Microstructural effects on the permeability of nitrocellulose membranes for paper diagnostics

As a simple, portable, cost-effective, and user-friendly diagnostic tool, paper-based detection platforms (e.g., paper diagnostics) have found widespread applications in diagnosis and monitoring of diseases, environment pollution, food safety, etc. The performance of paper diagnostics is notably affected by fluid flow and transport in porous nitrocellulose membranes (NC membranes). The complex and irregular pores of the NC membranes cause difficulties in experimental observation and numerical simulation of flow transport. The NC membrane is a granular porous medium that has a higher porosity (0.7-0.9) than the conventional granular porous medium (0.4-0.5). This paper aims to investigate the effects of microstructure on NC membrane permeability experimentally, theoretically and numerically. Rate-of-rise experiments were performed to measure the permeability of NC membranes. In order to characterize the microstructure of NC membranes, the porosity was measured using the weighting method. SEM images were then taken to statistically determine particle and pore size distributions. A novel particle-cubic unit cell was proposed to represent the pore geometry of NC membranes. Based on the model, the theoretical relation between porosity, pore size and particle size was derived, which agreed well with the experimental data. Fluid flow in the particle-cubic model was numerically simulated with periodic boundary conditions to calculate the permeability. In addition, a closed form solution of the permeability was developed as a function of representative microstructural parameters. Results obtained using the present numerical and theoretical models agreed well with experimentally measured permeability, whereas existing correlations were found to be inapplicable for NC membranes.