Phototunable tilt and levitation in catalytically active colloidal platelets

Chemically active colloids are synthetic microentities that self-propel by catalyzing surface reactions without the need for external fields. But, despite recent progress in steering field-driven colloids both near boundaries and in fluid bulk, achieving a similar control over chemically powered particles remains difficult. This is particularly challenging near boundaries, where phoretic and hydrodynamic interactions strongly couple to particle geometry. Here we introduce photoactivated hematite platelets capable of self-propulsion, lift, and levitation near a surface in a viscous fluid. These transport modes can be switched on or off and precisely controlled by light intensity. By combining experiments and analytical theory we show that the platelet tilted motion and levitation arise from the interplay of diffusiophoretic flow, gravitational forces, and geometric anisotropy. Using finite element simulations, we further unveil the interplay between phoretic and osmotic forces. The latter become increasingly important as the platelet slides close to the bottom surface due to gravity. In contrast to isotropic active colloids, the rich spectrum of dynamic regimes observed with our photoactivated platelets results from their large aspect ratio. This provides an alternative way to control and direct catalytic particles close to surfaces and in bulk fluids.