Can 3D-printed spacers improve filtration at the microscale?

Abstract

Despite their ubiquitous use in large-scale filtration processes, the benefits of adding spacers on the microscale—if any—remain unknown. At larger scales, spacers improve performance by directing the flow and inducing turbulent mixing. However, at low Reynolds numbers, it becomes increasingly difficult to initiate mixing because viscous forces dominate over inertial forces. In membrane filtration applications, concentration polarization and membrane fouling can severely limit filtration efficiency, and even a small amount of fluid mixing presents potential to mitigate these issues. In this study, three complex 3D-printed microspacer designs (with feature sizes in the range of 100–400 µm) were incorporated into narrow channels to consider their enhancement effects for microfiltration and ultrafiltration applications. These structures included two herringbone designs and one triply periodic minimal surface, e.g. a ‘gyroid’ spacer. Experiments and simulations found that the gyroid design achieved the highest membrane flux enhancement (i.e. 81 and 93% above a plain channel for blood mimicking and plasma mimicking solution tests, respectively). This was significantly better than the enhancement by herringbone designs. All of the spacers added back-pressure, with gyroid incurring a 23% higher pressure drop than the plain channel, which was considered as an acceptable performance trade-off. Based upon this work, 3D-printed microspacers were shown to enhance mixing and improve membrane filtration by reducing concentration polarization and fouling. Further, this study indicates that 3D printing can enable a promising new class of efficient, small-format devices for filtration processes.