Efficient micromixing is essential for lab-on-a-chip systems because microscale flows are typically laminar and diffusion-limited. This study numerically investigates electroosmotic micromixing in microchannels with emphasis on trapezoidal cross-sections and the role of cross-sectional geometry in enhancing species homogenization. Three-dimensional simulations were performed by solving the laminar flow field coupled with advection–diffusion of a dilute tracer concentration. Mixing performance was quantified using a statistical mixing index, computed from normalized concentration fluctuations over sampling points across a prescribed line/plane. A parametric analysis was then conducted to evaluate (i) the trapezoidal side angle θ and (ii) the shape factor (aspect ratio). The results show that mixing develops progressively along the channel, and geometric modifications significantly alter interfacial stretching and transverse transport. Increasing θ yields a systematic improvement in mixing index at all investigated downstream locations, with the strongest sensitivity observed farther from the inlet. In contrast, increasing S reduces the mixing index within the examined range, indicating that a more pronounced trapezoidal shaping (smaller S) is favorable for rapid homogenization in the entrance region. Outlet concentration profiles extracted along the horizontal midline confirm the geometry-dependent broadening of the mixing layer. These findings provide practical design guidelines for optimizing trapezoidal micromixers under electroosmotic actuation.
