Precise capillary flow for paper-based viscometry

The imbibition dynamics of aqueous solutions in paper substrates determines the performance of all the operations integrated in analytical paper-based devices. In particular, an accurate control of the flow rate is required for quantitative analysis such as viscometry. This work experimentally investigates paper filling dynamics in order to find a strategy to improve the precision and predictability of the imbibition process. The effect of performing successive wetting–drying cycles on the same strips is explored, since we have discovered that, after around four cycles, the filling kinematics is highly repetitive, and data closely follow the theoretical Lucas–Washburn model. It is found that the cyclic process enables quantitative assessment of the filling dynamics with uncertainties lower than 0.8 %. Implementing this protocol, paper-based viscometry with a precision around 1 % was experimentally demonstrated. This knowledge is of interest to develop paper-based microfluidic devices with a new level of precision.