A composite foam sample consists of a solid material of uniform density \(\rho_s\) interspersed with air-filled voids of negligible density. The void fraction \(\phi\) is defined as the ratio of the volume of the voids to the total outer volume of the foam block. The block is placed in a liquid of density \(\rho_f\), where \(\rho_s > \rho_f\). In terms of \(\rho_s\) and \(\rho_f\), what is the minimum void fraction \(\phi_{\text{min}}\) required for the foam block to float in the liquid?