Regularization of the slip length divergence in water nanoflows by inhomogeneities at the Angstrom scale

Marcello Sega,Mauro Sbragaglia,Sauro Succi,Luca Biferale

doi:10.1039/c3sm51508g

Abstract

We performed non-equilibrium Molecular Dynamics simulations of water flow in nano-channels with the aim of discriminating {\it static} from {\it dynamic} contributions of the solid surface to the slip length of the molecular flow. We show that the regularization of the slip length divergence at high shear rates, formerly attributed to the wall dynamics, is controlled instead by its static properties. Surprisingly, we find that atomic displacements at the Angstrom scale are sufficient to remove the divergence of the slip length and realize the no-slip condition. Since surface thermal fluctuations at room temperature are enough to generate these displacements, we argue that the no-slip condition for water can be achieved also for ideal surfaces, which do not present any surface roughness.

Highlights

We performed non-equilibrium Molecular Dynamics simulations of water flow in nano-channels with the aim of discriminating static from dynamic contributions of the solid surface to the slip length of the molecular flow
We show that the regularization of the slip length divergence at high shear rates, formerly attributed to the wall dynamics, is controlled instead by its static properties
Since surface thermal fluctuations at room temperature are enough to generate these displacements, we argue that the no-slip condition for water can be achieved for ideal surfaces, which do not present any surface roughness

Summary

The debate about slip length divergence

In the pioneering paper,[1] it was shown that the Navier slip boundary condition us 1⁄4 ‘sg_ can be regarded as the low-shear limit of a universal, nonlinear relation between slip velocity us, the slip length ‘s, and the local shear rate g_ that presents a divergence of the slip length upon approaching a critical value of the shear rate. The onset of divergence is observed at g_ x 1010 sÀ1, both for argon[1] and for water,[2] a value which has been accessible so far only to numerical simulations, the interest in the regime of ultrahigh shear rates is rapidly growing: commercial, industrial-grade xed-geometry uid processors like the micro uidizer (Micro uidics Corp, MA, USA) operate at shear rates exceeding 107 sÀ1 to achieve uniform particle size reduction in emulsions and dispersions, to create nanoencapsulations or to produce cell rupture. 43 concluded that heat and momentum transfer from the uid to the wall are responsible for the observed saturation of the slip length at high shear rates, as opposed to the divergent behavior which takes place using rigid walls This picture has been questioned in ref. We report numerical evidence that it is the presence of an even extremely tiny amount of disorder in the atomic positions at the surface, rather than wall exibility, which proves to be responsible for taming slip length divergence and making it shear-independent

Non-equilibrium Molecular Dynamics simulations of shear flow

The slip length divergence and its regularization

Findings

Conclusions