Free surface profiles and thermal convection in electrostatically levitated droplets (Q1579922)

For the title problem, the authors present a boundary element solution for electric potential outside the droplet, the weighted residual formulation of free-surface balance equation which includes electrostatic stresses, surface tension and gravity, and finite element solutions for internal fluid flow and temperature distribution in electrostatically deformed droplets. Various materials and operating conditions are considered. The results show that the applied electrostatic field generates a normal stress distribution along the droplet surface, which, combined with the surface tension, causes the droplet to deform into an ellipsoidal shape in microgravity, and into a block shape with a flat lower side under terrestrial conditions. Laser heating induces a nonuniform temperature distribution in the droplet, which produces a recirculating convection in the droplet. For the cases studied, Marangoni convection is the predominant mode, and buoyancy effects are negligible. There is a higher temperature gradient and stronger Marangoni convection in droplets with higher melting points, which requires more laser power. More uniform laser heating may reduce the internal recirculating flow. During undercooling of the droplet with heating turned off, both temperature and fluid flow fields evolve in time, such that the temperature gradient and tangential velocities along the droplet surface subside in magnitude and reverse their directions.