Simultaneous Measurements of Droplet Size and Transient Temperature Within Surface Water Droplets

A lifetime-based molecular tagging thermometry technique was developed for achieving simultaneous measurements of droplet size and transient temperature distribution within a small water droplet over a solid surface. For molecular tagging thermometry measurements, a pulsed laser is used to tag phosphorescent 1-BrNp M -CD ROH molecules premixed within a small water droplet. Long-lived laser-induced phosphorescence is imaged at two successive times after the same laser excitation pulse. While the size of the water droplet (in terms of volume, height, contact area, and contact angle of the water droplet) can be determined instantaneously from the acquired phosphorescence images, the transient temperature measurement is achieved by taking advantage of the temperature dependence of phosphorescence lifetime, which is estimated from the intensity ratio of the acquired phosphorescence image pair. The feasibility and implementation of the lifetime-based molecular tagging thermometry technique were demonstrated by conducting simultaneous measurements of droplet size and spatially and temporally resolved temperature distribution inside a convectively-cooled microsized water droplet over an aluminum test plate to quantify the unsteady mass and heat transfer processes within the small water droplet to elucidate underlying physics to improve our understanding aboutmicrophysical phenomena associatedwith aircraft icing studies.