Fig. 1: Propulsion mechanism of the rove beetle in genus Stenus and the bio-inspired Marangoni hydrogel rotor.

a When a rove beetle accidentally falls on the water, it can secrete surface-active substances at its tail to quickly move to the shore. b Similar to the rove beetle, bio-inspired hydrogel rotor can secrete organic fuel to keep itself moving at the air-water interface. c The hydrogel rotor is defined by three geometric parameters: radius r, thickness h and teeth number n. The profile of rotor tooth is a well-designed cubic spline curve, and its control point coordinates are shown in Supplementary Fig. 3. Inserted are SEM images of the porous structures in different parts of hydrogel rotor. d Fabricated hydrogel rotor top-view and side-view images with characteristic geometric parameters: r = 1000 µm, h = 629 µm, and n = 8. e Propulsion mechanism: HFIP fuel is initially trapped in the hydrogel chain, and it is released to the surrounding medium when the rotor comes in contact with water. Trapped HFIP molecules are replaced by water molecules. The scale bars are 20 μm in (c), 1 mm in (d).