Counter-intuitive evaporation in nanofluids droplets due to stick-slip nature

TL;DR

This study reveals that nanoparticle-induced contact line dynamics, rather than thermal conductivity, primarily influence nanofluid droplet evaporation rates, challenging conventional assumptions.

Contribution

It demonstrates that contact line behavior, not thermal conductivity, governs evaporation rates in nanofluids, with experimental evidence showing different nanoparticle effects.

Findings

Pinned contact lines lead to faster evaporation despite lower thermal conductivity.

Nanoparticle size and type significantly alter contact line dynamics and thermal patterns.

Counter-intuitive evaporation behavior is linked to contact line stick-slip phenomena.

Abstract

We experimentally investigate the evaporation characteristics of a sessile ethanol droplet containing Al$_2$O$_3$ and Cu nanoparticles of sizes 25 nm and 75 nm on a heated substrate using shadowgraphy and infrared imaging techniques. Our results demonstrate that the droplet contact line dynamics resulting from the presence of various nanoparticles plays a dominant role in the evaporation process. This is in contrast to the widely-held assumption that the enhanced evaporation rate observed in sessile nanofluid droplets is due to the higher thermal conductivity of the added nanoparticles. We observe that even though the thermal conductivity of Al$_2$O$_3$ is an order of magnitude lower than that of Cu, droplets containing 25 nm-sized Al$_2$O$_3$ exhibit pinned contact line dynamics and evaporate much more rapidly than droplets containing Cu nanoparticles of both sizes and 75 nm Al$_2$O$_3$ nanoparticles that exhibit stick-slip behaviour. We also found that the droplets with different nanoparticles display distinct thermal patterns due to the difference in contact line behaviour, which alters the heat transfer inside the droplets. We establish this counter-intuitive observation by analysing the temporal variations of the perimeter, free surface area, and deposition patterns on the substrate.