The asymmetric coalescence of two droplets with different surface tensions is caused by capillary waves

TL;DR

This study investigates how asymmetric capillary waves, caused by differing surface tensions, lead to asymmetric droplet coalescence, revealing the roles of surface tension differences, inertia, and Marangoni effects.

Contribution

The paper demonstrates that asymmetry in droplet coalescence is driven by intrinsic capillary wave differences, not Marangoni forces, and explores effects of inertia and surface tension.

Findings

Asymmetry increases with surface tension difference.

Inertia suppresses coalescence asymmetry.

Marangoni effect reduces asymmetry.

Abstract

When two droplets with different surface tensions collide, the shape evolution of the merging droplets is asymmetric. Using experimental and numerical techniques, we reveal that this asymmetry is caused by asymmetric capillary waves, which are the result of the different surface tensions of the droplets. We show that the asymmetry is enhanced by increasing the surface tension difference, and suppressed by increasing the inertia of the colliding droplets. Furthermore, we study capillary waves in the limit of no inertia. We reveal that the asymmetry is not directly caused by Marangoni forces. In fact, somehow counterintuitive, asymmetry is strongly reduced by the Marangoni effect. Rather, the different intrinsic capillary wave amplitudes and velocities associated with the different surface tensions of the droplets lie at the origin of the asymmetry during droplet coalescence.