电场抑制纳米液滴的Leidenfrost现象

Suppression of Leidenfrost phenomenon of nanodroplets through an external electric field

Leidenfrost现象是指液滴与过热表面接触时,快速汽化的蒸汽层将液滴与表面隔离,使液滴在表面悬浮的现象.为揭示表面润湿性对纳米液滴Leidenfrost现象的影响机制并实现纳米液滴的强化换热,本文采用分子动力学模拟研究了纳米液滴在不同润湿性表面上的Leidenfrost现象.结果表明,纳米液滴的Leidenfrost温度与表面润湿性密切相关,即表面越亲水, Leidenfrost温度越高.本文进一步研究了电场作用下纳米液滴在不同润湿性表面上的相变换热过程,发现施加电场可有效抑制Leidenfrost现象,电场增加液滴与表面的相互作用力是其抑制Leidenfrost现象的机理.

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.