System Size-Dependent Transport Properties in Materials of Nanoscale Dimension

Abstract

Fluid transport in finite-sized nanoporous materials is critically affected by apparent interfacial barriers, which severely restrict efficiency improvement on reduction in system size; however, the mechanism leading to this effect even in defect-free materials is unknown. Using large-scale atomistic simulations emulating carbon nanotube (CNT) and zeolite membranes up to a micrometer thick, we demonstrate that transport coefficients in finite nanomaterials are non-uniform and increase with distance from the external surface, attaining the bulk system value far from the surface. Attenuation of transport occurs in a finite entrance region of developing fluid flow, in which the fluid momentum de-correlates from that at the entry and whose extent depends on the texture of the pore surface. This effect disguises as interfacial resistance in nanomaterials and has remarkable effects on membrane selectivity. Thus, finite CNTs can be selective for H2 over CH4, in contrast to the corresponding infinite tube that is selective for CH4, due to the smaller apparent interfacial resistance for H2. The simulations, covering a wide range of conditions, reveal a quantitative correlation between the apparent relative interfacial resistance and Maxwell reflection coefficient for CNT membranes. These insights will enable optimal design of ultra-thin membranes, sensors, and other applications using nanoscale materials.