High-resolution neutron imaging reveals kinetics of water vapor uptake into a sessile water droplet

Abstract

The spatiotemporal distribution of multiple components and phases governs their evaporation and condensation at the liquid-vapor interface. However, in situ methods to characterize the distribution remain challenging, despite the significance of understanding the ubiquitous mass transport phenomena. Here, we introduce 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, we perform 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. Our observations are consistent with ex situ Fourier transform infrared spectroscopy measurements and our diffusion-based numerical model. Our 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.