Paper-based microfluidics: Analyte-driven imbibition under the lens

Paper-based microfluidic devices are widely used in point-of-care diagnostics, yet the fundamental mechanisms governing analyte transport under partially saturated conditions remain insufficiently characterized. Here, we systematically investigate the concentration-dependent imbibition dynamics and particle trapping behavior of analyte/colloid-laden fluids in porous paper substrates. Using model food-dye colloids of varying particle sizes (∼0.3-4.5 μm) and concentrations (0.5-2 mg/ml), we quantify key saturation-dependent parameters and reveal their strong influence on wicking length and analyte retention. A semiempirical numerical model incorporating experimentally derived van Genuchten and Brooks-Corey parameters is developed to predict analyte flow under varying conditions. Our study demonstrates that particle size, concentration, and paper properties critically modulate transport behavior, with implications for reproducibility and sensitivity in lateral flow assays. Furthermore, through Damköhler number analysis, we propose practical design guidelines for optimal test line placement based on flow and reaction dynamics. This combined experimental and modeling framework offers new insights for the rational design and optimization of paper-based diagnostic platforms.