On the basis of interfacial energies, the profile of a surface droplet or a liquid bridge between two surfaces can be described by the Young–Laplace equation. This equation is applicable for three-dimensional axisymmetric conditions and is highly non-linear. This is due to the mean curvature term which includes products of first- and second-order derivatives of the drop shape function f ( x , y ) : κ κ m = 1 2 ( 1 + f x 2 ) f y y − − 2 f x f y f x y + ( 1 + f y 2 ) f x x ( 1 + f x 2 + f y 2 ) 3 / 2 .