Nanoscience of Fluid/Material Interface

At micro and nano scale, the fluid flow over a surface presents inhomogeneities near the solid boundaries leading to the breakdown of the no-slip assumption and the occurrence of slippage effects. Quantifying the slip’s magnitude is essential for measuring, either experimentally or computationally, accurate flow characteristics such as shear rates and velocity profiles. The slip’s strength is usually expressed through a proportionality coefficient, called slip length, representing the extrapolated distance from the wall to the point where the tangential velocity component is equal to zero. Although the parameters affecting the generation of slip at the solid-liquid interface are not explicitly known, the following are considered among the main contributing factor: surface roughness, fluid properties, wettability, shear rate, and surface structure. Experimental studies of nanotransport phenomena and slippage effects present significant challenges primarily associated with the manufacturing difficulties of controlling the atomistic roughness and measuring accuracy of physical quantities at nanoscales. Due to the above reasons, atomistic numerical modeling is employed. We have performed a number of molecular dynamics (MD) aimed at shedding light on the mechanisms of momentum and thermal transport across solid-liquid interfaces and advancing our understanding regarding sli. We have identified surface stiffness as an important factor for the slip process that can contribute toward either slip or stick conditions. In particular, we have studied the combined effects of surface stiffness κ and wall particles’ mass mw on the slip length. Elastic spring potentials are employed to simulate the thermal solid walls and model the surface stiffness κ. We have found that for cases with variable wall mass the relation of slip length and thermal oscillating frequencies can be approximated by a “master” curve according to which the length initially increases, then approaches a peak value, and afterwards is reduced toward an asymptotic value.

"Master" curve describing the variation of the slip length as a fucntion of solid particles' oscillating frequency. The inset shows how the values Ls,max and the square root of (k/mw)max vary with the mass of the solid particles.

Other problems of fluid/material interface we are interested in include flows around and inside carbon nanotubes.

Flow of a liquid (simple Lennard-Jones potential) around single-walled carbon nanotubes
Flow of a liquid (simple Lennard-Jones potential) around single-walled carbon nanotubes
References
  • M. Frank, N. Asproulis, D. Drikakis, Crystal-like heat transfer of liquids in nanochannels, Physical Review Letters, under review, 2014.
  • M. Frank, N. Asproulis, D. Drikakis, Density effects on the ballistic heat transfer of confined liquids, Physical Review E, under review, 2014.
  • M. Kio, M. Frank, N. Asproulis, D. Drikakis, L. Könözsy, Wall-mass effects on thermal resistance at solid-liquid interface, under review, 2014.
  • N. Asproulis, D. Drikakis, Wall mass effects on hydrodynamic boundary slip, Physical Review E, 84, 031504, 2011.
  • N. Asproulis, D. Drikakis, Boundary slip dependency on surface stiffness, Physical Review E, 81, 061503, 2010.