Movie S1.

Flow over an SHS revealed by fluorescent microbeads in water imaged by spinning-disk confocal microscopy during a pressure-relaxation experiment. We record the flow ≈3 μm away from the plastron. The SHS consists of equispaced gratings, which are 30 mm long, 40 μm wide, separated by 20-μm wide ridges, and aligned with the flow direction (see Fig. 3 A–C and Tables S3, S4, and S6 for a detailed definition of the geometry and experimental parameters). The experiments begin with a loading phase, during which the flow (indicated by green arrows) is driven by a strong fixed pressure gradient and allowed to reach steady state (SI Experimental Protocols and Tables S6 and S7). The driving pressure is then rapidly decreased to zero while keeping the hydrostatic pressure approximately constant. A clear backflow, indicated by red arrows, in the direction opposite to the loading phase can be observed, thereby demonstrating the presence of Marangoni stresses.