More Reliable Measurements of the Slip Length with the Atomic Force Microscope

TL;DR

This paper enhances the data analysis algorithm for atomic force microscopy to more accurately measure slip lengths at solid-liquid interfaces, revealing smaller slip lengths than previously reported.

Contribution

It introduces a refined non-linear analysis method accounting for photo-diode non-linearity, leading to more reliable slip length measurements.

Findings

Slip lengths are approximately 2-3 nm for all tested surfaces.

Previous analyses overestimated slip lengths by a factor of 4-15.

Enhanced algorithm reduces measurement uncertainties to about 1 nm.

Abstract

Further improvements are made to the non-linear data analysis algorithm for the atomic force microscope [P. Attard, arXiv:1212.3019v2 (2012)]. The algorithm is required when there is curvature in the compliance region due to photo-diode non-linearity. Results are obtained for the hydrodynamic drainage force, for three surfaces: hydrophilic silica (symmetric, Si-Si), hydrophobic dichlorodimethylsilane (symmetric, DCDMS-DCDMS), and hydrophobic octadecyltrichlorosilane (asymmetric, Si-OTS). The drainage force was measured in the viscous liquid di-n-octylphthalate. The slip-lengths are found to be 3nm for Si, 2nm for DCDMS, and 2nm for OTS, with an uncertainty on the order of a nanometer. These slip lengths are a factor of 4--15 times smaller than those obtained from previous analysis of the same raw data [L. Zhu et al., Langmuir, 27, 6712 (2011). Ibid, 28, 7768 (2012)].