Due to limitations in experimental techniques, direct experimental studies on nano-scale capillary flow have been restricted to 10nm. Through high-precision large-scale molecular simulations, Professor Baifeng Bai and Professor Chengzhen Sun's team from the National Key Laboratory of Green Hydrogen and Fuel Cell Technology at Xi'an Jiaotong University investigated the capillary flow characteristics of water within nano-channels ranging from sub-nanometer to 30nm, revealing the scale dependence of capillary flow. This breakthrough challenges the conventional wisdom that smaller channels lead to greater resistance and slower flow. The research findings have recently been published in Physical Review Letters.
The study found that decreasing the nano-confinement space scale results in the capillary flow capacity of water being lower than predicted by the classical Lucas-Washburn theory, with deviations significantly increasing as the scale decreases, consistent with conventional experimental observations. However, when the characteristic scale of the channel further decreased to 3nm, an unexpected reversal in water flow occurred, exhibiting anomalous flow enhancement characteristics, leading to a decrease rather than an increase in theoretical deviations. Theoretical analysis revealed that the structure of nano-confined water depends on the confined scale, giving rise to two opposing scale-dependent effects: a long-range viscosity enhancement effect that increases flow resistance and a short-range separation pressure effect that enhances flow dynamics. The mismatch of scale effects results in the nano-scale capillary flow of water displaying a unique non-monotonic scale dependence, particularly observed widely in both hydrophilic and hydrophobic nano-channels.
Related paper information: https://doi.org/10.1103/PhysRevLett.132.184001
