# Drop Friction on Textured Lubricant-Coated Surfaces

## TL;DR

This study explores how liquid droplets interact with textured lubricant-coated surfaces, revealing how pillar height and density affect drop friction.

## Contribution

The study provides new insights into the presence of a lubricant film on tall dense pillars, contradicting previous findings.

## Key findings

- Drop friction on tall dense pillars is comparable to short pillars due to a lubricant film.
- The critical pillar density for the friction transition is approximately 50%.
- Friction on microholes and micropillars is similar when solid fraction is the same.

## Abstract

Understanding drop friction on textured surfaces has
implications
in microfluidics and lab-on-a-chip devices. In this work, we investigated
the drop friction on lubricant-coated pillars by systematically varying
pillar height and density. First, we measured the friction force on
a moving drop using a cantilever force sensor that has ±0.1 μN
sensitivity. This measurement shows that drop friction on tall dense
pillars is comparable to drop friction on short pillars, a significant
result that suggests the presence of a Landau–Levich–Derjaguin
(LLD) film underneath the moving drop. Second, we validated the force
measurement by estimating the lubricant layer thickness by using white-light
interferometry. Third, we visualized the lubricant film underneath
the moving drop using reflection interference contrast microscopy.
The three independent diagnostic tools and measurement techniques
complement each other and reaffirm that drops oleoplane on tall dense
pillars, while they graze over the pillar tops in tall sparse pillars.
The critical density that forces this transition to drop friction
is ≈50%. Furthermore, the experimental results show that friction
on microholes and micropillars is comparable when the solid fraction
is the same. The results reported in this study contradict past studies
that reported the absence of an oil layer on tall pillars. Besides
improving current understanding, the insights gained from this work
provide design guidelines for turning drop friction on–off
on demand for microfluidics applications.