Aqueous Laponite Dispersions are Attractive Gels, Not Repulsive Wigner Glasses: A Critical Commentary

Abstract

An aqueous dispersion of Laponite has been studied in the literature for over the past three decades. Typically, the aqueous dispersion of Laponite undergoes incessant evolution of its microstructure, wherein its elastic modulus and the mean relaxation time show a continuous increase as a function of time. A considerable amount of discussion has revolved around the nature of this dispersion, specifically whether it can be classified as a repulsive Wigner glass state, characterized by disconnected Laponite particles stabilized by electrostatic repulsions, or an attractive gel state, in which the particles form a percolated space-spanning network. The proponents of the Wigner glass state also conjecture that this system experiences a glass-glass transition after a period of two days has elapsed since its preparation. In this commentary, we explore this topic from a rheological point of view analyzing the published literature and performing new experiments. Aided by additional evidence from the literature, we propose that rheological behavior overwhelmingly suggests that an aqueous dispersion of Laponite undergoes sol - attractive gel transition and remains in the attractive gel state over at least up to 7 days without undergoing any additional transition. Importantly, rheology, despite being a macroscopic tool governed by principles of mechanics, offers profound insight into the microstructure of this particular system. The corresponding analysis conclusively determines the state of an aqueous dispersion of Laponite to be an attractive gel.