On the ultra-rapid mixing in two colliding Leidenfrost drops

Abstract

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.

Keywords: Binary collision; Coalescence; Leidenfrost drop; Mixing process; Surface oscillation; Unequal size.

Fig. 1

Schematics of the experimental setup.

Fig. 2

Fluorescence calibration of a 30 µl Leidenfrost drop: relations of normalized image intensity and normalized concentration of uranine in (a) water drop, (b) ethanol drop, and (c) ethanol-water drop.

Fig. 3

The µPIV diagnosis and fluorescence visualization for the coalescence of two 10 µl Leidenfrost drops of water: images taken from (a) the side view (used to evaluate ME), and (b) the top view.

Fig. 4

Comparison of the variation of mixing efficiency with time for the collision of two identical drops. The dash lines indicate the division between Stage II and Stage III; the color black and gray denotes the water drops and ethanol drops, whereas the solid symbols and the open symbols represent a drop volume of 10 µl and 20 µl, respectively.

Fig. 5

Comparison of the variation of the projected area with time for the collision of two identical drops. The gray dash lines delineate the division between different stages of mixing.

Fig. 6

The fluorescence visualization for the coalescence of (a) 10 µl ethanol drop and 30 µl water drop, and (b) 30 µl ethanol drop and 10 µl water drop.

Fig. 7

Comparison of the variation of mixing efficiency with time for the collision of two non-identical drops. The dash lines indicate the division between Stage II and Stage III; the black and gray denotes the drops of the same fluid type and different fluid types, whereas the short dash and the long dash represent lower and higher ethanol concentration, respectively. The shaded region is the transition between Stage II and Stage III for the combination of 10 µl ethanol and 30 µl water drops.

Fig. 8

Oscillation frequencies of coalesced drops vs. the Rayleigh frequency.