Stopping a Multilayered Co-Axial Flow in a 3D Printed Microchannel with Cascaded Nozzles

Three different flow control methods are investigated to stop a multilayered flow inside a 3D-printed microfluidic channel by bringing the average flow velocity from >100 mm s⁻¹ to below a critical velocity of 200 µm s⁻¹ within a certain delay time t_d of ≈2 s. A sequence of three concentric nozzles is 3D printed (≈75 µm) and embedded serially inside the microchannel (≈200 µm) using a two-photon polymerization method. The 3D printed device produces a structured coaxial flow of four streams with individual layer thicknesses of O(10 µm). The pressure gradient across the fluidic circuit is removed, from > 2 bar to ≈0 bar, to stop the multilayered flow and measure t_d to assess the performance of the three stop-flow methods. During the stop-flow phase, an inhomogeneous pressure gradient across different inlets resulted in a backflow to inlet channels with lower pressures. The fluidic capacitance is systematically managed to minimize a dimensionless backflow index (BFI) value from ≈0.3 (worst case) to ≈0.03 (best case) for a total flow rate ranging from 16.8 to 168 µL min⁻¹. Finally, the best stop-flow conditions are recommended, which resulted in a minimal delay time of t_d ≈ 2 s and a BFI < 0.05.