Transition dynamics from compact propagation to sustained stretching in aerodynamically driven sessile drops

Aerodynamically driven sessile drops can depin and propagate as compact bodies before entering a finite stretched-drop state. This transition is studied in wind tunnel experiments with Newtonian drops of glycerol–water mixtures of different viscosity on aluminum substrates of different roughness. The side-view front and rear contact-line positions show that motion inception and stretching are distinct stages. The inception Weber number Weinc marks macroscopic drop mobility and is mainly linked to contact-line retention, whereas the stretching threshold Westr marks sustained front–rear separation. Water and moderately viscous mixtures usually undergo bounded oscillatory deformation before stretching, whereas pure glycerol reaches the stretched-drop state through overdamped monotonic deformation. Viscosity therefore changes the route to sustained stretching, but the stretched-drop state is reached when the front and rear contact-line regions no longer move together. The transition is thus interpreted as a loss of coordinated contact-line motion, rather than as the attainment of a fixed geometric ratio.