High-resolution Neutron Imaging Reveals Kinetics of Water Vapor Uptake into Sessile Water Droplet

A research team, led by Professor Joonwoo Jeong in the Department of Physics at UNIST has announced that they have succeeded in observing how, under ambient conditions, a 7.7 μL sessile droplet of pure D2O evaporates while it absorbs ambient H2O vapor.

Understanding evaporation and condensation at a liquid-vapor interface is of vital importance. The spatiotemporal distribution of multiple components and phases governs their evaporation and condensation at the liquid-vapor interface. However, the use of in situ methods to characterize the distribution is still a challenge, despite the significance of understanding the ubiquitous mass transport phenomena, according to the research team.

Figure 1

Figure 1. The schematic of neutron imaging for the sessile droplet and the neutron imaging of the evaporating sessile droplets of H2O and D2O.

In this study, the research team introduced high-resolution neutron imaging as a versatile method to quantify the composition of a sessile droplet in situ, under evaporation and condensation. To prove the concept, they performed a neutron transmittance analysis of a sessile heavy water (D2O) droplet and measure the fraction change of H2O to D2O by the sorption of ambient H2O vapor during the evaporation. The ex situ Fourier-transform infrared (FTIR) spectroscopy results and their diffusion-based numerical model of an evaporating multi-component droplet supported their experimental observations using neutrons, noted the research team.

Their results demonstrate that, with deuterated components having a physicochemical similarity with their hydrogenated counterparts, high-resolution neutron imaging can trace composition changes in nonequilibrium phenomena, such as evaporation and condensation.

Figure 2

Figure 2. Neutron imaging of an evaporating sessile D2O droplet and the estimation of the attenuation coefficient from the quantitative analysis of neutron transmittance

“Our work is a proof of concept that high-resolution neutron imaging can reveal how the distribution of a component of interest changes in nonequilibrium phenomena with multiple components involved, such as the evaporation and condensation of a sessile droplet of a liquid mixture, e.g., water and ethanol,” noted the research team. “Neutron imaging can provide key information to understand the complex phenomena by tracing a crucial deuterated component, the physicochemical properties of a deuterated chemical are often comparable with its hydrogenated counterpart.”

Meanwhile, all neutron-imaging experiments have been performed, using the high-resolution neutron-imaging setup, called a neutron microscope at the Swiss spallation neutron source SINQ, Paul Scherrer Institute, Villigen, Switzerland.

This study was made available online in the May 2021 issue of Matter, a sister journal to Cell, ahead of its final publication on June 2, 2021. It has been supported through the Basic Research Laboratory (BRL) by the Ministry of Science and ICT (MSIT) and the National Research Foundation of Korea (NRF). It has also been financially supported by the IBS Center for Soft and Living Matter and the 2018 Research Fund of UNIST.

Journal Reference

Jae Kwan Im, Leekyo Jeong, Jan Crha, et al., “High-resolution neutron imaging reveals kinetics of water vapor uptake into a sessile water droplet,” Matter, (2021).

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