Investigation On Evaporation Process And Deformation Characteristics Of A Charged Droplet On The Solid Surface

Evaporation process of a droplet on the solid surface is an important phenomenon of heat and mass transfer which is widespread in industrial field. The enhancement of the heat transfer in the evaporation process could be helpful to overcome the diffusion limitation and improve the evaporation rate by applying an external electric field. The prospect of employment looks very promising in various industrial applications such as cooling in space or improving the uniformity of nanoparticle deposition. There exists complex phenomenon such as deformation, contraction, vibration and so on during the evaporation process of charged drops on the substrate. The study of the deformation characteristic and evaporation mode is very important for intensive research on the evaporation process under an external electric field.Most of these investigations have focused on the complexity of the physical dynamics, such as the influence of the substrate on the drop wettability, the evaporation flux at the interface and the triple line, the influence of substrate temperature on the evaporation rate. A detailed research on the deformation characteristics and the dynamic behavior of a charged droplet during the evaporation process are still lacking. In this thesis, basic research has been carried out on the evaporation process and electro hydrodynamic characteristics of a charged droplet on the solid surface by applying visualization system. The main research work is as follows.Firstly, visualization has been given on the evaporation process of a sessile droplet on the substrate under the applied electric field. The evaporation parameters such as solid-liquid contact angle and droplet height have been captured by high-speed microscopy and normalized by computer. The change of parameters of a charged droplet during the evaporation process is obtained. By combining high-speed microscopy with mini-PIV technology, the internal flow of a droplet is analyzed. Based on experimental data, the equation is deduced of the charge limit on the surface of the droplet.Secondly, the evaporation process of a sessile droplet under the applied electric field and temperature has been visualized. By coupling the influence of the temperature, the evaporation process of a charged water droplet mixing nanoparticles with high thermal and conductive has been studied by analyzing the change of mass, heat flux, evaporation time and so on. It is effective for promoting evaporation of a sessile droplet under temperature and electric field.Finally, the deformation characteristics of a charged droplet during the evaporation process have been studied. It is defined as two modes: shake evaporation mode and wave evaporation mode have been discussed. By comparing shaking margin and frequency, the differences between the two modes have been discussed. The charge distribution and deformation on the droplet surface of these two modes have been analyzed which gives a theoretical foundation for the future research.