Dissection of colloidal glasses using the laser as a lancet

Bo Li and Kai Kou, Research Fellows of IBS Center for Soft and Living Matter, along with Walter Kob, Professor of the University of Montpellier and Institute Universitaire de France, and Steve Granick, Director of the IBS Center for Soft and Living Matter, report together in Nature that the initiation of the glass transition is a highly non-trivial process involving complex nonlinear responses.

As a substance that gives a significant boost to both the convenience of our daily lives and the advancement of modern science and technology, eyewear perplexes us, however, on a basic scientific level. “Glasses see many incremental studies but rarely a breakthrough, regardless of the efforts of generations of scientists,” commented Granick.

The reported non-monotone dynamic length scale peaking at onset temperature subverts the prevailing understanding that cage formation is a simple cross between liquid and glass. “A central issue in glass science is the cage-forming process that gives glassy materials their unique optical and mechanical properties,” said Kob.

“And we addressed the problem directly by locally exciting a colloidal glass using laser beams,” Li said.

The emergence of the non-monotonous length scale results from the accumulation of domains with cooperative dynamics that become increasingly rigid and begin to dominate particle dynamics. “Just like Seurat’s painting, the mosaic of the dynamic grains and the formation of the cage is directly related to their fusion,” says Kob.

“The beauty of the science here is that we are able to see how glasses germinate from liquids under the microscope,” Li said.

The simple physical picture of enhanced cooperative dynamics for the non-monotone response suggests that the discovery should be general. Kob said: “It’s amazing that the physical rule behind such rich dynamics is so concise.”

Li adds: “Our results in a well-defined model system will help to better understand other glassy or disordered systems such as polymeric, granular and atomic glasses, etc.”

In addition to the non-monotonous behavior, a scale relationship between the morphology and the size of the excitation pattern is extracted based on a huge amount of experimental data. “The deviation of this relationship reflects the degree of heterogeneity of a material under certain conditions,” Li said. Granick noted: “This scaling law, in addition to its theoretical significance to physicists, will also appeal to chemists and materials scientists, offering them a ‘ruler’ that guides the design and synthesis of glass materials.”

In addition to clarifying the first step of the glass transition, this proof-of-concept experiment paves the way for the fundamental understanding of eyewear in the end. “Using the laser as a lancet, a glass sample can be precisely anatomized,” Granick said.

“This will evaluate increasingly exotic but at the same time disconcerting behaviors in eyewear,” Kob predicted.

This work is motivated by the long-standing challenges in glass science. Slow, highly coupled dynamics always hide the key effect. “If only I could shrink, jump into the system, and mix the environment,” Li said. The femtosecond holographic laser system originally developed by Kou perfectly meets the needs of local excitation. Valid theoretical support is obtained from Kob to refine complex experimental observations into concise physical principles. “The highly interdisciplinary environment at our center and successful international collaboration make brainstorming once unlikely real,” commented Granick.

Granick and Kob concluded: “The field of glass science, being classic but constantly challenging, is promoted by these experiments which clarify the beginning of the glass transition. The conceptual importance of cage formation for the properties of glass materials is revealed. . And the micro-rheological approach adopted here opens the door to an in-depth understanding of eyewear someday. ”