The traditional hand-held Japanese sparkler, Senko-hanabi, has been popular in Japan since the Edo period. The branching sparks, akin to pine-needles, are in fact the drops of a melt of potassium compounds accompanying their successive fragmentation with ever smaller droplets. This unique fragmentation cascade is self-sustained by the continuous heat from an exothermic surface reaction on the drop, in which internal nucleation growing as a bubble leads to it bursting at each step of cascade. In the present study, the drops of Senko-hanabi are found to be non-evaporative due to their low saturated vapor pressure. Then, the internal bubble radius is calculated. The bubble radius grows in proportion to the square root of the product of time and thermal diffusivity, which provides a direct evidence the thermal diffusion as the rate-controlling process. The thermal analysis indicates CO2 as the main component of the bubble, however, the gas production mechanism is still an open question.
sparkler, Senko-hanabi, bursting drop, bubble dynamics, visualization, thermal analysis