W. van Megen

Arrest of Flow and Emergence of Activated Processes at the Glass Transition of a Suspension of Particles with Hard Spherelike Interactions

By combining aspects of the coherent and self-intermediate scattering functions, we show that the arrest of particle number density fluctuations spreads from the position of the main structure factor peak. We propose that this arrest impairs the systems ability to respond to diffusing momentum currents, leading to an enhanced resistance to flow. From the stretching of the coherent intermediate scattering functions in the glass, we read a manifestation of the undissipated thermal energy-the source of the ergodicity restoring processes that short-circuit the sharp transition to a perfect glass.

Dynamics of hard sphere suspensions using dynamic light scattering and X-ray photon correlation spectroscopy: Dynamics and scaling of the intermediate scattering function

Intermediate scattering functions are measured for colloidal hard sphere systems using both dynamic light scattering and x-ray photon correlation spectroscopy. We compare the techniques, and discuss the advantages and disadvantages of each. Both techniques agree in the overlapping range of scattering vectors. We investigate the scaling behavior found by Segré and Pusey [Phys. Rev. Lett. 77, 771 (1996)] but challenged by Lurio et al. [Phys. Rev. Lett. 84, 785 (2000)]. We observe a scaling behavior over several decades in time but not in the long-time regime. Moreover, we do not observe long-time diffusive regimes at scattering vectors away from the peak of the structure factor and so question the existence of long-time diffusion coefficients at these scattering vectors.

Aging dynamics of colloidal hard sphere glasses

We report the results of dynamic light scattering measurements of the coherent intermediate scattering function (ISF) of glasses of colloidal hard spheres for several volume fractions and a range of scattering vectors around the primary peak of the static structure factor. The ISF shows a clear crossover from an initial fast decay to a slower nonstationary decay. Aging is quantified in several different ways. However, regardless of the method chosen, the perfect aged glass is approached in a power law fashion. In particular the coupling between the fast and slow decays, as measured by the degree of stretching of the ISF at the crossover, also decreases algebraically with waiting time. The nonstationarity of this coupling implies that even the fastest detectable processes are themselves nonstationary.

The cage effect in systems of hard spheres

The cage effect is generally invoked when discussing the delay in the decay of time correlation functions of dense fluids. In an attempt to examine the role of caging more closely, we consider the spread of the displacement distributions of Brownian particles. These distributions are necessarily biased by the presence of neighbouring particles. Accommodation of this bias by those neighbours conserves the displacement distribution locally and presents a collective mechanism for exploring configuration space that is more efficient than the intrinsic Brownian motion. Caging of some particles incurs, through the impost of global conservation of the displacement distribution, a delayed, non-local collective process. This non-locality compromises the efficiency with which configuration space is explored. Both collective mechanisms incur delay or stretching of time correlation functions, in particular the particle number and flux densities. This paper identifies and distinguishes these mechanisms in existing data from experiments and computer simulations on systems of particles with hard sphere interactions.The cage effect is generally invoked when discussing the delay in the decay of time correlation functions of dense fluids. In an attempt to examine the role of caging more closely we consider the spread of the displacement distributions of Brownian particles. These distributions are necessarily biased by the presence of neighbouring particles. Accommodation of this bias by those neighbours conserves the displacement distribution locally and presents a collective mechanism for exploring configuration space that is more efficient than the intrinsic Brownian motion. Caging of some particles incurs, through the impost of global conservation of the displacement distribution, a delayed, non-local collective process. This non-locality compromises the efficiency with which configuration space is explored. Both collective mechanisms incur delay or stretching of time correlation functions, in particular the particle number and flux densities. This paper identifies and distinguishes these mechanisms in existing data from experiments and computer simulations on systems of particles with hard sphere interactions.

Exposing a dynamical signature of the freezing transition through the sound propagation gap

The conventional view of freezing holds that nuclei of the crystal phase form in the metastable fluid through purely stochastic thermal density fluctuations. The possibility of a change in the character of the fluctuations as the freezing point is traversed is beyond the scope of this perspective. Here we show that this perspective may be incomplete by examination of the time autocorrelation function of the longitudinal current, computed by molecular dynamics for the hard-sphere fluid around its freezing point. In the spatial window where sound is overdamped, we identify a change in the long-time decay of the correlation function at the known freezing points of monodisperse and moderately polydisperse systems. The fact that these findings agree with previous experimental studies of colloidal systems in which particle are subject to diffusive dynamics, suggests that the dynamical signature we identify with the freezing transition is a consequence of packing effects alone.