Explaining the solution chemistry dependence of particle detachment by capillary forces

During two-phase flow, the fluid–fluid interface exerts a strong force on particles attached to the solid–water interface, resulting in some cases in their detachment. The capillary force exerted by the meniscus is more than sufficient to detach particles; however, existing experimental data indicate that the removal of particles by the interface is significantly affected by solution chemistry. Traditional models for detachment during two-phase flow predict complete detachment, independent of the solution chemistry. This discrepancy has yet to be resolved. Direct visualization of particle detachment was performed during drainage (air injection into water) and imbibition (water injection into air) on a particle-laden borosilicate glass substrate. Particle detachment was observed via optical microscopy. The novelty of the experimental procedure is the individual study of drainage and imbibition across a range of solution chemistries (controlled by NaCl concentration and pH). Almost complete particle removal was observed in all the cases except during drainage under favorable conditions. We experimentally and theoretically show that the capillary force is, in fact, large enough to detach all the particles but that during drainage, the particles are reattached to the substrate. Reattachment is observed directly and predicted theoretically with force balances and particle trajectory analysis. The existence of an energy barrier under unfavorable conditions prevents reattachment, which explains the solution chemistry dependence of particle removal. The result is a quantitative theory describing particle detachment, accounting for particle reattachment. The analysis in this work is an integral part of any quantitative description of particle detachment in two-phase flow.