Gyroscopic rotation of boiling droplets

The self-propelled motion of boiling droplets has attracted strong interest, and major discoveries are concentrated in the film boiling regime, e.g., translational motion of Leidenfrost drops on ratchets, Leidenfrost wheels. However, little attention was paid to the boiling regimes below the Leidenfrost point. Here, we focus on those boiling regimes and discover a gyroscopic rotation phenomenon of boiling droplets that is ubiquitous on various types of surfaces with diverse wettability and microstructures. The occurrence of gyroscopic rotation can be attributed to the viscous stress from vapor/bubble flows in the gaps of surface microstructures, verified by the results that for the experimental surfaces, the rougher the surface structures and the larger the solid–liquid contact area, the more probable it is to generate gyroscopic rotations. A theoretical model is established to investigate the effect of substrate temperature (boiling regime) on the spinning rate of boiling droplets, and the results further approve the proposed mechanism of gyroscopic rotation. The outcomes of this work help to deepen the understanding of droplet boiling and the corresponding dynamics on surfaces with microstructures.