Figure 4. Drop retraction and bouncing dynamics.

(a) Schematic drawings of the rim and central film on the symmetric and asymmetric surfaces, respectively. On the symmetric surface, the rim retracts uniformly inwards towards the central film. On the asymmetric surface, the central film is an ellipse with major axis b (in the azimuthal direction) and minor axis a (in the axial direction) and retraction is primarily along the axial direction. (b) Selected side-view images of drop retracting on the asymmetric surface in the axial direction. The drop rim is drawn inwards by the central film and the size of rim remains almost unchanged in the majority of retraction process. (c) Selected plan-view images of drop retracting on the asymmetric surface. (d) Selected side-view images of drop retracting in the azimuthal direction. Owing to preferential liquid pumping around the rim (blue arrows), there is a continuous increase in the height of the azimuthal rim. Moreover, as the drop retracts on the curved surface, the surface tension energy converts to kinetic energy with a velocity component in the vertical direction (red arrows). (e) Comparison of the time evolution of the normalized rim heights in the axial and azimuthal directions based on the simulation. The height is scaled by the drop diameter D₀ and time by the contact time t₀. During the retraction stage the axial rim height stays roughly constant while the azimuthal rim height increases greatly owing to the preferential flow and mass transfer. The reduced mass of the axial rim rendered by the symmetry-breaking flows results in a remarkably efficient pathway for fast drop retraction.