2.1. Fabrication and Treatment. The pillar-structured substrateswere fabricated as per the literature.27 To achieve hydrophobicity, thesubstrates were treated with an air-plasma generator (DT-02S, Opsplasma technology Co., Ltd.) at 200 W for 180 s. Then, the substrateswere modified with 1H,1H,2H,2H-perfluorodecyltrimethoxysilane in avacuum chamber through chemical vapor deposition. For preparing themultilayer bubbles, the pillar-structured substrate should have apublished and hydrophilic reverse side. The hydrophilicity in thebackside can be obtained by covering the backside with a plastic wrapduring the hydrophobic treatment above.