Tensorial slip of super-hydrophobic channels

TL;DR

This paper generalizes the tensorial slip boundary condition to channels of any thickness, developing a semi-analytical theory validated by simulations, which helps understand effective slip in super-hydrophobic channels.

Contribution

It extends the tensorial slip boundary condition to arbitrary channel thicknesses and provides a semi-analytical model validated by simulations.

Findings

Effective slip length tensor depends on channel gap and is a global property.

Semi-analytical theory matches lattice Boltzmann simulation results.

Method enables extraction of slip tensors from global measurements.

Abstract

We describe a generalization of the tensorial slip boundary condition, originally justified for a thick (compared to texture period) channel, to any channel thickness. The eigenvalues of the effective slip length tensor, however, in general case become dependent on the gap and cannot be viewed as a local property of the surface, being a global characteristic of the channel. To illustrate the use of the tensor formalism we develop a semi-analytical theory of an effective slip in a parallel-plate channel with one super-hydrophobic striped and one hydrophilic surface. Our approach is valid for any local slip at the gas sectors and an arbitrary distance between the plates, ranging from a thick to a thin channel. We then present results of lattice Boltzmann simulations to validate the analysis. Our results may be useful for extracting effective slip tensors from global measurements, such as the permeability of a channel, in experiments or simulations.