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Relaxation dynamics of glassy liquids: Meta-basins and democratic motionPowerPoint Presentation

Relaxation dynamics of glassy liquids: Meta-basins and democratic motion

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Relaxation dynamics of glassy liquids:

Meta-basins and democratic motion

G. Appignanesi, J.A. Rodríguez Fries, R.A. Montani

Laboratorio de Fisicoquímica, Bahía Blanca

W. Kob

Laboratoire des Colloïdes, Verres et Nanomatériaux

Université Montpellier 2

http://www.lcvn.univ-montp2.fr/kob

- motivation (long)
- strings
- democratic motion
- conclusions

Model and details of the simulation

Avoid crystallization binary mixture of Lennard-Jones particles;

particles of type A (80%) and of type B (20%)

parameters: AA= 1.0AB= 1.5BB= 0.5

AA= 1.0AB= 0.8BB= 0.85

- Simulation:
- Integration of Newton’s equations of motion (velocity Verlet algorithm)
- 150 – 8000 particles
- in the following: use reduced units
- length in AA
- energy in AA
- time in (m AA2/48 AA)1/2

Dynamics: The mean squared displacement

- Mean squared displacement is defined as
- r2(t)=|r(t) - r(0)|2

- short times: ballistic regime r2(t) t2
- long times: diffusive regime r2(t) t
- intermediate times at low T:
- cage effect
- with decreasing T the dynamics slows down quickly since the length of the plateau increases

- What is the nature of the motion of the particles when they start to become diffusive (-process)?

Time dependent correlation functions

- At every time there are equilibrium fluctuations in the density distribution; how do these fluctuations relax?
- consider the incoherent intermediate scattering function Fs(q,t)Fs(q,t) = N-1(-q,t) (q,0) with (q,t) = exp(qrk(t))

- high T: after the microscopic
- regime the correlation decays
- exponentially
- low T: existence of a plateau at
- intermediate time (reason: cage effect); at long times the correlator
- is not an exponential (can be fitted well by Kohlrausch-law)
- Fs(q,t) = A exp( - (t/ ))

- Why is the relaxation of the particles in the -process non-exponential? Motion of system in rugged landscape? Dynamical heterogeneities?

- 2(t) is large in the caging regime
- maximum of 2(t) increases with decreasing T evidence for the presence of DH at low T
- define t* as the time at which the maximum occurs

- One possibility to characterize the dynamical homogeneity of a system is the non-gaussian parameter
- 2(t) = 3r4(t) / 5(r2(t))2 –1
- with the mean particle displacement r(t) ( = self part of the van Hove correlation function Gs(r,t) )

- define the “mobile particles” as the 5% particles that have the largest displacement at the time t*
- visual inspection shows that these particles are not distributed uniformly in the simulation box, but instead form clusters
- size of clusters increases with decreasing T

Dynamical heterogeneities: III

- The mobile particles do not only form clusters, but their motion is also very cooperative:

ARE THESE STRINGS THE -PROCESS?

ARE THESE DH THE REASON FOR THE STRETCHTING IN THE -PROCESS ?

Similar result from simulations of polymers and experiments of colloids (Weeks et al.; Kegel et al.)

T=0.5

- we see meta-basins (MB)
- with decreasing T the residence time increases
- NB: Need to use small systems (150 particles) in order to avoid that the MB are washed out

- define the “distance matrix” (Ohmine 1995)
- 2(t’,t’’) = 1/N i|ri(t’) – ri(t’’)|2

ASD changes strongly when system leaves MB

Dynamics: I

- look at the averaged squared displacement in a time (ASD) of the particles in the same time interval:
- 2(t,) := 2(t- /2, t+ /2)
- = 1/N i|ri(t+/2) – ri(t-/2)|2

- look at Gs(r,t’,t’+ ) = 1/N i(ri(t’) – ri(t’+ ))2 for times t’ that are inside a meta-basin

- Gs(r,t’,t’+ ) is shifted to the left of the mean curve ( = Gs(r, ) ) and is more peaked

- look at Gs(r,t’,t’+ ) = 1/N i(ri(t’) – ri(t’+ ))2 for times t’ that are at the end of a meta-basin

- Gs(r,t’,t’+ ) is shifted to the right of the mean curve ( = Gs(r, ) )
- NB: This is not the signature of strings!

- define “mobile particles” as particles that move, within time , more than 0.3
- what is the fraction of such
- mobile particles?

- fraction of mobile in the MB-MB transition particles is quite substantial ( 20-30 %) ! (cf. strings: 5%)

Nature of the motion within a MB

- few particles move collectively; signature of strings (?)

Nature of the democratic motion in MB-MB transition

- many particles move collectively; no signature of strings

K. Binder and W. KobGlassy Materials and

Disordered Solids: An Introduction to their

Statistical Mechanics (World Scientific,

Singapore, 2005)

Summary

- For this system the -relaxation process does not correspond to the fast dynamics of a few particles (string-like motion with amplitude O() ) but to a cooperative movement of 20-50 particles that form a compact cluster
- candidate for the cooperatively rearranging regions of Adam and Gibbs
- Qualitatively similar results for a small system embedded in a larger system
- Reference:
- cond-mat/0506577

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