Nanobubble-Induced Aggregation of Ultrafine Particles: A Molecular Dynamics Study

Nanobubble-induced aggregation (NBIA)of fine and ultrafineparticlesin liquid is a promising method for enhancing floatation rates inmineral processing, cleaning contaminants from water, and revivingmarine ecosystems. Although the current experimental techniques canmeasure the nanobubble capillary force between two surfaces with controlledapproach speed, they are not capable of imaging NBIA dynamics of fine/ultrafineparticles by real-time observation with nanoscale spatial resolution.In this work, we use molecular dynamics (MD) simulations to studydynamics of NBIA of Ag particles in a Lennard-Jones fluid system.The molecular-level modeling allows us to study microscopic detailsof NBIA dynamics that are inaccessible by current experimental means.Using MD simulations, we investigated the effects of NB size, surfacewettability, surface roughness, and contact line pinning on NBIA dynamics.Our modeling results show that both concave NB bridges between twohydrophobic surfaces and convex NB bridges between two hydrophilicsurfaces can result in an attractive nanobubble capillary force (NBCF)that causes the aggregation of Ag particles in liquids. The equilibriumseparation between two fully aggregated particles can be well predictedby the improved capillary force model. We also observe that the changeof contact angle after the contact line pinning occurs at the sharpedge of a particle, which slows the aggregation process. Our thermodynamicsanalysis shows that there is a critical contact angle below whichthe merged surface NBs will detach from the surface instead of causingaggregation. The prediction of the critical contact angle is corroboratedby our MD simulation results.