Spatiotemporal control of a multilayered co-axial flow in a 3D printed microchannel with cascaded nozzles

Sculpting and stopping multilayered co-flowing streams is challenging due to inhomogeneous pressure distribution within a fluidic circuit composed of multiple interconnected microchannels having variable flow resistances. Here, we have investigated three different flow control methods to effectively 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 ∼2s. Firstly, we 3D printed a sequence of three concentric nozzles (∼75 µm) embedded serially inside the microchannel (∼200 µm) using a two-photon polymerization (2PP) method. Secondly, we used the 2PP-based 3D printed device to produce a structured coaxial flow of four streams with individual layer thicknesses of O (10 µm) within the outlet section of the microchannel. Thirdly, we removed the pressure gradient across the fluidic circuit, from > 2 bar to ∼0 bar, to stop the multilayered flow and measured 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. In the three stop-flow methods investigated, we systemically managed the fluidic capacitance 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 µl min ^-1 to 168 µl min ^-1 . Finally, we have recommended the best stop-flow conditions, which resulted in a minimal delay time of t _D ∼ 2s and a BFI < 0.05.