Experimental Study of Thin Liquid-Films Flowing Under Planar Substrates*

We undertake an experimental study of liquid films flowing under inverted planar substrates, and report on space- and time-resolved thickness measurements by laser-induced  fluorescence  at 330 mm from the inlet. The experiments span inclination angles β = -15 °, -30 ° and -45 °, Kapitza numbers Ka = 13.1 and 330, and Reynolds numbers Re = 0.6 - 193. When Ka = 13.1, the topology of the free surface transitions from smooth rivulets to sequences of solitary pulses with increasing Re. When Ka = 330, rivulets typically emerge above Re = 30 and the mean rivulet-amplitude increases non-monotonically. The peak amplitude shifts to higher Re at larger negative inclinations. Based on our results we conjecture that the development of rivulets can be attributed at smaller negative inclinations to a secondary Rayleigh-Taylor (RT) instability that destabilizes the suspended 2D wavefronts, and at larger negative inclinations to the primary RT instability of a flat film coating the underside of the plate.