Study of Jumping Droplets with Electrostatic Effects on Biphilic Surfaces for Thermal Energy Storage Systems

A thermal diode, a thermal device to control heat transfer in different directions, is a promising technology in thermal energy storage systems. Among various types of thermal diodes, phase-change thermal diodes show the best thermal rectification performance due to the large latent heat transfer of the working fluid during the phasechange process. In this study, an advanced phase-change thermal diode composed of a novel biphilic surface (i.e. hydrophilic nano-sites coated on a superhydrophobic surface to provide preferable nucleation sites) and superhydrophilic surface is developed and studied. To further enhance the thermal rectification performance of thermal diodes for thermal energy storage systems, an electrical voltage is applied between two parallel surfaces (i.e. the biphilic surface and superhydrophilic surface). However, a research gap exists in the study of jumping droplets with electrostatic effects on the biphilic surface adopted in thermal diodes. Therefore, this study aims to investigate both the effects of biphilic surfaces and an electric field on jumping droplet height in thermal diodes for rectification improvement. Our experimental results show that under an electric field of 50 kV/m, the droplet jumping height on the biphilic surface is improved by about 110% compared with the condition without electric fields. Additionally, in the biphilic thermal diode under influence from the electric field, the thermal rectification performance is also increased by approximately 9 times compared to the biphilic thermal diode operated without electric fields, which can potentially be adopted in thermal energy storage systems to enhance energy storage efficiency.