O water vapor uptake from air into sessile heavy water droplets*

At the surface of a liquid, the liquid molecules escape and enter the encompassing air as a gas, and the gas molecules join the liquid concurrently. Here we report the neutron imaging of a sessile heavy water (D₂O) droplet that absorbs light water (H₂O) vapor from the ambient air. A quantitative image analysis utilizing the Beer-Lambert law enables us to measure the attenuation coefficient of the specimen and estimate H₂O content within the droplet. We double-check this result with a quantitative Fourier-transform infrared spectroscopy and conclude that small D₂O droplets with a large surface-to-volume ratio can be very hygroscopic, e.g., the 15 percent of a D₂O droplet of 10 microliters can be replaced with H₂O in 10 minutes.