(299g) Shear Induced Breakage of Nanoparticle Agglomerates in a Supercriticle Fluid – A Simulation Study

A major challenge in making and utilizing materials based on nanoparticles and nanocomposites is the tendency for individual nanoparticle constituents to aggregate due to Van der Waal forces and form large fractal structures on the order of tens of microns. The agglomeration of nanoparticles makes them unusable for the synthesis of nanoparticle-based composites, and ultimately destroys those unique properties. Recent experiments conducted at NJIT group for deagglomeration of nanoparticles using rapid expansion of supercritical suspensions (RESS) with carbon dioxide (CO₂) revealed that this technique is very effective in breaking nanoparticle agglomerates, and can result in simultaneous mixing when two or more nanoparticle constituents are used in the suspension. To better understand the deagglomeration mechanisms in RESS, it is important to shed light on the interaction forces between nanoparticles in supercritical CO₂, which determine the dominant mechanisms for deagglomeration when the fluid expands rapidly. To the best of our knowledge, there have been no reported simulation studies of breakup forces for nanoscale silica agglomerates in the presence of supercritical CO₂. In this work we perform molecular simulations of silica nanoparticle agglomerates in supercritical CO₂ after exposing them to shear forces. We considered different types of aggregates (with different topologies and strengths) and studied how the agglomerate structure affects their final strength. Firstly, we used Grand Canonical Monte Carlo to validate the parameters for fluid-fluid and solid-fluid interactions with experimental studies, and then performed molecular dynamics (MD) simulations to investigate the behavior of nanoparticle aggregates under a range of shear rates, i.e. from 10⁵s^-1 (low) to 10⁶s^-1 (high). A group at Princeton studied this process theoretically, and obtained that the flow of the supercritical fluid inside the nozzle in the RESS device generates shear forces of the same order. We could quantitatively confirm that deagglomeration occurs under the influence of those values of the shear force.