Interfacial tension of a phi-four diffuse interface (source code)

= Interfacial tension of a phi-four diffuse interface
{title2=$\sigma$}

The excess free <energy> per <area> of the <phi-four diffuse interface> is
$$
\sigma=\int_{-\infty}^{\infty}\kappa(\phi')^2dx
=\sqrt{\frac{-8a^3\kappa}{9b^2}}.
$$