CORRELATION OF LOCAL ORDER AND DYNAMICS IN DENSE COLLOIDS
Despite intensive research for decades, the local order in liquids and glasses remains an open question. In particular, its role in crystallisation and vitrification is unknown. Here, results from a coherent X-ray scattering study on hard spheres reveal a clear connection between structure and dynamics when approaching the colloidal glass transition.
COMPLEX SYSTEMS AND BIOMEDICAL SCIENCES
The local orientational order of liquids and glasses has been studied intensively in the past, mostly with computational methods or real- space imaging techniques. In particular, colloidal hard spheres are a frequently investigated model system in pioneering studies on phase transitions . Depending on the particles volume fraction, all accessible phases comprising the fluid, crystal, and glassy states can be prepared. For microscopy studies, experimental limitations remain, limiting studies to rather large particles and slow relaxation time. Such limitations do not apply for X-ray scattering methods, which can thus provide complementary information.
One way to access local order in amorphous materials beyond the static structure factor is by X-ray cross correlation analysis (XCCA) based on coherent X-ray scattering. In XCCA experiments, the local orientational order is accessed by calculation of angular correlation functions C(q,∆) from coherent diffraction patterns, so- called speckle patterns . Furthermore, the sample dynamics is obtained from the same data using X-ray photon correlation spectroscopy (XPCS).
Local order and dynamics have been investigated from colloidal dispersions of poly(methylmethacrylate) particles of 125-nm radius in decalin that are a commonly-used experimental representation of a hard sphere system. At ID10, volume fractions f were studied between 0.49 and 0.58, covering liquid states (f < 0.52), supercooled liquid (f > 0.52) and glassy states (f > 0.56), which were complemented with measurements in the liquid state at SACLA. The schematics of the experiment in small-angle X-ray scattering geometry and the hard-sphere phase diagram are shown in Figures 51a and 51b, respectively.
First, the average structure was investigated by the static structure factor S(q) (Figure 51c). The position of the first maximum of S(q) corresponds to the typical next-neighbour distance and shifts toward larger q with increasing volume fraction, but the shape and amplitude does not vary. In contrast, the dynamics slow down by four orders of magnitude, indicating a glass transition around f ≈ 0.55 where the sample falls out of equilibrium. These results represent a typical feature of the liquid-glass transition the dynamics significantly slow down while the average structure remains unchanged.
To reveal the role of orientational order,
its degree was studied by XCCA . It was
quantified by the parameter
Fig. 51: a) Schematic of the experiment at ID10. b) Phase diagram of colloidal hard spheres, crosses represent the studied samples. c) Structure factors of all samples studied, the legend indicates the volume fractions.