Surface fluctuations of glassy colloidal suspensions

Christian Frétigny, François Lequeux, Victor Romero and Laurence Talini

Systems that are close to the glass transition evolve over time, they are said to “age”. The aging dynamics of such systems presents heterogeneities that are both spatial (regions of slow and fast dynamics coexist) and temporal (periods of relative calm are interspersed with periods of intense activity). Dynamical heterogeneities are not predicted by most theoretical models for the glass transition, and owing to the scarcity of quantitative measurements available, it is not possible to have a clear overview of the length and time scales they are associated with. Experimental documentation is therefore needed to elucidate the underlying physical mechanisms and improve the understanding of the phenomena involved in the glass transition.
We are interested in glassy phases of colloidal suspensions, which provide easier-to-study systems since the involved length scales are larger than in the ones in structural glasses or polymeric systems. We have evidenced intermittent dynamics during the aging of a glassy colloidal suspension by measuring the thermal fluctuations of its free surface. The technique uses the reflection of a laser beam on the free surface, the position of which is sensitive, at the first order, to the local slope of the surface.

 
At large enough aging times, the thermally induced fluctuations of the surface exhibit bursts of activity, characterized by non-Gaussian dynamics. The corresponding events, which can be thought of as surface quakes, induce large changes in the local slope of the free surface – at a scale larger than the 60µm of the beam size - and are uncorrelated.

The surface has nevertheless a short-time memory of each of those quakes, which is signalled by the two bumps that appear in the distribution of the surface slope before and after a quake. Those bumps demonstrate that the occurrence of a quake favours a strong change in the slope of the free surface.
We are currently developing a two beam set-up in order to characterize the length scale of the dynamical heterogeneities.
 

Publications

• Slow dynamics and intermittent quakes in soft glassy systems, T. Kajiya, T. Narita, V. Schmitt, F. Lequeux and L, Talini, Soft Matter 9, 11129 (2013).
  reprint

• Two beam surface fluctuation specular reflection spectroscopy. A. Raudsepp, C. Frétigny, F. Lequeux, L. Talini, Rev. Sci. Instrum. 83, 013111 (2012).

• Surface fluctuations of an aging colloidal suspension : evidence for intermittent quakes. A. Mamane, C. Frétigny, F. Lequeux, L. Talini, EPL 88, 58002 (2009).