Let us discuss the origin of the non-wetting property of the HDBSsurface. In our case, the Cassie-Baxter equation where a liquid droplet isin contact with both the substrate and air can be written aswhere fs is the relative fraction of a solid surface underneath a liquiddroplet, θY denotes the Young contact angle which is defined as theapparent (equilibrium) value of θc on a flat surface, and θc1 denotes thecontact angle predicted in the Cassie air-trapping wetting model. As theBS density increased, the wetting transition from the intrinsic polymerstate (the Young state) to the Cassie air-trapping state was induced onthe surface. For the BS with w=500 μm, the measured values ofθc=93.7° and θY=88.7° for the UV-curable polymer (NOA74)whereas for w=5μm, θc=123.3° and θc1=118.9°. This indicates thatthe hydrophobicity on the BS is significantly altered by the air pocketstrapped in the HDBS. In general, the effect of the air pockets on thesurface wettability critically depends on the amount of the roughness[22,23], which is consistent with our results.