  A liquid in contact with a textured surface can be found in two states,
Wenzel and Cassie. In the Wenzel state the liquid completely wets the
corrugations while in the Cassie state the liquid is suspended over the
corrugations with air or vapor trapped below. The superhydrophobic properties
of the Cassie state are exploited for self-cleaning, drug delivery etc., while
in the Wenzel state most of these properties are lost; it is therefore of
fundamental and technological interest to investigate the kinetics and
mechanism of the Cassie-Wenzel transition. Computationally, the Cassie-Wenzel
transition is often investigated using enhanced sampling techniques based on
the use of collective variables (CVs). The choice of the CVs is a crucial task
because it affects the free-energy profile, the estimation of the free-energy
barriers, and the evaluation of the mechanism of the process. Here we
investigate possible simulation artifacts introduced by common CVs adopted for
the study of the Cassie-Wenzel transition: the average particle density in the
corrugation of a textured surface and the coarse grained density field at
various levels of coarse graining. We also investigate possible additional
artifacts associated to finite size effects. We focus on a pillared surface. We
show that the use of the average density of fluid in the interpillar region
brings to severe artifacts in the relative Cassie-Wenzel stability and in the
transition barrier. A proper description of the wetting mechanism and its
energetics requires a rather fine discretization of the density field.
Concerning the finite size effects, we have found that the typical system
employed in the Cassie-Wenzel transition, containing a single pillar within
periodic boundary conditions, prevents the break of translational symmetry of
the liquid-vapor meniscus during the process.
