Biomimetic Crystalline Actuators: Structure–Kinematic Aspects of the Self-Actuation and Motility of Thermosalient Crystals

TLDR

Self‑actuation and motility are common in living organisms but rare in simple, homogeneous objects; thermosalient crystals, which jump thousands of times their size in less than 1 ms when heated, exemplify this phenomenon and remain poorly understood due to limited analytical methods. The study aims to systematically assess the interplay among thermodynamic, kinematic, structural, and macroscopic factors driving the thermosalient phenomenon. The authors develop a systematic assessment framework to elucidate the mechanistic aspects of thermosalient behavior. The results reveal a rapid, anisotropic unit‑cell deformation in two stages that leads to crystal explosion, debris separation, or reshaping, consistent with a martensitic‑like phase transition mechanism.

Abstract

While self-actuation and motility are habitual for humans and nonsessile animals, they are hardly intuitive for simple, lifeless, homogeneous objects. Among mechanically responsive materials, the few accidentally discovered examples of crystals that when heated suddenly jump, propelling themselves to distances that can reach thousands of times their own size in less than 1 ms, provide the most impressive display of the conversion of heat into mechanical work. Such thermosalient crystals are biomimetic, nonpolymeric self-actuators par excellence. Yet, due to the exclusivity and incongruity of the phenomenon, as well as because of the unavailability of ready analytical methodology for its characterization, the reasons behind this colossal self-actuation remain unexplained. Aimed at unraveling the mechanistic aspects of the related processes, herein we establish the first systematic assessment of the interplay among the thermodynamic, kinematic, structural, and macroscopic factors driving the thermosalient phenomenon. The collective results are consistent with a latent but very rapid anisotropic unit cell deformation in a two-stage process that ultimately results in crystal explosion, separation of debris, or crystal reshaping. The structural perturbations point to a mechanism similar to phase transitions of the martensitic family.