Fig. 2: Thin-film flow atop the undulator.
a A sketch of the actuator and a long-exposure image of a typical flow-field measurement, showing the motion of the tracer particles in the thin film. The free surface deforms in response to the flow. b Results of PIV for two different capillary numbers, Ca = 714 (top panel), Ca = 17 (bottom panel). In both these panels the bottom boundary is the actuator surface, while the top boundary is the liquid interface. The color coding represents the horizontal component of the velocity field, Vx; red signifies flow along the wave speed (Vw) while blue signifies flow opposite to Vw.
