Eight mercury drops, each of radius \( r \), coalesce to form a bigger drop. The surface energy released in this process is _____________________ . (\( S \) is the surface tension of mercury).
\( 8 \pi r^2 S \)
\( 16 \pi r^2 S \)
\( 64 \pi r^2 S \)
\( 4 \pi r^2 S \)