# Detection of self-generated nanowaves on the interface of an evaporating sessile water droplet

## TL;DR

This study employs optical trapping to detect nanometer-scale oscillations and wave-trains on evaporating water droplet interfaces, revealing new insights into surface dynamics during evaporation.

## Contribution

It introduces a novel optical trapping method to measure nanometer-scale interface oscillations and observes hydrothermal wave-trains in water, which were previously undetected.

## Key findings

- Detected 500 nm amplitude surface oscillations during evaporation.
- Observed traveling hydrothermal wave-trains on water droplet surface.
- Identified forces exerted by waves along the interface.

## Abstract

Evaporating sessile droplets have been known to exhibit oscillations on the air-liquid interface. These are generally over millimeter scales. Using a novel approach, we are able to measure surface height changes of 500 nm amplitude using optical trapping of a set of microscopic particles at the interface, particularly when the vertical thickness of the droplet reduces to less than 50 $\mu$m. We find that at the later stages of the droplet evaporation, particularly when the convection currents become large, the top air-water interface starts to spontaneously oscillate vertically as a function of time in consistency with predictions. We also detect travelling wave trains moving in the azimuthal direction of the drop surface which are consistent with hydrothermal waves at a different combination of Reynolds, Prandtl and Evaporation than previously observed. This is the first time that wave-trains have been observed in water, being extremely challenging to detect both interferometrically and with infra-red cameras. We also find that such waves apply a force parallel to the interface along the propagation direction.