Variation of the modulus of a foliation

The $p$–modulus ${\mathrm{m}\mathrm{o}\mathrm{d}}_p(\mathcal{F})$ of a foliation $\mathcal{F}$ on a Riemannian manifold $M$ is a generalization of extremal length of plane curves introduced by L. Ahlfors. We study the variation $t\mapsto{\rm mod}_p($\mathcal{F}$_t)$ of the modulus. In particular, we consider product of moduli of orthogonal foliations.