GELATION IN A MODEL NANOPARTICLE SYSTEM WITH THERMOREVERSIBLE SHORT-RANGE INTERACTIONS
Aaron P.R. Eberle, Ramon Castañeda-Priego, Norman J. Wagner, Paul D. Butler
Nanoparticle and colloidal dispersions exist in nearly every area of our daily lives from the paint applied to protect our homes to toothpaste. Hence, understanding the delicate interplay between the interparticle forces and structural states and phases is an important challenge to industry and scientists alike. Interestingly enough, experiments and simulations of model colloidal systems have shown that colloids can behave as “big atoms”. That is, colloidal systems can exist in various distinct states analogous to molecular systems such as an isolated gas, condensed liquid, or solid. Additionally, these systems can exhibit a wide range of equilibrium phases and non-equilibrium states such as gels, attractive driven glasses (ADGs), and repulsive driven glasses (RDGs), etc. While equilibrium phases are fairly well characterized by both experiments and simulations non-equilibrium states, such as gelation, are less-well understood and highly debated in literature.
In this work we study the fluid-to-gel phase transition of a well-characterized model system (particle radius, a ~ 15 nm) in which we control a short-range attraction via temperature. We define gelation as the point when particles first percolate through the system to form a continuous network which is in a state of dynamical arrest. We use a combination of small-amplitude oscillatory rheology and fiber optic quasi-elastic light scattering (FOQELS) to characterize the temperature at which dynamical arrest occurs for a given dispersion. We then use small-angle neutron scattering (SANS) to probe the nanostructure of the dispersion at and around the gel temperature. Analysis of the SANS scattering profiles provides an accurate method to extract the interparticle potential at the gel point. The experimental results demonstrate a connection between the gel and ADG lines defining the onset of dynamical arrest. Furthermore, the results show that the gel line intersects the liquid-vapor coexistence region at concentrations below the critical point. Monte Carlo (MC) simulations of the equilibrium structure using the experimentally determined interparticle potential allows for a direct visualization of the structure and a detailed numerical study of dynamic arrest. These results support a unifying interpretation of dynamical arrest for systems with a short-range interaction, namely, the gel-line is an extension of the ADG line to dilute concentrations and intersects the liquid-vapor coexistence region below the critical concentration.