Low fouling helical hollow fibers via rope coiling
Concentration polarization and membrane fouling are among the most significant bottlenecks in pressure-driven membrane processes. As species are rejected by the membrane, they deposit on the surface almost always to some extent, even when the feed flows tangential to the membrane surface to sweep the deposit, and decrease the mass transfer resistance. Applying high Reynolds number tangential flow can avoid concentration polarization and hence deposition of rejected material on the surface, but this is costly and may be undesirable due to negative effects of high shear on the feed components. One way of creating vortices at low Reynolds numbers is to use helically winding channels in which Dean vortices are generated as a result of the imbalance between centrifugal and inertial forces.
In a recent paper by the Çulfaz-Emecen Research Group, helical hollow fiber membranes were produced by making use of the liquid rope coiling phenomenon in a regular spinning system . Liquid rope coiling, a term first defined by Barnes and Woodcock in 1958  is the periodic buckling, which forms coiling fibers when a viscous fluid is poured onto a solid or liquid surface from a certain height. Coiling occurs as a result of the competition between axial compression and bending of the fluid "rope", and is influenced by the fluid's viscosity, density, flow rate and radius, as well as the height and gravity. This phenomenon was used in our study to produce helical hollow fibers, inside which Dean vortices form during filtration and reduce concentration polarization and fouling. A variety of spinning parameters including polymer dope and bore liquid flow rates, air gap distance, polymer dope viscosity and coagulation value were tested to map conditions where helical geometries occur.
It was demonstrated in filtration of yeast suspensions, as a model fouling medium containing biological cells, that the helical, or twisted, fibers have lower fouling tendency with the feed on either the bore or the shell side of the membrane, implying that the improved antifouling behaviour due to vortex formation in or around the curling structure can be an effective way of decreasing fouling during membrane filtrations. .
 H. Yücel, P. Z. Çulfaz-Emecen, Helical hollow fibers via rope coiling: Effect of spinning conditions on geometry and membrane morphology, J. Membr. Sci., 559, 2018, pp.54-62.
 G. Barnes, R. Woodcock, Liquid rope-coil effect, Am. J. Phys., 26 (4), 1958, pp. 205-209.
This study was funded by Middle East Technical University Research Fund Grant no. BAP-03-04-2016-003.