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Experimental physicist Peter Schall of the University of Amsterdam has made a three-dimensional reconstruction of the transformation an atomic structure undergoes when a solid in crystal form becomes liquid. The results of the study, published in the scientific journal Advanced Materials,are important for our understanding of melting and hardening, and the manufacture of new materials.

Experimental physicist Peter Schall of the University of Amsterdam has made a three-dimensional reconstruction of the transformation an atomic structure undergoes when a solid in crystal form becomes liquid. Never before has this process been revealed in so much detail and in 3D. The results of the study, published in the scientific journal Advanced Materials, are important for our understanding of melting and hardening, and the manufacture of new materials.

Physics distinguishes between three states of matter: gas, liquid and solid matter. Peter Schall studied, with colleagues from the Van der Waals-Zeeman Institute and a scientist from the University of North Texas, how the atomic structure of a crystal, a form comprising many solids, becomes a liquid. The atom motif of crystal can be compared to a structure made up of cubes that are stacked on top of each other side by side. Liquids, on the other hand, have an untidy structure, similar to marbles lying together in a box. The fundamental question is: How does the one atomic structure transform into the other structure?

The problem, when trying to answer that question, is that atoms move very fast and are also miniscule in size. Schall therefore imitated the transformation with colloidal particles that are one thousand times larger than atoms. He wanted to visually enhance the process to make it visible. By using thermo-sensitive colloidal particles, he could grow large colloidal crystals that are similar to their atomic counterparts. In this way, Schall obtained three-dimensional images of the transformation from crystals to liquids on the scale of the particles using a microscope. These images show how the geometric structure and symmetry of the crystals transform into the liquid structure.

Bristly interface

The structural transformation happens surprisingly quickly: the boundary between crystal and liquid is two to three atomic layers wide. The atomic motif of the crystal disappears within the atomic layers, such as the regular four faces (tetrahedron) and eight areas (octahedron) in the case of the cubic, evenly-aligned crystal in the study of Schall. That is when the atomic motifs typical of the liquid appear: the icosahedra. Moreover, the three-dimensional images show how bristly the interface between crystal and liquid is: the temperature counteracts the surface tension and ensures that the interface vibrates on an atomic scale like the membrane of a loudspeaker or the string of a violin.

In the longer term, the findings could lead to the design of new nano-materials such as photonic crystals that can steer light. This could make the Internet even faster through improved fiber optic cables.

Publication details

Van Duc Nguyen, Minh Triet Dang, Bart Weber, Zhibing Hu, Peter Schall: ‘Visualizing the Structural Solid-Liquid Transition at Colloidal Crystal/Fluid Interfaces.’ Advanced Materials (24 June 2011).