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.