Local molecular asymmetry mediated self-adaptive pinning force on the contact line

When a droplet is placed on a solid surface, the pinning force on the contact line is the key factor determining the dynamics of liquid droplet. As the apparent contact line position is not moving, the pinning force can still be adjusted to balance the changing driving force. However, the underlying mechanism of the self-adaptive vari-ations in the pinning force remains elusive. In this work, molecular dynamics simulations for evaporation of a liquid droplet on a chemically heterogenous substrate were carried out. Our investigation disclosed that the liquid atom on the solid surface experiences a tangential force from the substrate, unless it was located in an absolutely symmetrical position. There exists a non-uniform distribution of liquid atoms in the layer near the solid surface. The dependence of the capillary force on these liquid atoms on their locations relative to the solid atoms is the molecular origin of the self-adaptive pinning force on the contact line. The contact line position may be unchanged on the macroscopic scale, but the local distribution of liquid atoms had a subtle adjustment on the microscopic scale, giving rise to the pinning force on the contact line. A theoretical model was proposed to quantitatively describe the link between the pinning force on the contact line and the molecular asymmetry in the local distribution of liquid atoms.