Surfactant renormalization of the square-gradient coefficient

If a nonconserved surfactant polarization has local energy $\nu |\mathbf{p}{|}^2/2+\lambda \mathbf{p}\cdot \nabla \varphi$, eliminating $\mathbf{p}$ changes the composition-gradient coefficient from $\kappa$ to $\tilde{\kappa }=\kappa -{\lambda }^2/\nu$. This expresses the surfactant's reduction of interfacial energy.