Micro-mechanisms Responsible for Electrically-, Thermally-, and Mechanically- driven Flows of Continua

1. Electric charging in electrostatic atomization First part of the thesis is devoted to the study of the electrification mechanism of dielectric liquids, like hydrocarbon oils in applications of electrostatic atomization. Electrostatic atomization (EA) is an emerging technology with numerous applications for insulating liquids, like painting, spray coating, fuel injection to name a few. This technology is based on electric charge transfer to poorly conducting liquids, the mechanism of which however is not fully understood. Electrochemical reactions have been speculated as the charging mechanism in normal operation of electrostatic atomizers, however, there is no works studying them, as noted in the literature survey. We explore the similarities in the current-voltage characteristics and study the effect of electrode material on the charge transfer. We probed the nature of the electrode reactions and the resultant deposits on the electrodes were subjected to chemical analysis. The results proved that the electric charge transfer mechanism is of electrochemical nature, very much like in ordinary electrolytes. A novel method of measurement of electrical conductivity of oil is described. The method developed here is simple, straightforward, DC, and yet is sensitive for measurement of wide range of electrical conductivity values. The method developed has been compared with a commercial method available, albeit the present method can work in a much wider conductivity range. The resulting electrohydrodynamic flow was visualized using PIV which revealed toroidal Moffatt vortices at high applied voltages. Formation of such vortices was predicted theoretically before, but experimental observations are non-existant, as to our knowledge. 2. Evaporation-driven flow and pool boiling In the second part, interfacial phenomena of evaporative and boiling heat transfer are considered. We studied theoretically and experimentally the evaporation-driven surface flows in liquid layers of varying depths. Experimentally, we observed the temperature distribution using infrared thermography, which visualized the thermal Marangoni flows by means of hollow particles at the liquid surface. We study the effect of pool depth on the characteristics of boiling and the corresponding heat flux. In the case of small depth of the pool boiling, it is hypothesized that the behavior should reflect boiling in case of zero-gravity tests. In this regard, experiments were conducted to measure the heat flux and visualize the boiling patterns. We also studied the effect of polymer nano-texture and alcohol-based working fluids on boiling heat transfer. The polymer nano-texture was created on copper heater surface utilizing supersonic solution blowing technique. A significant improvement in boiling heat transfer was achieved with a hydrophobic nano-texture, and we explain the reason for that. 3. Rheological behavior of polymer fibers In this part, the aim is in the detailed understanding of polymer fiber deformation involving non-linear elasticity, plasticity and time-dependent characteristics: creep and viscoelasticity. A general tensorial phenomenological model incorporating all the above phenomena is developed. Tensile test and creep-recovery experiments are performed using Dynamic Mechanical Analyzer (DMA). Variety of polymer fibers is investigated and the model developed is applied to analyze the data. Furthermore, tests are conducted with yarned fibers and cellulose-based natural filaments. The applicability of the developed phenomenological model is demonstrated using the comparison with the experiments. The effect of humidity and temperature is investigated using nylon 6,6 and cotton fibers and the corresponding material parameters are established. Bicomponent fiber deformation is investigated and a preliminary theory explaining the observed peculiarity in the recovery process is presented.