Wind-Driven Rivulet Breakoff and Droplet Flows in Microgravity and Terrestrial-Gravity Conditions

A study is reported of the wind-driven breakoff of rivulets and subsequent droplet flows on a horizontal plate subject to different normal gravitational states, ranging from zero- to terrestrial-gravity conditions (1 g), and including some data for partial gravity conditions (between 0.1 g and 0.38 g). The study entailed experiments conducted in the authors’ laboratory at the University of Iowa and onboard the NASA KC-135, parabolic-flight aircraft. The wind-driven rivulets exhibited a breakoff phenomenon over a broad range of flow rates, in which a “head” at the tip of the rivulet broke off periodically to form a droplet that advected down the plate. The rivulet breakoff phenomena was sensitive to the normal gravitational force acting on the plate. For instance, the frequency of rivulet breakoff was nearly an order-of-magnitude greater for the 0 g condition than for the same flow in the 1 g condition. The droplet shape and behavior were observed to be quite different between the two cases. It was furthermore found in all cases examined that wind-driven rivulet and droplet flows are markedly different from gravitationally driven flows. These differences arise primarily from the role of form drag on the droplets and on the raised ridge of the rivulet and pool flows near the moving contact line.