Successful Observation of “Solid-like” Molecular Behaviors in Supercooled Liquid (Press Release)
- Release Date
- 15 Sep, 2012
- BL09XU (Nuclear Resonant Scattering)
Kyoto University
Japan Synchrotron Radiation Research Institute
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The research group at Kyoto University and Japan Synchrotron Radiation Research Institute (JASRI) conducted a study on molecular movements in a typical supercooled liquid*1 utilizing γ-rays scattered from an atomic nucleus resonantly-excited by synchrotron radiation. The study successfully revealed emerging microscopic “solid-like” behaviors as cooling advances. The study helped deepen our understanding of liquid state - one of the fundamental states of matter - and is expected to contribute to the elucidation of one of the remaining challenges in physics, i.e. the glass transition*1, which is the transition from liquid to glass state*1. Molecules in liquid have a relatively high degree of freedom to diffuse. However, as the liquid is supercooled beyond its freezing point, it is generally believed that microscopic “solid-like” regions gradually emerge. The evidence supporting this idea include: changes observed in diffusion behaviors (from simple diffusion to more complex diffusions with velocity distribution, including a very slow one) and the emergence of a hopping relaxation process*2, one of the characteristics inherent in solids. As these changes generally occur in the spatial domain of atoms and molecules in a very short period of time (typically 100ns), conventional measurement methods are virtually useless in capturing these microscopic movements because of many restrictions. Thus, the process through which a liquid starts to exhibit such properties has been murky to us up to the present. In this study, the behaviors of supercooled o-terphenyl were examined using quasi-elastic scattering measurement*5, whereby a highly monochromatic nuclear resonant scattering γ-ray*4 generated by synchrotron radiation was used as the probe (the nuclear resonance beam line, BL09XU, at SPring-8*3 was used for this study). The experiment clearly showed that the hopping relaxation process takes place only locally and under restrictions. It also revealed that the hopping relaxation does not start to appear right after the diffusion behavior starts to change, but only emerges well after further cooling when the solid-like regions have well developed. The research results not only provided evidence to support the idea that the liquid takes on solid-like properties as it is cooled down, but also demonstrated that the changes in molecular behavior of supercooled liquid take place in distinct steps, contrary to the conventional idea that they occur simultaneously. The research was conducted by a group of researchers including Makina Saito (graduate student at Kyoto University, Graduate School of Science; currently, research fellow at Sinchrotrone Trieste), Prof. Makoto Seto (Kyoto University Research Reactor Institute), and Dr. Yoshitaka Yoda (senior scientist, JASRI). The research projects/themes involved in this study include: “Studies on Nuclear Resonant Scattering Methods for Materials Science” (research director: Makoto Seto) (one of the research subjects included in “Novel Measuring and Analytical Technology Contributions to the Elucidation and Application of Material” [research supervisor: Professor Emeritus Michiyoshi Tanaka, Tohoku University] supported by JST Core Research for Evolutional Science and Technology [CREST]) and SPring-8 Power User’s Topic, “Studies on advanced nuclear resonant scattering methods for materials science” (leader: Makoto Seto). These results appeared in the online version of Physical Review Letters (a physics journal published in U.S.A.) on the 14th of September. Publication: |
<<Figures>>
(a) Temperature dependency of average relaxation time. The inset indicates the correspondence between the momentum transfer value (q=14, 23nm-1) and the static structure factor. The momentum transfer bar indicates the region of relaxation time measurement (q range), which is determined by the finite solid angle of the detector. The long dashed line indicates the slow β relaxation time associated with dielectric relaxation. The short dashed line indicates the slow β relaxation time obtained in this experiment (the data obtained using the q=23nm-1 condition was extrapolated to α relaxation time observed using the q=14nm-1 condition). (b) The q-dependency of average relaxation time at 265K. The short dashed line indicates the static structural factor, and the long dashed line indicates the slope in cases where the relaxation time changes according to q -2.
<<Glossary>>
*1 Supercooled liquid state and glass transition, glass state
The cooling of a liquid to its freezing point normally triggers crystallization. However, cooling under certain crystallization-inhibiting conditions may allow the liquid to remain fluid below the freezing temperature. This state is called the supercooled liquid state. Continued cooling beyond the outset of this state inhibits molecular diffusion, which is one of the characteristics seen in ordinary liquids. This change of state is called glass transition, meaning that the liquid assumes a vitreous state. In this state, the molecules are arranged in a random fashion as compared with those in a crystal.
*2 Hopping relaxation process
A type of relaxation process in which molecular hopping (jumping) from one site to another, rather than the molecular motions associated with diffusion, brings about relaxation.
*3 SPring-8
A RIKEN facility located in Harima Science Garden City (Hyogo prefecture) is capable of producing the world's highest intensity synchronous radiation. The management and promotion of utilization of this facility are undertaken by JASRI. The name “SPring-8” comes from “Super Photon ring-8GeV.” An electron flying at nearly the speed of light, if deflected from its original trajectory through the effect exerted by a magnet, emits an electromagnetic wave in a direction tangential to its trajectory, which is called radiation light (or synchrotron radiation). At present, there are three “3rd Generation” large scale synchronous radiation facilities in the world: SPring-8 (Japan), APS (USA) and ESRF (France). The acceleration energy available at SPring-8 (8 billion electron volts) enables the provision of an extremely wide spectrum of radiation light: from far infrared to visible, vacuum ultraviolet, and soft X-ray up to hard X-ray. SPring-8 provides a theater for collaborative works involving researchers inside and outside Japan, and the research conducted at this facility cover such diverse areas as material science, geoscience, life science, environmental science, and various applications in industrial sectors.
*4 Nuclear resonant scattering γ-ray
Atomic nucleus resonantly excited through exposure to synchrotron radiation or a radioactive isotope source can scatter a γ-ray, which is called the nuclear resonant scattering γ-ray. In the measurements reported here, a highly directional and monochromatic γ-ray was generated by exciting a 57Fe nucleus using the high intensity synchronous radiation available in SPring-8 (the 57Fe nucleus has an excited level life of 141ns, and energy uncertainty width of 4.6 neV). The energy of the γ-ray, 14.4 keV, is equivalent to 0.086 nm in wavelength, making it an optimal tool for precision measurement of atomic/molecular-scale structures.
*5 Quasi-elastic scattering measurement
Scattering that causes a colliding body, or a probe such as a photon, to change its energy is called inelastic scattering. Quasi-elastic scattering is a limited case of inelastic scattering characterized by energy changes being small, for example, energy transfers caused by the diffusion motion of the scattering body. The quasi-elastic scattering experiment using a nuclear resonant scattering γ-ray emitted from a 57Fe nucleus enables observation of very small energy changes of scattered photons. The energy change concerned is so small that it is almost equivalent to the γ-ray’s line width, typically at the neV level, which enables the probing of motions in a time-scale of 100ns, which is equivalent to the line width.
Careful selection of the angle from which to observe a scattered γ-ray allows a choice of size for the target structure for motion observation. These conditions enable examination of the motions of the structures of atomic/molecular-scale, i.e. the scale roughly equivalent to γ-ray wavelengths.
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