A new method for precise optical measurements of the sub-micron height of levitation of droplet clusters

Droplet clusters levitating over the locally heated water surface are considered a very promising phenomenon of microfluidics for the potential use of the droplets as unique microreactors for microbiological experiments. A new optical method is suggested for precise measurements of the sub-micron levitation height of single droplets. The method is based on the analysis of variable colors of the inter-ference halo around the droplet. For the first time, it is possible to measure the extremely low downward velocity of droplets, which grow due to steam condensation. This velocity was found to be 5-8 nm/s. The height of the levitation of various droplets just before their coalescence with a layer of water was also determined. Such measurements might be used for the verification of sophisticated models for the droplet levitation to be developed for a very thin layer of humid air under the droplet when the Knudsen effect should be taken into account.& COPY; 2023 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.