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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

Variance to mean ratio of the distribution of the number of jumps per particle.Time evolution (a) of the variance to mean ratio of the distribution of the number of jumps per particle, and temperature dependence of its asymptotic value (b). Data refer to species a. Analogous results for species b are reported in Fig. S4. Panel c illustrates that, in the deeply supercooled regime, the asymptotic value scales as , for both components of the KALJ mixture and for other model systems (see text). The full line is the CTRW prediction, .
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f5: Variance to mean ratio of the distribution of the number of jumps per particle.Time evolution (a) of the variance to mean ratio of the distribution of the number of jumps per particle, and temperature dependence of its asymptotic value (b). Data refer to species a. Analogous results for species b are reported in Fig. S4. Panel c illustrates that, in the deeply supercooled regime, the asymptotic value scales as , for both components of the KALJ mixture and for other model systems (see text). The full line is the CTRW prediction, .

Mentions: The time evolution of the distribution of the diffusivities gives further insights into the dynamics of the system. Indeed, Fig. 5a,b show that at long times the variance to mean ratio of P(nJ; t) reaches a plateau value , that grows on cooling. This plateau value can be related to the ratio of the two timescales and . In fact, within the CTRW framework3233 and , where . Given the relation between the persistence time and the waiting time distributions34 (see Appendix), it follows and thus . We have verified this prediction considering, beside the KA model, also a binary mixture of harmonic spheres18 and the kinetically constrained Kob–Andersen three dimensional lattice gas model1920, as illustrated in Fig. 5(inset). The lattice model confirms our predictions. The molecular dynamics simulations reproduce the asymptotically proportionality between g and , even though there are small deviations with respect to the CTRW prediction, suggesting the emergence of correlations between successive waiting times at low temperature.


Dynamic phase coexistence in glass-forming liquids.

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

Variance to mean ratio of the distribution of the number of jumps per particle.Time evolution (a) of the variance to mean ratio of the distribution of the number of jumps per particle, and temperature dependence of its asymptotic value (b). Data refer to species a. Analogous results for species b are reported in Fig. S4. Panel c illustrates that, in the deeply supercooled regime, the asymptotic value scales as , for both components of the KALJ mixture and for other model systems (see text). The full line is the CTRW prediction, .
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f5: Variance to mean ratio of the distribution of the number of jumps per particle.Time evolution (a) of the variance to mean ratio of the distribution of the number of jumps per particle, and temperature dependence of its asymptotic value (b). Data refer to species a. Analogous results for species b are reported in Fig. S4. Panel c illustrates that, in the deeply supercooled regime, the asymptotic value scales as , for both components of the KALJ mixture and for other model systems (see text). The full line is the CTRW prediction, .
Mentions: The time evolution of the distribution of the diffusivities gives further insights into the dynamics of the system. Indeed, Fig. 5a,b show that at long times the variance to mean ratio of P(nJ; t) reaches a plateau value , that grows on cooling. This plateau value can be related to the ratio of the two timescales and . In fact, within the CTRW framework3233 and , where . Given the relation between the persistence time and the waiting time distributions34 (see Appendix), it follows and thus . We have verified this prediction considering, beside the KA model, also a binary mixture of harmonic spheres18 and the kinetically constrained Kob–Andersen three dimensional lattice gas model1920, as illustrated in Fig. 5(inset). The lattice model confirms our predictions. The molecular dynamics simulations reproduce the asymptotically proportionality between g and , even though there are small deviations with respect to the CTRW prediction, suggesting the emergence of correlations between successive waiting times at low temperature.

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