Processing soft thin films on liquid surface for seamless creation of on-liquid walkable devices.

Walking on liquid surface is a unique locomotion ability of insects, but engineering on-liquid walkable devices currently requires disjointed, multistage fabrication and delicate deployment. Here, we introduce HydroSpread-a direct processing technology that enables seamless fabrication and patterning of soft films on liquid surface. It leverages the controlled spreading of liquid ink on liquid surface and combines with precise laser engraving supported by rapid heat transfer at the solid-liquid interface. Geometric shapes, including basic forms of straight lines, sharp turns and circles, and complex patterns, were fabricated with exceptional fidelity to design specifications. We propose two heat-driven hydrodynamic locomotion mechanisms, fin-like bending and leg-like buckling. By harnessing these principles, we engineered two walkable devices-HydroFlexor and HydroBuckler-and demonstrated robust on-water locomotion. This work eliminates fragile postfabrication transfers in soft device manufacturing, bridging the gap between soft films and structure fabrication, and establishes a streamlined pathway for designing and deploying functional soft devices directly in liquid environments.