Radial illumination enables the concentration, dispersion, lateral transport, and sorting of photocatalytic TiO2 microspheres.

The precise manipulation of a large number of colloidal particles in lateral (xy) dimensions is important for various applications. We introduce a simple way of concentrating, dispersing, moving, and sorting a large group of isotropic, photocatalytic TiO2 microspheres in 2D by moving the optical condenser of a microscope up and down. This operation varies the vertical (z) position of the beam waist of the illumination light and causes the light beam to be radially oblique, and the nonuniform photocatalysis on the surface of TiO2 microspheres causes them to move away from light via diffusiophoresis/osmosis.