Simulating droplet adhesion on superhydrophobic surfaces

A numerical model is proposed to simulate the adhesion, compression, and subsequent detachment of a micro-liter droplet from a superhydrophobic surface composed of chemically homogeneous pillars arranged in a periodic fashion, replicating a typical force probe microscopy experiment. We observe that as the droplet is pulled away from the surface, the net vertical force varies in a typical sawtooth manner with peculiar peaks and troughs, characteristic of the surface. The force first reaches a maximum before the droplet detaches from the surface with a comparatively lower force. The force variation predicted by the numerical model is in good agreement with the experimental results of Kumar et al. [1]. We also studied the effect of evaporation on the variation in the adhesion force by simulating an evaporating droplet on a superhydrophobic surface. For an evaporating droplet, the numerically predicted maximum and detachment force magnitudes are in good agreement with those obtained experimentally when we take into account the change in the droplet weight as it evaporates. The proposed method will be useful for the quantitative analysis and design of a variety of superhydrophobic surfaces and will pave the way for more accurate surface characterization based on droplet adhesion force measurements.