Liquid slip over gas nanofilms

Srinivasa B Ramisetti,Duncan A Lockerby,Jason M Reese,Matthew K Borg

doi:10.1103/physrevfluids.2.084003

Srinivasa B Ramisetti, Duncan A Lockerby + Show 2 more

Open Access

https://doi.org/10.1103/physrevfluids.2.084003

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Abstract

We propose the rarefied gas-cushion model (r-GCM), as an extended version of the gas-cushion model (GCM), to estimate the apparent slip of water flowing over a gas layer trapped at a solid surface. Nanobubbles or gas nanofilms may manifest rarefied gas effects, and the r-GCM incorporates kinetic boundary conditions for the gas component in the slip Knudsen regime. These enable an apparent hydrodynamic slip length to be calculated given the gas thickness, the Knudsen number and the bulk fluid viscosities. We assess the r-GCM through non-equilibrium molecular dynamics (NEMD) simulations of shear-driven liquid flow over an infinite gas nanofilm covering a solid surface, from the gas slip regime to the early transition regime, beyond which NEMD is computationally impractical. We find that, over the flow regimes examined, the r-GCM provides better predictions of the apparent liquid slip, and retrieves both the GCM and the free-molecular behaviour in the appropriate limits.

Highlights

A fundamental understanding of the flow of fluids over solid surfaces is important in many emerging technologies
The slip lengths predicted by our four nonequilibrium molecular dynamics (NEMD) simulations at steady state are compared with calculations from the gas-cushion model (GCM) [Eq (1)] and our proposed rarefied-gas-cushion model (r-GCM) [Eq (9)] in Table III, in the second, third, and fourth columns, respectively
We observe that the error in the slip length predictions of the GCM increases with Kn in the transition regime (∼10%–50% for these four cases), while our r-GCM error remains roughly constant at 4%–8%

Summary

INTRODUCTION

A fundamental understanding of the flow of fluids over solid surfaces is important in many emerging technologies. Nizkaya et al [8] used atomic force microscopy to confirm the GCM theoretical prediction by measuring the local slip length in the gas regions In their experiments the gas film thickness Yg = 1.9 μm is much larger than the gas mean free path λ (e.g., at standard atmospheric conditions, air molecules have λ = 68 nm [9]), which is given by λ = μg π ,. In this paper we propose a modification to the GCM to include thermodynamic nonequilibrium gas effects in any gas film that interfaces with the liquid-solid regions in a shear flow. We term this model the rarefied-gas-cushion model (r-GCM). Velocity slips at the two interfaces are modeled by the classical Maxwell slip boundary condition

RAREFIED GAS-CUSHION MODEL

MOLECULAR SIMULATIONS

RESULTS AND DISCUSSION

CONCLUSION