Generation Mechanism and Reynolds Number Regulation of Multi-Peak Oscillatory Concentration Gradients in Multi-Layer Vertical-Stepped Microchannels

This study systematically investigates the flow characteristics, mixing efficiency, and concentration gradient generation (CGG) capabilities of three types of vertical-stepped main-channel microfluidic concentration gradient generators-the upward vertical-step (UVS-GG), downward vertical-step (DVS-GG), and straight horizontal channel (SHC-GG)-under different Reynolds numbers (Re) through numerical simulation and comparative analysis. Using numerical simulations, the research reveals the universal transition of flow regimes from diffusion-dominated to convection-dominated and reports the emergence of a "multi-peak oscillatory concentration gradient" phenomenon under stepped geometries and high Re (Re = 100, 200). The results indicate that the SHC-GG can generate monotonic gradients at low Re, making it an ideal baseline configuration. In contrast, UVS-GG and DVS-GG enhance mixing and enable the programming of complex concentration distributions through unique inertia-geometry coupling effects. The synergistic interaction between geometric configuration and Re is identified as the core mechanism for regulating concentration field morphology and device performance. This study provides key theoretical and design foundations for the rational design of microfluidic gradient generators targeting applications such as biological screening, chemical analysis, and material synthesis.