Gradient variational problems in ${\mathbb{R}}^2$

We prove a new integrability principle for gradient variational problems in ${\mathbb{R}}^2$, showing that solutions are explicitly parameterized by $\kappa$-harmonic functions, that is, functions which are harmonic for the laplacian with varying conductivity $\kappa$, where $\kappa$ is the square root of the Hessian determinant of the surface tension.