“Jumping flame propagation”: Almost-periodic combustion dynamics of iron microwires

A “jumping” combustion phenomenon of iron microwires was discovered and characterized by using an advanced optical diagnostic setup capable of 10k fps shadowgraph imaging and synchronized two-color thermometry. We systematically examined the iron microwires with a fixed length (6 mm) but varying cross-sectional areas (3000–23400 µm 2 ). The results reveal that all the tested cases exhibit an almost-periodic “jumping” combustion propagation, which can be divided into four distinct phases: tip combustion—continuous heat release from the molten droplet melts the downstream microwire; neck formation—melted downstream material merges with the tip droplet, creating a characteristic neck structure; droplet jumping—rapid droplet movement (<1 ms) driven by surface tension, leaving partially unmelted microwire behind; tip retraction—post-jump combustion completes melting of the residual solid microwire through heat release. Furthermore, the microwire’s cross-sectional area significantly influences the combustion dynamics, with thicker microwires exhibiting slower average propagation speed, reduced “jumping” frequency, and shorter quenching distances, attributed to the size-dependent heat capacity and heat loss.