  In this paper, the pinning and depinning mechanism of the contact line during
droplet evaporation on chemically stripe-patterned surfaces is numerically
investigated using a thermal multiphase lattice Boltzmann (LB) model with
liquid-vapor phase change. A local force balance in the context of diffuse
interfaces is introduced to explain the equilibrium states of droplets on
chemically patterned surfaces. It is shown that, when the contact line is
pinned on a hydrophobic-hydrophilic boundary, different contact angles can be
interpreted as the variation of the length of the contact line occupied by each
component. The stick-slip-jump behavior of evaporating droplets on chemically
patterned surfaces is well captured by the LB simulations. Particularly, a slow
movement of the contact line is clearly observed during the stick (pinning)
mode, which shows that the pinning of the contact line during droplet
evaporation on chemically stripe-patterned surfaces is actually a dynamic
pinning process and the dynamic equilibrium is achieved by the self-adjustment
of the contact lines occupied by each component. Moreover, it is shown that,
when the surface tension varies with the temperature, the Marangoni effect has
an important influence on the depinning of the contact line, which occurs when
the horizontal component (towards the center of the droplet) of the force
caused by the Marangoni stress overcomes the unbalanced Young's force towards
outside.
