The potential of an X-ray spectroscopy technique to shed light on the mysterious phenomena that occur when a liquid nears a glass-like state has been demonstrated by four RIKEN physicists. On cooling, many liquids undergo a sharp switch at their freezing points, snapping into crystalline solids. The most famous example is water, with a freezing point of 0 degrees Celsius.

In contrast, many liquid polymers and other materials go through a more graceful transition known as the glass transition. The solids they form have structures closer to the random order of a liquid than the ordered structure of such as ice and metals. Glass is a classic example: it is a solid at room temperature but its molecules are arranged in a disordered manner.

There are many unanswered questions about the glass transition. « The phenomenon of glass transition is one of the biggest mysteries of soft-matter physics, » notes Taiki Hoshino of the RIKEN SPring-8 Center. « Some scientists even question whether the glass transition is really a transition or if it just looks like one. »

One key that may help unlock the mysteries about the glass transition is the concept of dynamical heterogeneity—fluctuations in space and time in the local dynamic behavior of molecules. « Many researchers believe that the glass transition can be explained in terms of dynamical heterogeneity, » says Hoshino.

Now, Hoshino and three RIKEN SPring-8 Center colleagues have used synchrotron-generated X-rays to measure dynamical heterogeneity in a liquid polymer near its glass-transition temperature.

During the measurements, the polymer was squeezed between a stationary cylindrical rod and a moving substrate. Liquid closer to the substrate moved faster than liquid near the rod, resulting in a velocity gradient across the liquid. The team found that the dynamical heterogeneity decreased as the velocity gradient was increased. This confirmed the predictions of a molecular dynamics simulation published more than 20 years ago.

The researchers used a technique called X-ray photon correlation spectroscopy (XPCS). Because the that make up a all peak and trough in sync with each other, laser light scattered from an object generates a speckle pattern on a screen. XPCS uses the speckle pattern generated by X-rays to obtain information about a sample. « If the scatterers in the sample move, the scattering pattern changes, » explains Hoshino. « These fluctuations reveal information about the motion of the scatterers. »

Hoshino notes that XPCS hasn’t enjoyed as much popularity among soft-matter physicists as other techniques, but he hopes this study will convince others of its potential. « Our results show that XPCS is a powerful technique for studying transition, » he says.

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