On the ultra-rapid mixing in two colliding Leidenfrost drops

Leidenfrost drop can accelerate chemical reactions, offering great potential for droplet-based chemical reactors. By leveraging their motion on heated surfaces with micro-rachets, we demonstrate that mixing can be further enhanced through head-on collisions of two Leidenfrost drops. This study identifies three mixing stages. In Stage I, collision dynamics and film drainage between drops control coalescence, with surface tension disparities prolonging Stage I for heterogeneous drops. Stage II is driven by advective transport, though viscous effect from deformation can slow internal flow. In Stage III, oscillations promote mixing. For identical drops, complete mixing can be achieved within 2-3 oscillations. However, when drops with different boiling points collide, bubble nucleation emerges from the contact surface. Boiling not only prolongs the transition to Stage III or even disrupts stable oscillations but also hinders mixing through selective evaporation. In this study, the most rapid mixing occurs when two 10 µl Leidenfrost drops of water coalesce. Complete mixing is achieved within 100 ms, about two orders of magnitude faster than conventional methods. The results provide insights into optimizing Leidenfrost drop reactors and highlight the benefits of the extreme mobility of the Leidenfrost state for applications requiring rapid mixing.