"Traditionally, it has been challenging to microscopically observe the dynamic process of martensitic transitions in soft materials on a single-particle level. One must devise a means to do so in a way that quickly initiates the transition without harmful perturbation to the system."Hajime Tanaka, University of Tokyo,

Believe it or not, steel has something in common with bacterial appendages: they both undergo a special type of physical transformation that remains puzzling. Now, researchers from Japan and China have used direct microscopic observations to provide more clarity on how this transformation occurs.

In a paper in Nature Communications, researchers from the University of Tokyo in Japan and Fudan University in China have revealed previously unknown physical details that underpin crystalline solid-to-solid phase transitions in soft materials. These details could possibly allow researchers to exploit more fully the properties of advanced materials.

A special type of solid-to-solid phase transition, known as a martensitic transition, is an exciting frontier in medicine, technology and other fields. The martensitic transition involves the coordinated movement of atoms in a material, which change the properties of the material without changing its chemical composition. Metal alloys and proteins can both undergo this type of transition, but researchers hypothesize that it may occur differently in easily deformable soft materials than in hard materials with stable defects, where it has been observed. At present, this hypothesis is difficult to test, something the researchers aimed to address.

"Traditionally, it has been challenging to microscopically observe the dynamic process of martensitic transitions in soft materials on a single-particle level," says Hajime Tanaka at the University of Tokyo, who is co-senior author of the paper. "One must devise a means to do so in a way that quickly initiates the transition without harmful perturbation to the system."

To do this, the researchers used a gentle technique known as ion exchange –the same basic method that is used to remove calcium and magnesium ions from water – to quickly change the crystal structure of polymeric microparticles. Using a microscope, they could then observe the kinetics of the resulting martensitic transitions with single-particle resolution.

"The microscopy results were unambiguous," explains Peng Tan at Fudan University, who was the other co-senior author of the paper. "We observed three previously unknown mechanisms by which body-centered cubic soft colloidal crystals form from face-centered cubic ones, depending on the condition."

The researchers examined the features of these three mechanisms – termed thermally activated in-grain nucleation, grain-boundary-premelting-assisted nucleation and wall-assisted growth – with particular focus on how the energy barrier to the transition is reduced in each case.

"Softness of a crystal plays a critical role in thermally activated in-grain nucleation," explains Tanaka. "Whereas the other two pathways may occur even in hard materials."

These findings could have multiple uses. For example, some pharmaceuticals can alter their availability in the body via solid-to-solid phase transitions, and so understanding how to control when and where such transitions occur could provide a new means of targeted drug delivery. In addition, a greater understanding of the physical mechanisms of solid-to-solid transformations supports the development of new materials that can be tailored for specific applications.

This story is adapted from material from the University of Tokyo, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.