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Effects of surface roughness and interface wettability on nanoscale flow in a nanochannel

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Abstract

Non-equilibrium molecular dynamics simulations have been carried out to investigate the effect of surface roughness and interface wettability on the nanorheology and slip boundary condition of simple fluids in a nanochannel of several atomic diameters width. The solid surfaces decorated with periodic nanostrips are considered as the rough surface in this study. The simulation results showed that the interface wettability and the surface roughness are important in determining the nanorheology of the nanochannel and fluid slip at solid–fluid interface. It is observed that the presence of surface roughness always suppresses the fluid slip for hydrophilic and hydrophobic surface nanochannels. For fluids over smooth and hydrophobic surfaces, the snapshots of fluid molecules show that an air gap or nanobubble exists at the fluid–solid interface, resulting in the apparent slip velocity. For a given surface with fixed interface wettability, the fluid velocities increase by increasing the driving force, while the driving force has no significant influence on the density structure of fluid molecules. The fluid slip and the flow rate are measured for hydrophilic and hydrophobic nanochannels. The flow rates in rough surface nanochannels are smaller than those of smooth surface walls due to the increase of drag resistance at the solid–fluid interface. The dependence between fluid slip and flow rate showed that the slip length increases approximately linearly with the flow rate for both the hydrophobic and hydrophilic surface nanochannels.

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Abbreviations

c fw :

parameter of surface wettability

F ij :

intermolecular force between molecules i and j

F e :

external driving force

h :

height of the parallel plates

h d :

height of the nanostrip at rough surface wall

k s :

spring constant

L x :

lateral cell dimension of nanochannel in the x direction

L y :

lateral cell dimension of nanochannel in the y direction

L z :

distance between the first layer of the solid walls

m f :

mass of fluid atom

m w :

mass of wall atom

p i :

momentum of fluid molecular i

Q :

volumetric flow rate

r c :

cutoff distance of Lennard-Jones potential

r ij :

distance between molecular i and j

r iw :

position of the wall atom i

U :

Lennard-Jones potential

u x :

fluid streaming velocity

u slip :

slip velocity at the solid–fluid interface

v i :

velocity of fluid molecular i

w :

width of the parallel plates

w d :

width of the nanostrip at rough surface wall

\(\dot{\gamma}\) :

fluid shear rate

δ:

slip length at the solid–fluid interface

ɛf :

energy parameter of fluid atom

θ:

contact angle of fluid atom and solid wall

μf :

fluid shear viscosity

ρ:

average fluid number density

σf :

diameter of fluid atom

σw :

diameter of wall atom

τ:

reduced unit of time

ϕharm :

harmonic restoring force of solid wall

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Authors and Affiliations

  1. Department of Mechanical Engineering, ChienKuo Technology University, Changhua, 50094, Taiwan, ROC

    S. C. Yang

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  1. S. C. Yang
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Yang, S.C. Effects of surface roughness and interface wettability on nanoscale flow in a nanochannel. Microfluid Nanofluid 2, 501–511 (2006). https://doi.org/10.1007/s10404-006-0096-5

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  • DOI: https://doi.org/10.1007/s10404-006-0096-5

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