Suppression of Leidenfrost phenomenon of nanodroplets through an external electric field

When a droplet comes into contact with a hot sold surface, heat is conducted from the surface to the droplet, triggering phase change of the droplet. As the surface temperature increases, the droplet undergoes successively evaporation, nucleate boiling, and transition boiling. When the surface temperature increases further and exceeds a critical one, a vapor layer immediately generates between the droplet and surface, and separates the droplet from the surface, making the droplet depart from the surface. This phenomenon is commonly termed the Leidenfrost phenomenon and the critical temperature is referred as Leidenfrost point (LFP). The Leidenfrost phenomenon significantly reduces heat dissipation from a surface because heat is transported from the surface to a droplet only by heat conduction of the vapor film, so that the surface temperature abruptly increases, which may lead to surface burnout. Various methods have been developed to suppress the Leidenfrost phenomenon. Among these methods, electrostatic suppression of the Leidenfrost phenomenon by external electric fields exhibits many advantages, and hence is considered as a very promising method. Many efforts have been devoted to understanding the mechanisms behind electrostatic suppression and optimizing the parameters of electric fields. These studies focused on droplets with sizes ranging from a few millimeters to a few tens of microns. With the development of micro/nano technique, heat transfer enhancement by manipulation of micro/nanodroplets has attracted a great deal of attention. Thus, how to suppress the Leidenfrost phenomenon of micro/nanodroplets becomes an interesting problem. In this work, the Leidenfrost phenomenon of a nanoscale water droplet on gold plates with different wettabilities is first investigated via molecular dynamics simulations. The main emphasis is focused on how the surface wettability affects the LFP. Our simulations show that, with the same plate temperature of 700 K, only evaporation on the free surface of the droplet takes place on a hydrophilic surface, whereas the Leidenfrost phenomenon is triggered on a moderate wettability surface and a hydrophobic surface. Moreover, the triggering time is earlier on the hydrophobic surface. The further simulations demonstrate that the Leidenfrost phenomenon can be triggered on the hydrophilic surface by increasing the surface temperature to 1132 K, whereas it can also be suppressed on both the moderate wettability surface and the hydrophobic surface by decreasing the surface temperature. Therefore, it is concluded that the LFP increases as the increased surface hydrophilicity. By charging gold atoms in two specific region of the gold plate bottom, an electric field parallel to the surface is generated. This electric field is employed to suppress the Leidenfrost phenomenon. With the same plate temperature of 700 K, it is found that the Leidenfrost phenomenon is successfully suppressed on both the moderate wettability surface and the hydrophobic surface by the electric field, but a stronger electric field is required by the hydrophobic surface. The electric field enhances the interaction between water molecules and gold atoms, making the surface more wettable by the droplet. In other words, the electric field enhances the surface hydrophilicity, which is responsible for the electrostatic suppression of the Leidenfrost phenomenon.