Dynamics of Nanoparticles in a Supercooled Liquid Chiara Caronna, 1,2 Yuriy Chushkin, 1 Anders Madsen, 1, * and Antonio Cupane 2 1 European Synchrotron Radiation Facility, B.P. 220, F-38043 Grenoble, France 2 Department of Physical and Astronomical Sciences, via Archirafi 36, I-90123, Palermo, Italy (Received 30 July 2007; revised manuscript received 19 December 2007; published 8 February 2008) The dynamic properties of nanoparticles suspended in a supercooled glass forming liquid are studied by x-ray photon correlation spectroscopy. While at high temperatures the particles undergo Brownian motion the measurements closer to the glass transition indicate hyperdiffusive behavior. In this state the dynamics is independent of the local structural arrangement of nanoparticles, suggesting a cooperative behavior governed by the near-vitreous solvent. DOI: 10.1103/PhysRevLett.100.055702 PACS numbers: 64.70.P , 61.05.cf, 64.70.D The glass transition of supercooled liquids [1] is an intriguing phenomenon which continues to receive a con- siderable amount of interest, both from an experimental and a theoretical point of view. For molecular liquid glass formers, supercooling leads to an increase in rigidity and finally a vitrification due to complex molecular processes which still are poorly understood. It is believed that ther- mally activated motion (hopping) [2] between different minima configurations of the energy landscape [3] gets important in the supercooled state. The transition to land- scape dominated dynamics and dynamical heterogeneity was found to happen at temperatures higher than the calo- rimetric glass transition temperature T g [4,5]. Several stud- ies have been reported where probe molecules were used to investigate the local molecular dynamics of liquid glass formers (see, e.g., Refs. [6,7] ), but here we focus on the hydrodynamic behavior of larger tracer particles in a super- cooled solvent. X-ray photon correlation spectroscopy (XPCS) was employed to probe the dynamics of silica nanoparticles suspended in the glass-forming liquid pro- panediol, and the data indicate a transition from Brownian motion to hyperdiffusive behavior as T g is approached. Sterically stabilized colloidal silica, from EKA Chemicals (16 nm radius) and Duke Scientific (250 nm radius), was delivered suspended in water and has previ- ously been characterized by small angle x-ray scattering (SAXS) [8,9]. The water solvent was substituted by 1,2 propanediol (Sigma-Aldrich) using a thorough distillation and centrifugation procedure and the samples were filled into cylindrical quartz capillaries (diam. 1:5 mm) which were sealed to prevent evaporation. The melting tempera- ture of pure propanediol is T m  245 K and T g was mea- sured by differential scanning calorimetry [10] to 170  2K for all investigated samples (including pure propanediol). For the x-ray measurements the capillaries were placed in a thermostated Cu sample holder and good thermal contact was ensured by application of low tem- perature conducting grease. The sample holder was situ- ated in a vacuum chamber ( 10 7 mbar) operating in the range 100–400 K with a stability better than 0.01 K. Static SAXS measurements showed that the size and poly- dispersity of the silica particles and their interactions did not change after the solvent substitution. The experiments were performed at the Troika beam line ID10A of the European Synchrotron Radiation Facility (ESRF). A collimated 8 keV undulator beam was reflected from a single bounce Si(111) monochromator and a down- stream Si mirror was employed to suppress higher order light. An adjustable aperture was used to select the ‘‘co- herent part’’ of the beam. The fringes produced by the aperture scattering were removed by careful positioning of guard slits a few cm upstream of the sample. A pixelated 2D detector (Medipix-2 [11] ) was recording the scattering patterns 2:3m downstream of the sample. The detector consists of 256  256 pixels each having an area of 55  55 m 2 . Each pixel is single photon sensitive and the detector has essentially no dark current because the upper and lower detection limits and gain can be set for each pixel. For every measurement thousands of frames were recorded and subsequently analyzed by the multispeckle XPCS method [12] to obtain a set of temporal correlation functions in the accessible q range. Figure 1 shows a series of correlation functions taken on a dilute (  1%) suspension of R  250 nm particles (corresponding to an average interparticle distance of 0.0 0.2 0.4 0.6 0.8 1.0 10 -1 10 0 10 1 10 2 10 3 [g (2) (τ)-1] / β τ (sec) 205 K 215 K 220 K 230 K 240 K FIG. 1 (color online). Normalized correlation functions (sym- bols) measured at q  0:027 nm 1 on a dilute suspension of 250 nm spheres at different temperatures. Solid lines are fits with the KWW expression. PRL 100, 055702 (2008) PHYSICAL REVIEW LETTERS week ending 8 FEBRUARY 2008 0031-9007= 08=100(5)=055702(4) 055702-1  2008 The American Physical Society