Light Activated Self-Propelled Colloids

TL;DR

This study presents a versatile method to synthesize light-activated self-propelled colloids using photoactive materials, demonstrating multiwavelength activation, controlled motion, and emergent collective behaviors like living crystals.

Contribution

The paper introduces a new approach for creating multiwavelength light-activated colloids with controllable propulsion and collective organization, advancing the design of active matter systems.

Findings

Particles exhibit persistent random walks under light activation.

Formation of mobile 'living crystals' at high surface density.

External magnetic fields can steer particle organization.

Abstract

Light-activated self-propelled colloids are synthesized and their active motion is studied using optical microscopy. We propose a versatile route using different photoactive materials, and demonstrate a multiwavelength activation and propulsion. Thanks to the photoelectrochemical properties of two semiconductor materials (\alpha Fe2 O3 and TiO2 ), a light with an energy higher than the bandgap triggers the reaction of decomposition of hydrogen peroxide and produces a chemical cloud around the particle. It induces a phoretic attraction with neighbouring colloids as well as an osmotic self- propulsion of the particle on the substrate. We use these mechanisms to form colloidal cargos as well as self-propelled particles where the light-activated component is embedded into a dielectric sphere. The particles are self-propelled along a direction otherwise randomized by thermal fluctuations, and exhibit a persistent random walk. For sufficient surface density, the particles spontaneously form "living crystals" which are mobile, break apart and reform. Steering the particle with an external magnetic field, we show that the formation of the dense phase results from the collisions heads-on of the particles. This effect is intrinsically non-equilibrium and a novel principle of organization for systems without detailed balance. Engineering families of particles self-propelled by different wavelength demonstrate a good understanding of both the physics and the chemistry behind the system and points to a general route for designing new families of self-propelled particles.