Evaporation of Squeezed Droplets between Two Non-Parallel Hydrophobic Surfaces

When a droplet is squeezed by two non-parallel hydrophobic surfaces, both the constant contact angle (CCA) mode and constant contact radius (CCR) mode are observed during evaporation. However, the evaporating droplet between two non-parallel surfaces would laterally transport towards the cusp due to its asymmetric shape. At the beginning of CCA mode, the contact line motion of the droplet is asymmetric and only the receding contact line keeps moving whereas the advancing contact line is fixed. Once droplet volume is reduced to certain volume, both the advancing and receding contact lines begin to move while the whole droplet is self-propelled towards the cusp. Finally, the evaporating droplet will be pinned then rupture near the cusp, leaving two daughter droplets at the upper and lower surfaces, respectively. We use Surface Evolver to obtain the shapes and positions of squeezed droplets during the CCA mode. The evolution of Laplace pressure difference and adhesion force exerted on the droplet are calculated. The droplet mass scales as ~R³ during evaporation, while the length of contact line and surface scale as R and R², respectively. It results in the increase of droplet acceleration during the evaporation which is the origin of lateral transport of a smaller droplet.