Fouling minimization at membranes having a 3D surface topology with microgels as soft model colloids

Cross-flow ultrafiltration of colloidal suspensions is mainly limited by the accumulation of retained particles adjacent to the membrane surface. The hydrodynamic conditions in the feed channel strongly affect the mass transfer rate of colloids from the membrane surface to the bulk and is therefore an important control measure. A common approach for enhancing mass transfer through hydrodynamics is to modify the geometry of the feed channel. In flat-sheet membrane configurations, feed spacers are typically installed, which function as turbulence promoters. In tubular and hollow-fiber membranes implementation of feed spacers is difficult, thus different strategies are sought for. Here we study the effect of a spiralling surface pattern (Helix) on the hydrodynamics and flux performance of tubular ultrafiltration membrane operated in cross-flow mode at inside-out configuration. We apply flow Magnetic Resonance Imaging (MRI) to visualize the flow-field inside the lumen and asses the length dependency of the permeate flux. We combine this information with classical filtration experiments at a wider range of operational parameters. Microgel suspensions were used as a model for soft colloidal systems. The transmembrane flux for the Helix as compared to the reference tubular membrane only increases when non-laminar flow developed inside the lumen channel. Yet the onset of transient non-laminar flow conditions starts at much lower Reynolds numbers of above 1700 for the Helix, while the regular tube shows a transition at high velocities and Re>3150. In consecutive filtration experiments, the Helix shows less flux decline as well. This improved fouling resistance is based on less flux decline at the end of the Helix membrane. Flow-MRI proves that the helical corrugations impose swirling flow patterns with contributions normal to the membrane surface opposing the deposition of microgels onto the membrane surface.