Multiple path-dependent routes for phase-transition kinetics in thermoresponsive and field-responsive ultrasoft colloids

Abstract

The nature of solid-solid phase transformations has been a long-standing question spanning the fields of metallurgy and condensed-matter physics, with applications from metallic alloys and ceramics to modern shape-memory materials. In spite of the importance of solid-to-solid transformations in many areas of materials science and condensed-matter physics and the numerous experimental and theoretical studies, a deep understanding of the microstructural changes and the underlying kinetic mechanisms is still missing. In this work, we establish a versatile model system composed of micron-scale ionic microgel colloids, where we not only probe the single-particle kinetics in real space and real time but also tune the phase transition in a multiple-parameter space. In the presence of an imposed electric field, a face-centered cubic (FCC) crystal transforms diffusively into a body-centered tetragonal (BCT) crystal via nucleation and growth. In the reverse direction, however, the BCT phase transforms cooperatively into a long-lived metastable body-centered orthorhombic phase, which only relaxes back to the equilibrium FCC when annealed at higher temperatures. The kinetics is thus either diffusive or martensitic depending on the path, and we believe that these two path-dependent transitions provide the first real-space, particle-level insights of diffusive and martensitic transformations, respectively, in a single system.