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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

Temporal correlation lengths.Scaling of the times at which the correlation lengths ξ0 and ξd acquire their maximum value, with the persistence correlation time, . The inset shows the rescaling of the data of Fig. 5a, for temperatures T ≤ 0.55.
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f7: Temporal correlation lengths.Scaling of the times at which the correlation lengths ξ0 and ξd acquire their maximum value, with the persistence correlation time, . The inset shows the rescaling of the data of Fig. 5a, for temperatures T ≤ 0.55.

Mentions: We find both correlations functions to decay exponentially, with correlation ξ0(t) and ξd(t), respectively. Their time dependence is illustrated in Fig. 6, for selected temperatures. Both correlation lengths have a maximum as a function of time. We indicate with and , and with and , the time of occurrence and the value of the maxima of the two correlation lengths. As apparent from Fig. 6, both correlations length are small, as usual in structural glasses, and increases on cooling, being much more temperature dependent than . We characterize the temperature dependence of and investigating their scaling with respect to the average persistence time, 〈tp〉. Figure 7 shows that , in agreement with previous results suggesting that the time of the maximum of the dynamical heterogeneities scale as the relaxation time. Conversely, we approximately find . We note that the relation between and is model dependent, as for instance we observe in the Kob–Andersen lattice gas model. Since controls the diffusivity correlations, we expect it to also control the approach of the diffusivity distribution to its asymptotic Gaussian shape, and thus to be the time scale at which the variance to mean ratio reaches its asymptotic value g, as in Fig. 5a. Indeed, the data of Fig. 5a are successfully rescaled when normalized and plotted versus , as in Fig. 7(inset).


Dynamic phase coexistence in glass-forming liquids.

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

Temporal correlation lengths.Scaling of the times at which the correlation lengths ξ0 and ξd acquire their maximum value, with the persistence correlation time, . The inset shows the rescaling of the data of Fig. 5a, for temperatures T ≤ 0.55.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f7: Temporal correlation lengths.Scaling of the times at which the correlation lengths ξ0 and ξd acquire their maximum value, with the persistence correlation time, . The inset shows the rescaling of the data of Fig. 5a, for temperatures T ≤ 0.55.
Mentions: We find both correlations functions to decay exponentially, with correlation ξ0(t) and ξd(t), respectively. Their time dependence is illustrated in Fig. 6, for selected temperatures. Both correlation lengths have a maximum as a function of time. We indicate with and , and with and , the time of occurrence and the value of the maxima of the two correlation lengths. As apparent from Fig. 6, both correlations length are small, as usual in structural glasses, and increases on cooling, being much more temperature dependent than . We characterize the temperature dependence of and investigating their scaling with respect to the average persistence time, 〈tp〉. Figure 7 shows that , in agreement with previous results suggesting that the time of the maximum of the dynamical heterogeneities scale as the relaxation time. Conversely, we approximately find . We note that the relation between and is model dependent, as for instance we observe in the Kob–Andersen lattice gas model. Since controls the diffusivity correlations, we expect it to also control the approach of the diffusivity distribution to its asymptotic Gaussian shape, and thus to be the time scale at which the variance to mean ratio reaches its asymptotic value g, as in Fig. 5a. Indeed, the data of Fig. 5a are successfully rescaled when normalized and plotted versus , as in Fig. 7(inset).

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