Pinning and depinning mechanism of the contact line during evaporation of nano-droplets sessile on textured surfaces

The contact line dynamics during the evaporation of nano-droplets sessile on textured solid substrates is investigated using molecular dynamics method. Our simulations reproduce the nanoscale physical phenomena such as the constant contact angle (CCA) and constant contact radius (CCR) evaporation modes which have been reported in recent experiments. The results also show that the localized liquid-solid interaction in the vicinity of the contact line dominates the wetting behavior on the textured surfaces, which also indicates a molecular origin of the pinning force. Increase of the characteristic width of the inhomogeneity enlarges the energy barrier to jumping out a hydrophilic textured element, which further results in a larger pinning force. For the evaporation of a nano-droplet on the surface with large pinning force, the anchoring of the contact line and the CCR mode are observed at the first onset. While on the textured surfaces with smaller characteristic width of the inhomogeneity, only the CCA and mixed modes can be found during the evaporation. This work provides a foundation on which the sessile nano-droplet evaporation can be understood from atomistic scale.