= Capillary parameter for a falling film
{title2=$S=2\gamma/(3\rho gL^2\sin\alpha)$}
For downslope length scale $L$, film thickness scale $h_0$ and surface <velocity> scale $U_0=\rho gh_0^2\sin\alpha/(2\mu)$, this normalization makes the capillary part of the <Benney equation> flux $\varepsilon S h^3h_{xxx}$. The dimensionless capillary <pressure> is $-3S h_{xx}/2$. Keeping $S=O(1)$ specifies a distinguished <long-wave approximation> in which <surface tension> remains relevant.
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