Contact-line instabilities of driven liquid films

Liquid-film contact lines driven over a surface (substrate) by gravity, centrifugal force, Marangoni force, or wind, exhibit a wealth of instabilities that control many coating and cleaning processes. Starting from a sheet with an approximately straight contact line advancing over the substrate, these instabilities lead to fingering of the sheet to form rivulets, meandering and oscillation of the rivulets, and break-up of the rivulets to form droplets or multiple rivulets. Droplet flows on a flat substrate can themselves exhibit deformation, elongation and eventual bifurcation of the droplets to form smaller droplets. The chapter presents a review whose objective is to give a broad explanation of the underlying physics of these various phenomena and to present an overview of targeted literature on these subjects. The chapter also examines some new results regarding the effect of surface inhomogeneity on rivulet formation, the physics of wind-blown rivulets (including their break-up as droplet flows), and rivulets subject to microgravity.