The study of out-of-equilibrium systems is of fundamental scientific interest and industrial applicability, and, as such, characterizing and controlling these systems is considered by the Department of Energy as one of the five grand challenges facing scientists today [
1]. Colloidal dispersions are usually out-of-equilibrium systems that are of particular relevance as they are found in a wide variety of natural and synthetic consumer products, such as foods, pharmaceuticals, paints and cosmetics [
2]. Moreover, colloidal dispersions are often employed as model systems to probe and understand non-equilibrium behavior in other systems of interest, such as atomic or molecular systems, because colloidal dispersions have more experimentally accessible length scales and time scales [
3]. Importantly, the properties of colloidal systems 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. 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 [
4,
5]. In particular, we quantitatively relate rheological aging to structural aging using concurrent rheometry and time-resolved small angle neutron scattering (Rheo-SANS) measurements in both glasses and gels [6]. Moreover, by probing the system with different thermal profiles and shear rates, we investigate the effect of thermal and shear history on the mechanical properties and microstructure during aging to assess the efficacy of shear as a rejuvenation method. Our results showed that after mechanical rejuvenation, the evolution of the microstructure and bulk properties progressed more rapidly compared to the thermal rejuvenation, indicating that either the microstructure may not be completely erased by shear or that the microstructure continues to evolve under shear. While the formation and aging trajectory in the both the gel and glass samples are significantly affected immediately following a temperature quench or the application of stress, the samples appear to eventually evolve along a similar aging trajectory, independent of the thermal and shear history at long times.
References:
[1] ‘‘Directing Matter and Energy: Five Challenges for Science and the Imagination,’’ Basic Energy Sciences Advisory Committee Report, U.S. Department of Energy (2007).
[2] Joshi, Y. M., ‘‘Dynamics of Colloidal Glasses and Gels,’’ Annual Review of Chemical and Biomolecular Engineering, Vol 5 5, 181-202 (2014).
[3] Lu, P. J., and Weitz, D. A., ‘‘Colloidal Particles: Crystals, Glasses, and Gels,’’ Annual Review of Condensed Matter Physics, Vol 4 4, 217-233 (2013).
[4] 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).
[5] 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).
[6] Gordon, M.B., Kloxin, C.J., and Wagner, N.J., "Rheology and Microstructure of an Aging Thermoreversible Colloidal Gel," Journal of Rheology (2017).
