The present invention relates to method for modifying or treating solid particles which includes the steps of: (a) forming a gelled mixture by mixing either simultaneously or sequentially in any order: (1) hollow microspheres each comprising a deformable polymeric shell having entrapped therein an expandable fluid, (2) a polar organic solvent capable of swelling but not dissolving the polymeric shells of the hollow microspheres, and (3) solid particles, wherein micro-channels are formed in the swelled polymer shells to allow entry of the solid particles into the hollow microspheres and exit of the expandable fluid therefrom, thereby forming microspheres that each comprises a collapsed polymeric shell in a gelled state and has one or more of said solid particles entrapped therein; (b) removing the expandable fluid and the polar organic solvent from the gelled mixture; and (c) coating the microspheres thus-produced with a film-forming material to form a liquid-impermeable membrane thereon.