Detection of self-generated nanowaves on the interface of an evaporating sessile water droplet
Posted on 18.10.2019 - 17:06
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 numbers 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.
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Bhatt, Dhanush; Vaipully, Rahul; kharbanda, Bhavesh; Ranjan, Anand; R, Sulochana; Dharod, Viraj; et al. (2019): Detection of self-generated nanowaves on the interface of an evaporating sessile water droplet. The Optical Society. Collection. https://doi.org/10.6084/m9.figshare.c.4603292.v1
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