Knudsen minimum disappearance in molecular-confined flows

It is well known that the Poiseuille mass flow rate along microchannels shows a stationary point as fluid density decreases, referred to Knudsen minimum. Surprisingly, if characteristic length comparable molecular size, minimum disappears, reported for first time by Wu et al. ( J. Fluid Mech. , vol. 794, 2016, pp. 252–266). However, there still no fundamental understanding why monotonically increases throughout entire range of regimes. Although diffusion believed dominate transport at nanoscale, here we show Fick's law fails in capturing this behaviour, and so alone insufficient explain confined phenomenon. Rather, disappears tight confinements because decay due decreasing effects overcome enhancing contribution provided velocity slip wall.