Capillary filling under nanoconfinement: The relationship between effective viscosity and water-wall interactions

Abstract

Understanding the capillary filling in nanopores is crucial for a broad range of science and engineering problems. Because of the dominant importance of surface effects at the nanoscale, the properties of confined water in nanopores must substantially differ from that of bulk water. Here, a novel model for effective viscosity in hydrophilic nanopores was proposed by further considering the water-wall interactions and density layering phenomena to modify the capillary filling at the nanoscale, every parameter in the proposed model has clear physical meaning. The presented model is successfully validated against existing experimental data collected from published literatures. The theoretical analysis and comparison denote that pore size, pore shape and nanomaterial properties have significant effects on the effective viscosity: (1) due to the stronger water-wall interactions, the higher the energy surface is, the larger the effective viscosity will be; (2) the effective viscosity in nanocapillaries is larger than that in nanochannels at the same pore size; (3) the effective viscosity of confined water in hydrophilic nanopores can exhibit a dramatic increase, the value could be even three times or one order magnitude greater than that of bulk water for silica or clay nanocapillaries within 10 nm. Meanwhile, compared with the traditional imbibition model in cylindrical capillaries, the coupled effects of pore shape and water-wall interactions would make the capillary filling characteristic more complex, which is one of the reasons for the deviation between real imbibition condition and traditional value for nanomaterial.