Importance of pinning effect of wetting in super water-repellent surfaces

We have first found that the pinning effect of wetting plays an important role as the 4th mechanism for the super water-repellent surfaces in some cases, particularly in the surfaces consisting of pillar structures. In such cases the plot of cos θR (θR: contact angle of a liquid on a rough surface) versus cos θ (θ: that on the flat surface) deviates remarkably from the expected curves of the Wenzel and the Cassie/Baxter theories, and one can judge the dominance of pinning effect in the super water-repellency from this plot. In addition the super water-repellent surfaces resulting from the pinning effect is not in the thermodynamically equilibrium state, but in the metastable one when the contact angle on the flat surface is smaller than 90°.