Spontaneous droplet transport on shape-evolving microfiber rails.

Directional droplet motion on solid surfaces, governed by Gibbs' theory of minimizing surface free energy, traditionally relies on static surface gradients. However, such approaches face intrinsic constraints, including fixed transport directions and interdependence between transport distance and speed. Here, we introduce shape-evolving microfiber rails (SEMRs) that enable spontaneous droplet transport on initially non-gradient surfaces. Inspired by a domino-like mechanism, the SEMRs dynamically generate cascading gradients through shape deformations in response to droplet-surface interactions, diverging from conventional static-gradient designs. This adaptive strategy allows for steerable droplet motion, independent control over transport distance and velocity, and design flexibility for on-demand performance optimization. The SEMR's versatility heralds advanced applications across analytical chemistry, cargo transport, electronics, and diagnostics, paving the way for advanced intelligent fluidic systems.