let q be the charge on each drop : $$ V_1 = \frac {kq}{r} $$
Let R be radius of single drop. then
$$ \frac {4}{3} \pi R^3 = 27 \frac {4}{3} \pi r^3 \Rightarrow R = 3r $$
$$ V_2 = \frac { k(27q) }{R} = \frac { k27q}{3r} = 9V_1 \Rightarrow \frac {V_2}{V_1} = 9 $$