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Dynamic phase coexistence in glass-forming liquids.

Pastore R, Coniglio A, Ciamarra MP - Sci Rep (2015)

Bottom Line: Indeed, although the non-Gaussian shape of the van-Hove distribution suggests the transient existence of a diffusivity distribution, it is not possible to infer from this quantity whether two or more dynamical phases coexist.We relate this distribution to the heterogeneity of the dynamics and to the breakdown of the Stokes-Einstein relation, and we show that the coexistence of two dynamical phases occurs up to a timescale growing faster than the relaxation time on cooling, for some of the considered models.Our work offers a basis for rationalizing the dynamics of supercooled liquids and for relating their structural and dynamical properties.

View Article: PubMed Central - PubMed

Affiliation: CNR-SPIN, Dipartimento di Scienze Fisiche, Universitá di Napoli Federico II, Italy.

ABSTRACT
One of the most controversial hypotheses for explaining the heterogeneous dynamics of glasses postulates the temporary coexistence of two phases characterized by a high and by a low diffusivity. In this scenario, two phases with different diffusivities coexist for a time of the order of the relaxation time and mix afterwards. Unfortunately, it is difficult to measure the single-particle diffusivities to test this hypothesis. Indeed, although the non-Gaussian shape of the van-Hove distribution suggests the transient existence of a diffusivity distribution, it is not possible to infer from this quantity whether two or more dynamical phases coexist. Here we provide the first direct observation of the dynamical coexistence of two phases with different diffusivities, by showing that in the deeply supercooled regime the distribution of the single-particle diffusivities acquires a transient bimodal shape. We relate this distribution to the heterogeneity of the dynamics and to the breakdown of the Stokes-Einstein relation, and we show that the coexistence of two dynamical phases occurs up to a timescale growing faster than the relaxation time on cooling, for some of the considered models. Our work offers a basis for rationalizing the dynamics of supercooled liquids and for relating their structural and dynamical properties.

No MeSH data available.


Related in: MedlinePlus

Persistence and cage–jump properties.Panels a,b and c show the probability distributions of the persistence time tp, of the jump length ΔrJ, and of the waiting time tw. Panel d illustrates the decay of the persistence. Full lines in panel d are fits to stretched exponentials, while those in panel a are the corresponding predictions for F(tp) (see text). All data refer to species a of the KALJ mixture. Analogous results for species b are shown in Fig. S1.
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f1: Persistence and cage–jump properties.Panels a,b and c show the probability distributions of the persistence time tp, of the jump length ΔrJ, and of the waiting time tw. Panel d illustrates the decay of the persistence. Full lines in panel d are fits to stretched exponentials, while those in panel a are the corresponding predictions for F(tp) (see text). All data refer to species a of the KALJ mixture. Analogous results for species b are shown in Fig. S1.

Mentions: In order to prove that the CTRW approach provides a quantitative description of the dynamics of the KA mixture, we have performed a careful analysis of the single particle cage–jump intermittent motion, for temperatures slightly above the mode–coupling one151617, . Figure 1a–c illustrate the distribution of the persistence time F(tp) and of the jump length P(Δr), that fix the temporal and spatial features of the system in the CTRW approach, as well as the distribution of the time particles wait in their cages before making a jump, P(tw). No correlations between the persistence time and the jump length have been found, in agreement with the CTRW scenario. Panel d illustrates the decay of the persistence. At short times all jumps contribute to the decay of the persistence; we therefore observe , as is the rate at which particles jump, and . At long times the persistence is found to decay with a stretched exponential, p(t) ∝ exp(−(t/τ)β). This implies F(tp) = −dp(t)/dt ∝ τ−βtβ−1 exp(−(t/τ)β) as verified in Fig. 1a.


Dynamic phase coexistence in glass-forming liquids.

Pastore R, Coniglio A, Ciamarra MP - Sci Rep (2015)

Persistence and cage–jump properties.Panels a,b and c show the probability distributions of the persistence time tp, of the jump length ΔrJ, and of the waiting time tw. Panel d illustrates the decay of the persistence. Full lines in panel d are fits to stretched exponentials, while those in panel a are the corresponding predictions for F(tp) (see text). All data refer to species a of the KALJ mixture. Analogous results for species b are shown in Fig. S1.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4496729&req=5

f1: Persistence and cage–jump properties.Panels a,b and c show the probability distributions of the persistence time tp, of the jump length ΔrJ, and of the waiting time tw. Panel d illustrates the decay of the persistence. Full lines in panel d are fits to stretched exponentials, while those in panel a are the corresponding predictions for F(tp) (see text). All data refer to species a of the KALJ mixture. Analogous results for species b are shown in Fig. S1.
Mentions: In order to prove that the CTRW approach provides a quantitative description of the dynamics of the KA mixture, we have performed a careful analysis of the single particle cage–jump intermittent motion, for temperatures slightly above the mode–coupling one151617, . Figure 1a–c illustrate the distribution of the persistence time F(tp) and of the jump length P(Δr), that fix the temporal and spatial features of the system in the CTRW approach, as well as the distribution of the time particles wait in their cages before making a jump, P(tw). No correlations between the persistence time and the jump length have been found, in agreement with the CTRW scenario. Panel d illustrates the decay of the persistence. At short times all jumps contribute to the decay of the persistence; we therefore observe , as is the rate at which particles jump, and . At long times the persistence is found to decay with a stretched exponential, p(t) ∝ exp(−(t/τ)β). This implies F(tp) = −dp(t)/dt ∝ τ−βtβ−1 exp(−(t/τ)β) as verified in Fig. 1a.

Bottom Line: Indeed, although the non-Gaussian shape of the van-Hove distribution suggests the transient existence of a diffusivity distribution, it is not possible to infer from this quantity whether two or more dynamical phases coexist.We relate this distribution to the heterogeneity of the dynamics and to the breakdown of the Stokes-Einstein relation, and we show that the coexistence of two dynamical phases occurs up to a timescale growing faster than the relaxation time on cooling, for some of the considered models.Our work offers a basis for rationalizing the dynamics of supercooled liquids and for relating their structural and dynamical properties.

View Article: PubMed Central - PubMed

Affiliation: CNR-SPIN, Dipartimento di Scienze Fisiche, Universitá di Napoli Federico II, Italy.

ABSTRACT
One of the most controversial hypotheses for explaining the heterogeneous dynamics of glasses postulates the temporary coexistence of two phases characterized by a high and by a low diffusivity. In this scenario, two phases with different diffusivities coexist for a time of the order of the relaxation time and mix afterwards. Unfortunately, it is difficult to measure the single-particle diffusivities to test this hypothesis. Indeed, although the non-Gaussian shape of the van-Hove distribution suggests the transient existence of a diffusivity distribution, it is not possible to infer from this quantity whether two or more dynamical phases coexist. Here we provide the first direct observation of the dynamical coexistence of two phases with different diffusivities, by showing that in the deeply supercooled regime the distribution of the single-particle diffusivities acquires a transient bimodal shape. We relate this distribution to the heterogeneity of the dynamics and to the breakdown of the Stokes-Einstein relation, and we show that the coexistence of two dynamical phases occurs up to a timescale growing faster than the relaxation time on cooling, for some of the considered models. Our work offers a basis for rationalizing the dynamics of supercooled liquids and for relating their structural and dynamical properties.

No MeSH data available.


Related in: MedlinePlus