Effects of wall roughness on flow in nanochannels

Abstract

Nonequilibrium molecular dynamics simulation is applied to investigate the effect of periodic wall roughness on the flow of liquid argon through krypton nanochannels. The effect of the length of a rectangular protrusion on the flow is investigated and compared to the case of nanochannels with flat walls. The results show a clear trapping of fluid atoms inside the rectangular cavities that are formed between successive protrusions. The size of the cavities affects the potential energy map and, consequently, fluid atom localization. This localization results in a reduction of velocity values inside the cavities, as well as a reduction of the slip length near the rough wall.

Hydrodynamic force measurements under precisely controlled conditions: Correlation of slip parameters with the mean free path

“…2. In agreement with the literature, 4,15,17 we find that the slip length (and also the viscous slip coefficient) is larger on surfaces with smaller roughness. 3.…”

“…On the contrary, for λ f < 2 the roughness peaks extend into the slip regime, thereby adding friction and effectively reducing slippage. A reduction of slip on surfaces with larger roughness has been found also previously, 4,15,17 but the effect seems to depend upon the wettability of the surface, 2, 46 which we cannot assess here.…”

“…This point of view is supported by molecular dynamics simulations. 15,67 For λ f , the roughness profile could then act more like curvature, and α decreases with larger wavelength of the surface corrugations.…”

“…In numerous modern experiments the influence of tribological parameters, such as roughness, [3][4][5][6] wettability, 7 surface contaminations, 8,9 nanobubbles, 2, 10 and shear velocity, 5,[11][12][13] on the flow of fluids on solid surfaces has been studied. Numerically, molecular dynamics and Monte Carlo simulations [14][15][16][17][18] have improved our understanding of the effects of gas trapping, roughness scales, and momentum exchange at surfaces (accommodation), thereby shedding new light on experimental problems and their origins. However, results still remain rather inconclusive.…”

Non-equilibrium phase behavior and friction of confined molecular films under shear: A non-equilibrium molecular dynamics study

“…There have been experimental and some NEMD studies of this effect (see for example Refs. 3,[69][70][71][72] ). Roughness can increase or decrease the flow rate and slip length depending on the chemical nature of the wall and liquid (e.g., broadly hydrophobic or hydrophilic in interaction), and the relative size of the wall roughness and its periodicity compared to that of the dimensions of the molecules of the confined system.…”