  We evaluate the electrostatic interaction energy between two surfaces, one
flat and the other slightly curved, in terms of the two-point autocorrelation
functions for patch potentials on each one of them, and of a single function
$\psi$ which defines the curved surface. The resulting interaction energy, a
functional of $\psi$, is evaluated up to the second order in a derivative
expansion approach. We derive explicit formulae for the coefficients of that
expansion as simple integrals involving the autocorrelation functions, and
evaluate them for some relevant patch-potential profiles and geometries
