Refractive microswimmers with a symmetry-broken refractive index profile

We present a novel type of active matter system consisting of a suspension of microparticles fabricated from a transparent polymer with a symmetry-broken refractive index profile. Under illumination these particles are subject to a driving force due to the momentum transfer associated with light refraction. Using the refractive index profile instead of the particle shape to break particle symmetry allows a decoupling of the propulsion mechanism from the hydrodynamic properties of the particles, which distinguishes this archetype of active particles from, e.g., acoustically driven particles. With structured light, a high degree of both spatial and temporal control of particle propulsion can be achieved. In contrast to optothermally driven particles, refractive microswimmers possess sensitivity also to the phase component of the light field giving a richer space of control parameters. Additionally, we find that particles can interact on much larger distances via mediation by the light field compared to simple steric repulsion. Combining this with external feedback mechanisms allows for high complexity particle networks, thus making this kind of active particle a promising candidate for novel, adaptive materials.