Velocity profiles, couette flow simulations

Barrat and Bocquet [6] carried out the molecular dynamics simulation of Couette and Poiseuille flows. In Couette flow, the upper wall is moved with a cmistant velocity, and in Poiseuille flow an external force drives the flow. Sample results from molecular dynamics simulation are reproduced in Fig. 7. The application of no-slip boundary condition leads to the expected linear and parabolic profiles, respectively, for Couette and Poiseuille flows. However, the velocity profile obtained from molecular dynamics simulation shows a sudden change of velocity in the near-waU region indicating the slip flow. The velocity profile for Couette flow away from the solid surface is linear with different slope than that of the no-slip case. The velocity for slip flow case is higher than that observed in the no-slip case for Poiseuille flow. For both Couette and Poiseuille flows, the partial slip boundary condition at the wall predict similar bulk flow as that observed by molecular dynamics simulation. Some discrepancy in the velocity profile is observed in the near-wall region. 

The slip length also depends on the shear rate imposed on the fluid particle. Thompson and Troian [8] have reported the molecular dynamics simulation of Couette flow at different shear rates. At lower shear rate, the velocity profile follows the no-slip boundary condition. The slip length increases with increase in shear rate. The critical shear rate for slip is very high for simple liquids, i.e., 10 s for water, indicating that slip flow can be achieved experimentally in very small devices at very high speeds. Experiments performed with the SEA and AFM have also showed shear dependence slip in the hydrodynamic force measurements. 

Boundary Slip of Liquids, Fig. 7 Velocity profile for (a) Couette and (b) Poiseuille flow comparison between molecular dynamics simulation, no-slip boimdary condition, and partial slip boundary condition... 

Barrat and Bocquet [6] reported slip in Couette and Poiseuille flows using molecular dynamics simulation (Fig. 7). Tretheway and Meinhart [4] reported micron-resolution velocity profile in hydrophilic and hydrophobic microchannels of cross section 30 x 300 pm using the p-PIV technique (Fig. 5a). Their results showed significant fluid velocity near a hydrophobic (octadecyltrichlorosilane or... 

Figure 9 Molecular dynamics simulation of a Lennard-Jones, bead-spring model, (a) Slip length, 8, as a function of the strength, [ an, of attraction between a hard, corrugated substrate and liquid for temperature, kgT/ [ =. 2. The solid line with circles is obtained from the Couette and Poiseuille profiles (NEMO) according toeqn [37], whereas the dashed line with squares, from the Green-Kubo (GK) relation, eqn [39]. The curve marks the behavior 1/ [ jii in accord with eqn [40]. The inset illustrates the velocity profiles of the Couette and Poiseuille flows, from which the slip length has been estimated for [mii = 0.6, measured in units of the Lennard-Jones parameter, [. Adapted from Servantie, J. Muller, M. Phys. Rev. Lett. 2008, 101,...

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