cassie/wenzel
The Cassie and Wenzel states refer to the stages of a water droplet on a surface. A water droplet in its Cassie state is still spherical and sits above the surface, regardless of surface roughness. Alternatively, a water droplet in its Wenzel state has started to fill the small crevices of a roughened or micro-textured surface. The transition between these two states is a key component for researches when constructing icephobic surfaces. When the droplet moves into the Wenzel state, it stabilizes to a point where it may begin to freeze, adhering to the surface. Therefore, it is vital that water repellency of the icephobic surface avoids the Wenzel state.
wenzel state
cassie state
water droplets on textured solid surfaces
Researchers often place water droplets on textured solid surfaces to observe their evaporation, and change in droplet size during and after evaporation, in order to see what their geometric texture is doing in terms of water repellency. In other words, how 'sticky' the surface is to water is a function of water repellency and can be measured by how much water evaporates off of the surface. In addition, researchers study the shape of the water droplets post evaporation to explore the possibility of transitioning water from the Wenzel to the Cassie state which would highly benefit the hydrophobic, and therefore the icephobic, abilities of the surface. Geometric and roughened textures can also lead to air being trapped below the water droplet which would fit the Cassie model and even contribute to superhydrophobic behavior.
Based on some research, there is a possibility of an energy barrier between the Wenzel and Cassie states meaning a certain energy threshold would have to be met for the state transition to take place. If this were true, constructing a textured geometric surface that maintained the energy barrier, not allowing the state transition, would be possible. It is clear that this could potentially be a driving design element of textured geometric surfaces striving to be hydrophobic and, by avoiding the Wenzel state, icephobic as well.