  A lattice Boltzmann algorithm is used to simulate the slow spreading of drops
on a surface patterned with slanted micro-posts. Gibb's pinning of the
interface on the sides or top of the posts leads to unidirectional spreading
over a wide range of contact angles and inclination angles of the posts.
Regimes for spreading in no, one or two directions are identified, and shown to
agree well with a two-dimensional theory proposed in Chu, Xiao and Wang (Nature
Materials, 9, 413). A more detailed numerical analysis of the contact line
shapes allows us to understand deviations from the two dimensional model, and
to identify the shapes of the pinned interfaces.
