Mixing Characterization in a Microfluidics Pearl-Chain Channel using μ-PIV

Micromixing is a critical process in a wide range of lab-on-a-chip and micro total analysis systems (μTAS), which are extensively used in chemical and biological applications such as sample preparation, protein folding, DNA analysis, and cell separation. Due to the inherently small scales and low flow rates in these systems, fluid dynamics are typically governed by laminar flow, characterized by very low Reynolds (Re) numbers. This regime poses a significant challenge for efficient mixing, as turbulence is minimal and mixing must rely primarily on diffusion, which is inherently slow. To address this limitation, this study investigates a passive micromixer based on a pearl-chain microchannel geometry with two inlets and a single outlet. The micromixer was integrated into a microfluidic platform equipped with a flow control circuit composed of two valves and two syringe pumps, delivering fluids at various and precise flow rates. Mixing performance was characterized using a synchronized inverted fluorescence microscope, allowing real-time visualization of fluid interdiffusion. Additionally, micro-particle image velocimetry (μ-PIV) was employed to quantify the velocity fields and to observe the development of flow structures indicative of stretching and folding, which are key mechanisms that enhance mixing in the absence of turbulence. Measurements were taken at distinct locations along the pearl-chain geometry. Additionally, a fluorescence intensity probe was used to assess concentration profiles and quantify mixing efficiency at the locations. The combined use of μ-PIV and intensity measurements provides a comprehensive evaluation of the micromixer's performance and the effectiveness of the pearl-chain geometry in promoting rapid mixing under low Re conditions.