Radial illumination enables the concentration, dispersion, lateral transport, and sorting of photocatalytic TiO ₂ microspheres

The precise manipulation of a large number of colloidal particles in lateral (<i>xy</i>) 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 (<i>z</i>) 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 <i>via</i> diffusiophoresis/osmosis.