Aging processes and scale dependence in soft glassy colloidal suspensions

Abstract

The aging behavior of colloidal suspensions of laponite, a model synthetic clay, is investigated using light scattering techniques. In order to measure the complete dynamic structure factor as a function of time and of wave vector, we have developed an original optical setup using a multispeckle technique for simple light scattering. We have thus measured the correlation of the scattered light intensity as a function of the age of the sample t(w) for various concentrations. For sufficiently concentrated samples, we observe a two-stage relaxation process. The fast relaxation is diffusive, stationary, and reminiscent of the liquidlike behavior observed in less concentrated samples. The slow relaxation behavior, however, is more complex. It exhibits two successive regimes as the sample ages. In the first regime, the decay time tau(a) increases exponentially with t(w) as long as tau(a)<t(w). In the second one, "full aging" is observed in which tau(a) is proportional to t(w). In this second regime, the relaxation of concentration fluctuations are hyperdiffusive, scaling as exp[-(t/tau(a))(beta)] with beta=1.35+/-0.15. In addition, the spatial dependence of this relaxation time scales as tau(a)(q) approximately q(-x) with x approximately 1.3.

Gels and Glasses in a Single System: Evidence for an Intricate Free-Energy Landscape of Glassy Materials

“…The full lines show the glassy and dotted lines show the gel line, obtained by averaging over all the samples measured. [6,10] have been invoked and were thought to be mutually exclusive [6,9]. We here provided clear experimental evidence for how the distinction of gel and glassy states can be made [16].…”

“…The system we consider is a suspension of anisotropic and charged colloidal particles suspended in water: Laponite, a synthetic clay [5][6][7][8][9][10]. The study of colloids allowed for a significant contribution to elucidating the basic physics of glass transition [11,12].…”

More on the Phase Diagram of Laponite

“…This equation is not derived from any first principle argument, rather it embodies the exponential growth of the τ m for short t w (as previously observed [6,19,24]) and the existence of a divergence of τ m at a finite value of t w , i.e. t ∞ w .…”

“…The most studied part of the phase diagram is the one at low salt concentration (usually samples are prepared in pure deionized water, C s ≃ 1 × 10 −4 M) and intermediate clay concentration (C w ≃ 3%). In this region several dynamical light scattering measurements have shown that the samples initially liquids age towards an arrested phase [2,3,4,5,6] but the mechanism of aggregation is not clear. The gel/glass state has been in fact attributed by different authors to Wigner glass transition [3], frustrated nematic transition [1,7,8,9], micro-segregation [1,10,11], gelation [12,13,14], etc..…”

Novel experimentally observed phenomena in soft matter

“…(d) G (ω) (squares) and G (ω) (circles), acquired from a 5 wt.% montmorillonite clay (Bentonite) suspension [152], display the features predicted in [153]. relaxation τ ∼ t µ w , with µ ≈1 [156]. For smaller waiting times, hyper-aging dynamics (µ > 1) has been observed [157].…”