We report the continuous growth of spherical assemblies of silica and titania nanoparticles, which were pre-activated to produce hydroxyl/peroxy groups on their surface, by flowing a colloid-in-dioctyl phthalate emulsion into a polydimethylsiloxane microfluidic chip, which is held at a temperature of 120–130 °C. When the colloidal microdroplets enter the chip, the instantaneous heating results in vapour-based convective agitation and emulsion breakup, leading to microdroplet fusion and subsequent merging of their nanoparticle content. This progressively results in a robust large assembly obtained after sintering at 600 °C owing to strong chemical interparticle bonds.