Microscopic dynamics of flow in molecularly narrow pores

Abstract

Flow of fluids confined in molecularly narrow pores is studied by molecular dynamics. Strong density variations across the pore render the usual dependence of the local viscosity on local density inappropriate. At separations greater than four molecular diameters flow can be described by a simple redefinition of local viscosity. In narrower pores a dramatic increase of effective viscosities is observed and is due to the inability of fluid layers to undergo the gliding motion of planar flow. This effect is partially responsible for the strong viscosity increases observed experimentally in thin films that still maintain their fluidity.

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

“…The LADM gave satisfactory results in predicting the flow of moderately-confined fluids [28]. However, the LADM model failed to predict many of the properties of strongly-confined films (≤ 4 molecular layers), most notably the enhanced viscosity, where the density variation alone cannot explain the local dynamic properties of the film [137].…”

“…Conversely, the dynamics of confined fluids are also influenced by factors that are not intrinsic to the fluid, so the definition of a bulk viscosity is not strictly appropriate [138]. However, the ratio of the shear stress to the shear rate still gives insight into the resistance to flow of confined films and can be compared directly to experiments [137]. Most experiments and NEMD simulations of confined films report this value as the "effective" shear viscosity,  eff .…”

Nonvascular, Symplasmic Diffusion of Sucrose Cannot Satisfy the Carbon Demands of Growth in the Primary Root Tip of Zea mays L

“…The main contributors to elevated cytoplasmic viscosity are proteins that would be excluded from channels 1 nm in radius. However, the effective viscosity of water for pressure-driven flow in such small channels would be higher than that of water moving through macroscopic channels (1.0 X 10-2 poise), because of steric hindrance and nonuniform density created by the layering of molecules next to the wall of the channel (Bitsanis et al, 1990). A channel 1 nm in radius is as wide as 5.2 water molecules (the radius of a sphere with a volume equal to the average volume of one water molecule is 0.192 nm; Renkin [1954]).…”

“…A channel 1 nm in radius is as wide as 5.2 water molecules (the radius of a sphere with a volume equal to the average volume of one water molecule is 0.192 nm; Renkin [1954]). From molecular dynamic simulations (Bitsanis et al, 1990), this width implies an effective viscosity for pressure-driven flow 3.4 times higher than that of water flowing through macroscopic channels. Hence, we used 3.4 x 10-2 poise as our estimate of qo.…”

“…Hagen-Poiseuille flow should occur even at microscopic dimensions, although the effective viscosity of the moving fluid increases dramatically as pore diameter becomes narrower than four molecules of the fluid (Bitsanis et al, 1990). For the portion of the plasmodesma that contains ultrastructure, the conductance should be p(r; -r:)k2…”