Heterogeneous dynamics in liquids: fluctuations in space and time

Abstract

The disordered nature of glass-forming melts gives rise to non-Arrhenius and non-exponential behaviour of their dynamics. With respect to the microscopic details involved in the structural relaxation, these materials have remained an unsolved puzzle for over a century. The observation of spatial heterogeneity regarding the dynamics provides an important step towards understanding the relation between the macroscopic properties of soft condensed matter and the molecular mechanisms involved. On the other hand, dynamic heterogeneity is the source of several new questions: What is the length scale and persistence time associated with such clusters of relaxation time? What is the signature of heterogeneity at high temperatures and in the glassy state? How do these features depend on the particular material and on the correlation function used for probing these heterogeneities? This work attempts to review the various approaches to heterogeneous dynamics and the generally accepted results, as well as some controversial issues. Undoubtedly, heterogeneity has provoked a number of novel experimental techniques targeted at studying glass-forming liquids at the molecular level. It will be emphasized that the picture of heterogeneity is a requirement for rationalizing an increasing number of experimental observations rather than just an alternative model for the dynamics of molecules.

Slow dynamics, aging, and glassy rheology in soft and living matter

“…A feature of systems near the jamming transition, such as supercooled liquids and glasses, is the presence of dynamical heterogeneities [20,61]. The slow and heterogeneous dynamics in jammed systems such as sand [62], colloidal hard sphere glasses [63] and coarsening foams [64] have been studied in detail.…”

Dynamical Heterogeneity and Jamming in Glass-Forming Liquids

“…Simulations [21,22,23,24,25,26,27] and experiments [28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43] have demonstrated the cooperative and spatially heterogeneous nature of the liquid dynamics (for reviews of the experimental evidence for spatially heterogeneous dynamics, see, e. g., Refs. [44,45,46]). …”

Emergence of glassy-like dynamics in an orientationally ordered phase

“…The cooperative character of the α-relaxation is commonly associated with the nonexponentiality, described by the stretching β KWW parameter 47 due to the many-body coupling 48 or to dynamic heterogeneities. 49,50 On crossing from phase I to phase II around 200 K, the stretching parameter suddenly changes from values close to 1 (indicating Debye behavior) to less than 0.6 and, on further cooling within phase II, the value still reduces to less than 0.5 at T g and this is an effect to be ascribed to the cooperativity of many-body molecular motions. A nonexponential relaxation or a broad loss peak as a consequence of the addition of different distinct noncooperative modes with distinct energy barriers must be discarded in the present case due to the conflict with temperature variation of the relaxation shape.…”