The invention generally relates to an apparatus and a method for mixing of liquids (50) or of particles with a liquid (50). In a volume of liquid (50), a thermal convection flow is generated at at least one surface of the volume of liquid by irradiating IR radiation (30) into the volume of liquid. Thereby it is possible to avoid a depletion zone at the surface and to more accurately measure interactions of the particles with the surface by means of surface-based measurement methods.