Light-propelled microparticles based on symmetry-broken refractive index profiles

TL;DR

This paper introduces 3D-printable, symmetry-broken refractive index particles that achieve light-driven propulsion through asymmetric refraction, enabling deep light penetration and potential for adaptive optical materials.

Contribution

It presents a novel transparency-based propulsion mechanism using internal refractive index gradients, decoupling motion from particle shape and reducing heating effects.

Findings

Validated momentum transfer mechanism experimentally.

Numerical simulations confirm propulsion via asymmetric light refraction.

High transparency allows deep light penetration for volumetric active matter.

Abstract

Active colloidal microparticles require reliable actuation to sustain directed motion. Light-based propulsion is particularly attractive as it provides persistent energy supply and enables direct spatiotemporal control. Here, we introduce 3D-printable particles with symmetry-broken refractive index profiles (SBRIP particles) that achieve propulsion through direct momentum transfer from asymmetric light refraction. Internal refractive-index gradients provide optical symmetry breaking independent of external shape, fundamentally decoupling propulsion from particle geometry. Geometrically symmetry-broken particles with a homogeneous refractive index are another special case, where propulsion originates from refractive contrast at the boundary instead of within the particle. Unlike conventional systems relying on absorption or reflection, this transparency-based mechanism minimizes heating and mitigates shadowing in bulk suspensions. We present a theoretical framework for refractive propulsion as well as numerical simulations of the SBRIP particles using raytracing and the finite volume method. This is complemented by experiments, validating the momentum transfer mechanism using particles with geometric symmetry breaking. The high transparency of our particles ensures deep light penetration, enabling the realization of volumetric active matter. This opens pathways toward adaptive nonlinear optical materials where light-driven particle reorganization modulates the local refractive index, establishing a dynamic feedback loop between the optical field and the material structure.