Molecular Tagging Techniques for Micro-Flow and Micro-Scale Heat Transfer Studies

We report recent progresses made in development of novel molecule-based flow diagnostic techniques, named as Molecular Tagging techniques, to achieve simultaneous measurements of multiple important flow variables (such as flow velocity and temperature) for micro-flows and micro-scale heat transfer studies. Instead of using tiny particles, specially-designed phosphorescent molecules, which can be turned into long-lasting glowing molecules upon excitation by photons of appropriate wavelength, are used as tracers for both velocity and temperature measurements. A pulsed laser is used to “tag” the tracer molecules in the regions of interest, and the movements of the tagged molecules are imaged at two successive times within the photoluminescence lifetime of the tracer molecules. The measured Lagrangian displacement of the tagged molecules between the two image acquisitions provides the estimate of the fluid velocity vector. The simultaneous temperature measurement is achieved by taking advantage of the temperature dependence of phosphorescence lifetime, which is estimated from the intensity ratio of the tagged molecules in the two images. The implementation and application of the MTV&T technique are demonstrated by conducting simultaneous velocity and temperature measurements to qunatify the transient behavior of electroosmotic flow (EOF) inside a microchannel and to reveal the unsteady heat transfer, mass transfer and phase changing process inside micro-sized water droplets pertinent to wind turbine icing phenomena.