# The Water Droplet Contact Line Probed with Multiwalled Carbon Nanotubes at the Air–Water Interface

## TL;DR

This paper explores how carbon nanotubes behave at the edge of evaporating water droplets, offering insights into droplet pinning and interface dynamics.

## Contribution

The study introduces the use of multiwalled carbon nanotubes to probe droplet contact line behavior during evaporation.

## Key findings

- MWNTs form chain structures at the air–water interface due to capillary interactions.
- Chain structures align perpendicular to the contact line before droplet retraction.
- Van der Waals interactions and capillary forces explain MWNT behavior at the interface.

## Abstract

Within the issue of sessile droplet evaporation, particularly
important
is the behavior of the triple phase contact line governed by pinning
phenomena. We demonstrate how pristine, insoluble multiwalled carbon
nanotubes (MWNT) can exhibit ordering phenomena at the air–water
interface, contribute to droplet pinning, and are also responsive
to the evolving shape of evaporating water droplets at the contact
line. The arc-discharge-synthesized MWNTs were of high quality, but
they were mixed with graphitic impurity particles in the 10–100
nm size ranges. The MWNTs were ordered into chain structures by capillary
interactions at the air–water interface. Moreover, we observed
how the chain structures regularly turned perpendicular to the contact
line a short time prior to the withdrawal of the strongly pinned contact
line, which we explain with the capillary force acting in a region
with nonconstant interface curvature. The MWNT chains thus offer a
unique way to probe the local behavior of the droplet. We modeled
the van der Waals interactions of MWNTs and graphitic impurities in
the vicinity of the contact line. According to these, the related
energies are large enough to explain issues related to the transfer
onto the air–water interface. Capillary effects can be qualitatively
explained by existing theories. As the MWNTs are strictly confined
to the air–water interface, these results are complementary
to the separate but closely related coffee ring effect.