The Rheology and Microstructure of an Aging Thermoreversible Colloidal Gel

The properties of colloidal gels are known to change with gel age, but the processes and mechanisms by which aging occurs is not well understood and limits our ability to predict macroscopic behavior in these systems. Colloidal gels are disordered systems that are trapped in a dynamically arrested, non-equilibrium state. It is generally accepted that the aging of colloidal gels is governed by their potential energy landscape of which the thermodynamic equilibrium is the global minimum; however, the system cannot reach equilibrium on an experimental timescale [1]. Instead, the gel samples local potential energy minima resulting in aging behavior. Due to its fundamental significance and industrial applicability, we investigate the microstructural basis of aging in a well-studied, model adhesive hard sphere (AHS) system, consisting of silica nanoparticles grafted with an octadecyl brush dispersed in tetradecane [2, 3].

In this work, we quantitatively relate rheological aging to structural aging by simultaneously measuring the bulk properties and gel microstructure using rheometry and small angle neutron scattering (Rheo-SANS), respectively [4]. Specifically, we develop a quantitative and predictive relationship between the macroscopic properties and the underlying microstructure (i.e.the effective strength of attraction) of an aging colloidal gel, which is independent of the gel’s thermal and shear history. Analysis with mode coupling theory is consistent with local particle rearrangements as the mechanism of aging, which lead to monotonically increasing interaction strengths in a continuously evolving gel and strongly support aging as a trajectory in the free energy landscape. The analyses and conclusions of this study may be 1) industrially relevant to products that age on commercial timescales, such as paints and pharmaceuticals, 2) applicable to other dynamically arrested systems, such as metallic glasses, and 3) used in the design of new materials.

References:

[1] Royall, C. P., Williams, S. R., Ohtsuka, T., and Tanaka, H., ‘‘Direct observation of a local structural mechanism for dynamic arrest,’’ Nature Materials 7, 556-561 (2008).

[2] Eberle, A. P. R., Castaneda-Priego, R., Kim, J. M., and Wagner, N. J., ‘‘Dynamical Arrest, Percolation, Gelation, and Glass Formation in Model Nanoparticle Dispersions with Thermoreversible Adhesive Interactions,’’ Langmuir 28, 1866-1878 (2012).

[3] Eberle, A. P. R., Wagner, N. J., Akgun, B., and Satija, S. K., ‘‘Temperature-Dependent Nanostructure of an End-Tethered Octadecane Brush in Tetradecane and Nanoparticle Phase Behavior,’’ Langmuir 26, 3003-3007 (2010).

[4] Gordon, M.B., Kloxin, C.J., and Wagner, N.J., "Rheology and Microstructure of an Aging Thermoreversible Colloidal Gel," Journal of Rheology. Submitted.