Fig. 3. Schematics and confocal microscopic images of liquid-gas interfaces on single level surfaces (top panel: (a)$\sim$(d)) and hierarchically structured surfaces with nanoparticles-coated sidewalls (bottom panel: (e)$\sim$(h)). The depinned metastable state is shown in (a), where $\theta $ and $h$ are the contact angle and sagging depth, respectively. The pinned state is shown in (e)