Direct measurements of hydrophobic slippage using double-focus fluorescence cross-correlation

TL;DR

This study introduces a high-precision fluorescence cross-correlation method to measure flow velocity profiles in microchannels, revealing true hydrophobic slip lengths of tens of nanometers unaffected by electrolyte concentration or shear rate.

Contribution

The paper presents a novel measurement technique that accurately distinguishes true slip from dispersion effects, providing direct evidence of hydrophobic slip lengths in microfluidic flows.

Findings

Velocity profiles near walls show apparent slip due to Taylor dispersion.

True slip length is on the order of tens of nanometers.

Slip length is independent of electrolyte concentration and shear rate.

Abstract

We report results of direct measurements of velocity profiles in a microchannel with hydrophobic and hydrophilic walls, using a new high precision method of double-focus spacial fluorescence cross-correlation under a confocal microscope. In the vicinity of both walls the measured velocity profiles do not turn to zero by giving a plateau of constant velocity. This apparent slip is proven to be due to a Taylor dispersion, an augmented by shear diffusion of nanotracers in the direction of flow. Comparing the velocity profiles near the hydrophobic and hydrophilic walls for various conditions shows that there is a true slip length due to hydrophobicity. This length, of the order of several tens of nanometers, is independent on electrolyte concentration and shear rate.