An apparatus and a process for modifying the surface energy of particles using a reactive gas or reactive energetic species generated in an excited and/or unstable gas stream is provided. The apparatus and process incorporates a process container, a portion of which is movable and having a shape in which the particles are tumbled during exposure to the energetic species. The resulting surface treatment allows the energy modified particles to be more easily dispersed in a liquid medium with a reduction in the occurrence of particle agglomeration or flocculation.