Using Planar Laser Induced Fluorescence and Micro Particle Image Velocimetry to study the heating of a droplet with different tracers and schemes of attaching it on a holder

The Planar Laser Induced Fluorescence optical technique is used to measure the droplet temperature field, and the Micro Particle Image Velocimetry - the velocities of natural convection in the droplet. We focus on the convection, vortex formation centers, convection velocities, and the influence of convection on the temperature in different droplet sections. The most common tracer particles are used (Al2O3, TiO2, polyamide). Differences in the temperature fields of evaporating droplets with tracers and without them are analyzed. We compare the reliability of measuring droplet temperature using Planar Laser Induced Fluorescence and fast-response thermocouples. The concentrations of tracers are varied to establish the ranges in which the fields of temperature and convection velocities can be reliably measured in rapidly heated and evaporating droplets. We study the influence of typical holder materials on droplet heating rates. Metal rods, wires, and nichrome wires are used. The focus is on illustrating the influence of the holder type and material on the droplet temperature field, its heating and evaporation rates, as well as its lifetime. We consider typical schemes of droplet fixation on holders, as well as the main differences in the fields of temperature and convection velocities in different droplet sections. As a result, we have obtained the concentrations of Al2O3 and TiO2 tracers that make it possible to use both Planar Laser Induced Fluorescence and Micro Particle Image Velocimetry at the same time. Examples are given of particle agglomeration and photobleaching. The use of polyamide particles as tracers is found not feasible for evaluating the convection rates in droplets, because they are displaced from the deep layers of a droplet to the surface. It is established that if a Rhodamine B solution contains Al2O3 and TiO2 tracers, the fluorophore settles on those particles, which may degrade the accuracy of temperature measurement using Planar Laser Induced Fluorescence. The experimental findings are important to develop the fields of research into the conditions and characteristics of heating droplets, films, and liquid jets, using Planar Laser Induced Fluorescence and Micro Particle Image Velocimetry.