Molecular dynamics simulation on the mobilization of deposited nanoparticles by a moving interface

It is now generally accepted that aside from liquid and gaseous phases, the solid phase is also responsible for contaminant transport in subsurface media. Understanding the mobilization of colloidal particles induced by moving interfaces is relevant to contaminant spreading and water purification in the subsurface. The transport and fate of particles in the presence of dynamic interfaces, present in the unsaturated subsurface or vadose zone, is one of the considerable uncertainties in predicting particulate mobilization. Although forces and torques are relatively well characterized for a particle at an interface near thermodynamic equilibrium, non-equilibrium effects due to moving interfaces and contact lines on the re-mobilization of a colloidal particle deposited on a solid surface remain not entirely understood. A better understanding of the mechanisms driving the transport of particulates in unsaturated porous media would contribute to the design of water purification processes and the management of contamination risk. Using molecular dynamics simulations, we investigate the transport and fate of a nanoparticle deposited on a solid surface as a liquid-liquid interface moves past it, depending on the wetting of the solid by the two liquids and the magnitude of the driving force. The particle, the wall, and the fluids are all modeled as atomic systems, where the wetting properties of fluids are determined by the interactions between liquid and solid atoms. We explore how the interfacial transient dynamics alters the equilibrium deposition of particles to a solid surface by a moving fluid-fluid interface in parallel plates channel by looking into the driving force and static contact angles. The deposited particle interacts with two types of interfaces: an advancing interface where the wetting fluid replaces the non-wetting fluid and a receding interface that the non-wetting fluid invades the wetting one. Theoretically, for a static force balance model, when the vertical upward net force is positive, lifting of colloid from the solid substrate can be observed, otherwise, particle remains attached to the substrate or sliding along the substrate occurs based on the horizontal forces. In this work, particle interfacial pinning is observed at sufficiently small values driving forces that below a critical value predicted by a static force balance. Above the critical driving force for pinning and for large contact angle valuePh.D.Includes bibliographical referencesThis work was partially supported by the National Science Foundation Grant no. CBET-1437478.This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation Grant No. ACI-1548562