Zero-contact-angle wall meniscus ($z_0=\sqrt{2}{\ell }_c$)

A static liquid meniscus rising on a perfectly wetting vertical wall has dimensional height $z_0=\sqrt{2}{\ell }_c$, where ${\ell }_c=\sqrt{\sigma /(\rho g)}$ is the capillary length. In capillary-length coordinates, write the interface $x=h(z)$. The Young–Laplace equation gives $h_{zz}/(1+h_z^2{)}^{3/2}=z$ and the first integral $h_z/\sqrt{1+h_z^2}=z^2/2-1$. The integration constant follows from the horizontal far bath, where $z\to 0$ and $h_z\to -\infty$. Zero contact angle means $h=h_z=0$ at the wall, so $z_0=\sqrt{2}$. Expanding and integrating there gives $h\sim (z-z_0{)}^2/\sqrt{2}$, the parabolic shape used to match an entrained film.