Aging in short-ranged attractive colloids

G. FoffiINFM, Universita’ “La Sapienza”, Roma

Overview

• We study a short-ranged attractive system in the reentrant region of the glass phase diagram

• We quench the system deep in the glass by lowering and increasing the temperature

• The aging properties of this two states have been studied by molecular dynamics

Simulated system

• N=700• 50%-50% concentration = 1.2 = 3%

The numerical experiment

• We prepared 60 independent configurations equilibrated at T=0.6

• We quenched the system at two point state:

T=1.2 in the repulsive glass T=0.3 in the attractive glass

• After that the system evolves at constant temperature.

AgingWe are studying an out-of-equilibrium process!

• Time translational invariance does not hold

• One time observable (i.g. energy and static structure factors) will depend on the waiting time

• Correlation function will depend on the observation time and the waiting time:

Energy evolution

• After the quench the system present two different evolution of the energy:– @T=1.2 the energy

increase and can be fitted with a log.

– @T=0.3 the energy decrease and follows a power law:

Static structure factors

• The overall shape of the S(q) does not change with a waiting time

• Only a slightly increase (decrease) in the first peak is present

Density-density correlation function

Scaling Theoretical studies (Cugliandolo cond-mat/0210312) suggest the possibility of rescaling the correlators ad different waiting time

For example:

Simple aging case

Proposed for BMLJ with 0.88 [1]

[1] Kob and Barrat PRL 78, 4581 (1997)

Scaling for the attractive glass

Even if the scaling time can be fitted with a power law (with approx. 0.38) the collapse of the correlators into a master curve is not satisfactory.

Mean Squared Displacement

@T=1.2

There is a clear plateau that grows with the waiting time

@T=0.3

There is no clear plateau but rather a power law increase in that follows a power law (with an exponent that grows with waiting time)

Experimental FindingsT=1.2 T=0.3

Pham et al. cond-mat/0308250, to be published on PRE

Conclusion

The repulsive and the attractive glasses aging scenario has been numerically investigated

• Static structure factors do not change significantly with time

• Density-density correlation functions evolves with the waiting time in a completely different way

• No scaling properties with waiting time• Good agreement with experimental results.

Collaborators:E. Zaccarelli, F. Sciortino and P.Tartaglia (Roma)

S. Buldyrev (Boston)

Acknowledgment:

W.C.K. Poon and K.N. Pham for useful discussions and for bringing their results to our attention.

Reference:G. Foffi et al. cond-mat/0311621 submitted to JCP