An air bubble of radius \( 0.1 \, \text{cm} \) lies at a depth of \( 20 \, \text{cm} \) below the free surface of a liquid of density \( 1000 \, \text{kg/m}^3 \). If the pressure inside the bubble is \( 2100 \, \text{N/m}^2 \) greater than the atmospheric pressure, then the surface tension of the liquid in SI unit is (use \( g = 10 \, \text{m/s}^2 \)).
