Effect of hydrodynamics on nanoparticles stability in Pickering emulsions stabilized by metal organic frameworks (MOFs) in reversing shear flow

The effect of flow hydrodynamic on particle-droplet stability in Pickering emulsions (PE) is investigated both theoretically and experimentally under oscillatory shear conditions. The emulsions are stabilized by two types of metal organic frameworks (MOFs) nanoparticles (NPs) with different hydrophilic/hydrophobic nature. The more hydrophobic NPs (Zeolite Imidazolate-ZIF-8) remained attached to the droplets surface, and resulted in droplet sizes consistent with those predicted by a torque balance (TB) model. On the other hand, emulsions stabilized by the more hydrophilic NPs (Amine Materials of Institute of Lavoisier-MIL-101) had smaller size droplets than TB model predictions and broader size distributions at high oscillatory shear stresses. This was attributed to the stripping of the stabilizing NPs of the droplet surface at high shear leading to possible droplets coalescence and subsequent fragmentation. The analysis showed that, in a drop-by-drop emulsification using membranes or microporous surfaces, forces due to fluid shear stress as well as flow eddies acting on a droplet during formation are the most significant contributor to particles detachment from a droplet surface. Forces due to an eddy viscous shear or post detachment fluid shear are orders of magnitude smaller than desorption energy and do not contribute to particles detachment. Although the analysis is done for an oscillatory shear flow, the approach and the stability concept can be applied to other types of flow fields. (C) 2015 Elsevier B.V. All rights reserved.