which indicates d increases proportionally to R. This means the friction decreases with the microdroplet radius (R) increased (Figure ). Therefore, the VPS process would be accelerated in the larger W/O microdroplets, resulting in the increase in the percentage of porous structures in the larger W/O microdroplets (Figure ). This hypothesis is also supported by Figure , which indicates the lower DOTAP ratio condition (i.e., less electrostatic interaction, less wettability, and less friction) resulted in a higher probability of porous structure formation.