 Here σ is the surface tension, n, t and s are unit vectors in a local orthogonal coordinate system (n,t,s) at the free surface (n is outward normal to the free surface while the other two lie in the tangential plane and are mutually orthogonal).The indices 'l' and 'g' denote liquid and gas, respectively and K is the curvature of the free surface.with Rt and Rs being radii of curvature along coordinates t and s.

The surface tension σ is force per unit length of a surface element and acts tangential to the free surface.