We report on a novel concept of microreactor for the synthesis of metal nanoparticles (nP). The principle is to make grow by electrocrystallization long metallic ramified branches, made of metal nanocrystals, in a Hele-Shaw cell. The activation of an integrated vibrating element (PZT disk) induces the fragmentation of these fragile branches and so the nanocrystal release. One advantage is the possibility to flush the branches (and so the nP), prior to fragmentation, to avoid purification step. This principle is applied here to the synthesis of iron nP and focus is put on the branch fragmentation. High speed visualisations highlight the key role of H ₂ bubbles (co-formed during the branch growth). An effective fragmentation is obtained only using a square signal with which the initial bubbles coalesce. The resulting large bubbles exhibit shape oscillations and they induce microstreaming. This latter brings the branches close to bubble surface, where they are fragmented into fine particles. The required initial coalescence events are explained by a “dancing bubbles” effect made easier to achieve because initial small bubbles are excited at their resonance frequency by the sufficiently stiff signal steps. Transmission electron microscopy (TEM) reveals that dendritic particles, ∼ 2 μ m long and ∼ 1 μ m wide (broken secondary dendrites) with very high specific surface, and needle-like particles, ∼ 200 nm long and ∼ 20 nm in diameter (broken tertiary dendrites), are produced. A force balance, between the mechanical constraint applied by the fast flow near bubble surface and the material resistance, allows highlighting the key role of shape oscillations in the breakage of the dendrites.