A thin film mill for simultaneous reduction in size as by separation of agglomerates of particle clusters and dispersion of the fine particles in a liquid carrier, constructed of a plurality of rotatable cylindrical sections arranged in a row, each section having a helical groove cut in its cylindrical surface, the grooves being variable in depth becoming shallower and wider with progressively increasing pitch from inlet to outlet, the sections being mounted for application eccentrically of radial pressure upon a thin film of the dispersion between each section and the walls of the confining casing.