Heat Flux Density Evaluation in the Region of Contact Line of Drop on a Sapphire Surface Using Infrared Thermography Measurements

The paper presents a new tool “the method of IR transparent thick plate” that can be used to study the heat and mass transfer processes in the air-liquid-solid contact line area. Its distinctive feature as compared to the previously known methods is the solution of the initial-boundary problem for the heat conductivity equation, which in terms of mathematics is a correct problem. Currently, the heat and mass transfer processes in the area of the contact line are not completely understood because of its small size and a limited set of applied research methods. The challenges in modeling of contact line phenomena have to do with the fact that several physical effects such as evaporation, viscous flow, surface tension, thermocapillary stresses, London-van der Waals forces, disjoining pressure, nonequilibrium effects are coupled together and all significant in this highly localized region. This leads to difficulties in both mathematical modeling and design of experiments. The experimental part of the study includes the evaporation of a liquid drop on a sapphire substrate. The upper part of the sapphire glass is coated with a high heat-resistant black graphite paint (Graphit 33), which is a non-transparent for visual and IR-rays. Measurements of various physical, chemical and geometrical properties of this coating have been done by electron microscopy and other techniques. Trial experiments on the drop evaporation were carried out. The sapphire surface temperature fields after single drop deposition were obtained using the IR-scanner. The experimental local heat flux distribution at drop evaporation on the sapphire surface with two small local highs close to the contact line regions has been measured.