  Understanding the critical condition and mechanism of the droplet wetting
transition between Cassie-Baxter state and Wenzel state triggered by an
external electric field is of considerable importance because of its numerous
applications in industry and engineering. However, such a wetting transition on
a patterned surface is still not fully understood, e.g., the effects of
electro-wetting number, geometry of the patterned surfaces, and droplet volume
on the transition have not been systematically investigated. In this paper, we
propose a theoretical model for the Cassie-Baxter- Wenzel wetting transition
triggered by applying an external voltage on a droplet placed on a
mircopillared surface or a porous substrate. It is found that the transition is
realized by lowering the energy barrier created by the intermediate composite
state considerably, which enables the droplet to cross the energy barrier and
complete the transition process. Our calculations also indicate that for fixed
droplet volume, the critical electrowetting number (voltage) will increase
(decrease) along with the surface roughness for a micro-pillar patterned
(porous) surface, and if the surface roughness is fixed, a small droplet tends
to ease the critical electrowetting condition for the transition. Besides,
three dimensional phase diagrams in terms of electrowetting number, surface
roughness, and droplet volume are constructed to illustrate the
Cassie-Baxter-Wenzel wetting transition. Our theoretical model can be used to
explain the previous experimental results about the Cassie-Baxter-Wenzel
wetting transition reported in the literature.
