Wicking dynamics of two-ply channels in porous medium-based microfluidic devices

In the advancement of paper-based microfluidic devices, it is reported that the two-ply channel transports fluid noticeably faster than traditional single-ply channels. In this work, the capillary flows in two-ply channels, consisting of a gap of width w between two porous sheets with porosity ϵ and thickness d, are investigated through many-body dissipative particle dynamics simulations. The advancing meniscus varies with position, characterized by the penetration lengths in the gap (L_g), the porous sheet (L_p), and the maximum value (L_max). L_max is always located within the porous sheet but near the gap. The time evolution of the penetration lengths can be described by Washburn's expression, L² = (S)t, and the imbibition rates S_p, S_g, and S_max depend on ϵ, d, and w, differing from each other. Two distinct imbibition characteristics are identified: S_g > S_p for low porosities and S_g < S_p for high porosities. Both S_g and S_p decrease with d but increase with w. As ϵ increases, a minimum of S_max occurs due to the synergistic competition between S_p and S_g. Compared to the single-ply channel, which consists of a single porous sheet, the imbibition rate of the two-ply channel is significantly enhanced by at least four times due to side-imbibition from the gap (acting as a reservoir) toward the porous sheet.