Gravity-driven thin film on an incline
= Gravity-driven thin film on an incline
For a film of thickness $h(x,t)$ flowing down a plane inclined by $\alpha$, lubrication theory gives the flux
$$
q=\frac{\rho gh^3}{3\mu}
\left(\sin\alpha-\cos\alpha\,h_x\right)
$$
when surface tension is neglected. Mass conservation gives the thin-film equation $h_t+q_x=0$.