Manning condensed fraction ($f_c=\max (1-1/\eta ,0)$)

In the mean-field infinite-dilution cylinder model, the fraction of neutralizing charge that remains localized is $f_c=\max (1-1/\eta ,0)$, where $\eta$ is the Manning parameter. For $\eta >1$, integrating the cylindrical counterion-only Poisson-Boltzmann profile gives $Q_c=\lambda -q/{\ell }_B$. Below threshold, the local limiting density vanishes even though every finite neutral cell contains the full counterion charge.