Enhanced wicking dynamics of paper-based microfluidics by a nonporous wall

A paper-based microfluidic device, comprising a nonporous wall and a porous sheet separated by a gap, is investigated through many-body dissipative particle dynamics simulations. The time evolutions of the mean penetration lengths within the gap and the porous sheet are monitored separately, characterized by Lg(t) and Lp(t), respectively. They can be described by Lg2 = Sgt and Lp2 = Spt, consistent with the Lucas–Washburn equation. The influences of gap width (w) and the surface wettability of the nonporous wall (θw) on the imbibition rates Sg and Sp are systematically examined. Due to the presence of the gap, Sp is facilitated to exceed the imbibition rate of isolated paper, and Sp is always larger than Sg. It is found that Sp increases linearly with increasing w but decreases with increasing θw. The enhancement of the imbibition rate results from side imbibition, which introduces extra capillary flow from the gap to the porous sheet, in addition to the capillary flow from the bottom reservoir. The contribution from side imbibition to the liquid absorbed within the porous sheet increases over time and becomes the dominant source, surpassing the direct contribution from the bottom reservoir.