communication at 10.1002/syst.201900021.

The Front Cover shows active Ag3PO4 nanoparticles herding bigger inactive SiO2 particles into colloidal crystals that exhibit oscillatory motion. The crystals disperse then rejoin/reorganize, undergoing “autonomous annealing”. More information can be found in the Communication by A. Altemose et al.

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