Hydrodynamics of liquid–liquid parallel flow in novel microextractors: Review

Parallel flows on microfluidic platforms enable continuous liquid–liquid operations and inline separation of effluent streams, bearing immense scope in integration of miniaturized separation processes. However, these flows face major challenges including low mass transfer efficiency due to lack of transverse convection and flow instability at low flow rates, which undermine their operating range and utility. The limitations have inspired dedicated research, delving into the fundamentals of fluid flow and transport mechanism and exploring novel configurations of microextractors. The current article summarizes the hydrodynamics of parallel flows and relevant process intensification strategies in microfluidic extractors, evolving from the use of straight to curved and helical geometries, besides elucidating unique secondary flow patterns observed in-state-of-the-art designs. It includes exclusive sections addressing various aspects of parallel flows: (i) flow inception and theoretical modeling of flow fields and phase hold up, (ii) challenges concerning interfacial stability and flow intensification, (iii) curvature effects in planar curved geometries, and (iv) curvature cum torsional effects in unique multi-helical configurations. The theoretical perspective of this review presents a roadmap that can provide further insights into design modifications for developing improved integrated microextractors based on parallel flows.