Marangoni effect of surfactant droplet in transition boiling and formation of secondary droplet

The dynamic processes of surfactant droplets impacting onto substrates of varied temperatures have been widely studied in heat transfer, cooling and printing. In this work, we observe the impacting process of aqueous droplets of surfactants SDS, CTAB, and Triton X-100 on a hot aluminum plate via a high-speed camera, in order to study the dynamics of different surfactant droplets impacting on a hot aluminum substrate. Experimentally, it is discovered that the surfactant droplets in transition boiling produce a secondary droplet of non-wetting state in the final stage of evaporation. The analysis demonstrates that after the droplet impacts the substrate, a temperature gradient is created between the top of the droplet and the triple-phase contact line, increasing the surfactant concentration near the triple-phase contact line as compared with that of the top. The top liquid is maintained by the Marangoni effect, which is caused by the concentration gradient. In the final stage of the evaporation process, the residual droplet gradually shrinks into a sphere. It is detached from the substrate and taken off under the impulse force of the bubble explosion at the bottom, generating the secondary droplet. The radius of the secondary drop increases with the raising of initial concentration of the drop, but ultimately reaches the saturation size. This work explains the role of surfactants in forming secondary droplets. Additionally, this work provides a reference for understanding the physical mechanism of Leidenfrost effect and the controlling of boiling heat transmission.