A fiber is provided that has been thermally drawn from a fiber preform, having a longitudinal-axis length and including at least one core that has a longitudinal core axis parallel to the longitudinal axis and internally disposed to at least one outer fiber cladding material layer along the fiber length. The fiber is fed through a localized heating site having a heating site temperature, T, that is above a melting temperature of the fiber core, with a feed speed, υf, that melts a portion of the fiber core at the heating site, causing molten droplets to pinch off of fiber core material, one droplet at a time, with a time period of molten droplet formation set by the fiber feed speed, υf. The fiber is fed through the localized heating site to move the molten droplets out of the heating site and solidify the molten droplets into solid in-fiber particles.