Rotational Self-Assembly of Hydrogel Microstructures via Induced Asymmetric Capillary Dynamics

Capillary force, as a key driving mechanism for micro/nanostructure self-assembly, has been widely employed in constructing complex micro/nanostructured architectures. However, existing studies have primarily focused on capillary-force-driven vertical collapse and horizontal displacement of microstructures, while systematic exploration of in-plane rotational motion under constrained conditions remains scarce. This study proposes an in-plane rotation model for microstructures based on the capillary force self-assembly (CFSA) mechanism, which achieves controllable and directional in-plane rotational motion of microstructures. This approach employs femtosecond laser direct writing to facilely fabricate rotational microstructures, in which an initial deflection angle is intentionally introduced to induce asymmetric capillary force distribution on both sides of the structure, thereby generating the torque required for rotation. Governed by the interplay between "driving force" and "structural resistance," the fabricated microstructures undergo large-angle in-plane rotation during liquid evaporation. Furthermore, by precisely tuning the laser power and scanning pitch, anisotropic microstructures with tunable cross-linking density were successfully fabricated. The subtle hydration differences induced by this anisotropic characteristic can form a deterministic guiding mechanism for the rotation direction, thereby enabling the promotion or inhibition regulation of the rotational behavior of microstructures and the manipulation of chiral self-assembly.