Communication: Observation of ultra-slow relaxation in supercooled selenium and related glass-forming liquids

Rheology of supercooled P-Se glass-forming liquids: From networks to molecules and the emergence of power-law relaxation behavior

The effect of the network-to-molecular structural transformation with increasing phosphorus content in P_xSe_100-x (30 ≤ x ≤ 67) supercooled liquids on their shear-mechanical response is investigated using oscillatory shear rheometry. While network liquids with 30 ≤ x ≤ 40 are characterized by shear relaxation via a network bond scission/renewal process, a Maxwell scaling of the storage (G') and loss (G″) shear moduli, and a frequency-independent viscosity at low frequencies, a new relaxation process emerges in liquids with intermediate compositions (45 ≤ x ≤ 50). This process is attributed to an interconversion between network and molecular structural moieties. Predominantly molecular liquids with x ≥ 63, on the other hand, are characterized by a departure from Maxwell behavior as the storage modulus shows a linear frequency scaling G'(ω) ∼ ω over nearly the entire frequency range below the G'-G″ crossover and a nearly constant ratio of G″/G' in the terminal region. Moreover, the dynamic viscosity of these rather fragile molecular liquids shows significant enhancement over that of network liquids at frequencies below the dynamical onset and does not reach a frequency-independent regime even at frequencies that are four orders of magnitude lower than that of the onset. Such power-law relaxation behavior of the molecular liquids is ascribed to an extremely broad distribution of relaxation timescales with the coexistence of rapid rotational motion of individual molecules and cooperative dynamics of transient molecular clusters, with the latter being significantly slower than the shear relaxation timescale.

Rheological behavior of molecular vs network chalcogenide supercooled liquids

The viscoelastic behavior of supercooled glass-forming liquids along the binary join As₄S₃-GeS₂ with As₄S₃ contents varying from 81.25 to 9 mol. % and correspondingly with structures varying from predominantly molecular to a three-dimensional tetrahedral network is studied by small-amplitude oscillatory shear parallel plate rheometry. The storage shear modulus G' shows a scaling behavior of G'(ω) ∼ ωⁿ in the terminal (low-frequency) regime, where n varies between 1 and 2 and shows an increasingly anomalous departure from the expected value of 2 (Maxwell scaling) with increasing molecule content. A concomitant departure from the Maxwell scaling is also observed for the loss modulus G″ at frequencies above the G'-G″ crossover. On the other hand, the variation in the phase angle δ with the complex modulus indicates that the molecular liquid does not display a purely viscous response even at the lowest frequencies. These results, combined with an analysis of the relaxation spectra of these liquids, suggest that the anomalous behavior of molecular liquids may be linked to their rather broad relaxation spectrum and the presence of slow relaxation processes associated with molecular clusters. Additionally, these liquids are also characterized by a wide high-frequency plateau in the relaxation spectral density that can be linked to the rotational dynamics of the constituent molecules. Such fundamental differences between the rheological behavior of molecular and network liquids may explain the significantly higher fragility of the former.

Observation of soft glassy behavior in a magnetic colloid exposed to an external magnetic field

We provide the first experimental evidence for soft glassy behavior in a sterically stabilized magnetic colloid (ferrofluid) of relatively low volume fraction (φ = 0.037) when a uniform magnetic field is applied at a sufficiently high rate (fast quench). Fast magnetic-field quenches favor structural arrest of field-induced aggregates, owing to insufficient time to settle into lower energy states, thereby pushing the system to a frustrated metastable configuration like a repulsive glass. Brownian dynamics simulations are used to show that the polydisperse ferrofluid (as in experiments) forms thick ropes aligned along the field direction, while a monodisperse ferrofluid does not. The simulations show that there is practically no ordering of the thin, monodisperse chains, while the thick, polydisperse ropes show positional ordering with a typical center-center separation between the particles in different ropes of about 0.39 μm. As a consequence of structural arrest, the ferrofluid exhibits aging with broken time-translational invariance, a hallmark of glassy dynamics. The superposition of strain and creep compliance curves obtained from rheological measurements at different waiting times in the effective time domain corroborates the soft glassy behavior when exposed to a magnetic field applied at a fast ramp rate.