A liquid column of height 0.04cm balances excess pressure of soap bubble of certain radius. If density of liquid is $8 \times 10^3 \mathrm{~kg} \mathrm{~m}^{-3}$ and surface tension of soap solution is $0.28 \mathrm{Nm}^{-1}$, then diameter of the soap bubble is ______cm. (if $\mathrm{g}=10 \mathrm{~ms}^{-2}$ )