Vapor Mapping in a Microscopic Space with a Scanning Nanoprobe Interferometer

Evaporation of a liquid inside a microscopic space plays a key role to drive various natural phenomena or chemical/ thermal processes in microfluidic systems. So far, fundamental studies in such fields have been indirect due to the lack of techniques geometrically accessible in microscopic spaces. Here, we report on a nanoprobe interferometer that can directly visualize vapor in 3D (dimensions) in a microscopic space. We first optimize the tip geometry of the nanoprobe to enhance its sensitivity just by analyzing the interference patterns during the nanoprobe growth in real time. Based on scanning with the nanoprobe interferometer, vapor concentration from a water meniscus in a glass microcapillary is experimentally mapped, which leads to the measurement of the local evaporation flux from the meniscus. The evaporation flux increases from the capillary center to the wall and decreases with the capillary diameter, mostly due to the Kelvin effect and strong evaporation at the three-phase contact line. Specifically, the flux at the center follows J(v(center)) proportional to e(-7.7x10-4d), in sharp contrast to the power-law decrease reported. Our nanotechnological methodology would pave the way to explore various questions associated with microscopic, geometrically confined evaporation dynamics which has been experimentally inaccessible so far.