Fragile-to-strong crossover, growing length scales, and dynamic heterogeneity in Wigner glasses

Abstract

Colloidal particles, which are ubiquitous, have become ideal testing grounds for the structural glass transition (SGT) theories. In these systems glassy behavior is manifested as the density of the particles is increased. Thus, soft colloidal particles with varying degree of softness capture diverse glass forming properties, observed normally in molecular glasses. By performing Brownian dynamics simulations for a binary mixture of micron-sized charged colloidal suspensions, known to form Wigner glasses, we show that by tuning the softness of the interaction potential, achievable by changing the monovalent salt concentration, there is a continuous transition between fragile to strong behavior. Remarkably, this is found in a system where the well characterized interaction potential between the colloidal particles is isotropic. We also show that the predictions of the random first order transition (RFOT) theory quantitatively describes the universal features such as the growing correlation length,where φK , the analogue of the Kauzmann temperature, depends on the salt concentration. As anticipated by the RFOT predictions, we establish a causal relationship between the growing correlation length and a steep increase in the relaxation time and dynamic heterogeneity as the system is compressed. The broad range of fragility observed in Wigner glasses, which can be induced by merely changing the salt concentration, is used to draw analogies with molecular and polymer glasses. The large variations in the fragility is found only when the temperature dependence of the viscosity is examined for a large class of diverse glass forming materials. In sharp contrast, this is vividly illustrated in a single system that can be experimentally probed. Our work also shows that the RFOT predictions are accurate in describing the dynamics over the entire density range, regardless of the fragility of the glasses, implying that the physics describing the structural glass transition is universal. arXiv:1909.03463v1 [cond-mat.soft]

RFOT theory for glassy dynamics in a single condensed polymer

“…Such a large increase in τ α in a finite sized polymer is substantial. The sharp increase in τ α in the SCP as well as in metallic [40], colloidal systems [41], and molecular glasses [42], is described well by the Vogel-Tamman-Flucher (VFT) equation, τ α = τ 0 exp[ D0T0 T −T0 ], where τ 0 , D 0 and T 0 are fitting parameters. We fit the calculated τ α as a function of T to the VFT equation (the dashed line in Figure 3 The divergence of τ α is closely associated with the decrease in S conf .…”

Understanding charged vesicle suspensions as Wigner glasses: dynamical aspects

“…Notice that both the interaction strength, through ψ 0 , and range, λ, are taken from the aforementioned experiments of reference [14], and are shown in table 1 as function of salt content. As a matter of fact, a slight correction (around 10%) for the Debye length at the lowest salt concentration was here made, to take into account the non-negligible sulfate ions concentration (from DIPEDMAMS molecules) giving an additional force screening between vesicles [16,22]. Finally, notice that equation (1) describes vesicle interaction only when l > 0.…”

“…These relatively low-density glassy states, known as Wigner glasses, were first experimentally observed [16] and then theoretically described [17]. Following these pioneering works, Wigner glass-forming systems were deeply investigated in a variety of simulations and experiments [18][19][20][21][22][23] during the last two decades. The qualitative behaviour of Wigner glass-formers is now well established, but it is also clear that quantitative aspects are very sensitive to specific system properties.…”

Rigorous treatment of pairwise and many-body electrostatic interactions among dielectric spheres at the Debye–Hückel level

“…While the DLVO energy is beautiful in its simplicity and physical clarity and remains in use (see, for a recent example, Ref. [27]), its numerical accuracy and the range of phenomena it can capture are inherently limited.…”