Deformation hysteresis of a water nano-droplet in an electric field.

Electric field is an effective method to manipulate droplets in micro/nano-scale, and various physical phenomena have been found due to the interaction of electric field and fluid flow. In this study, we developed a molecular dynamic model to investigate the deforming behavior of a nano-droplet in a uniform electric field. The nano-droplet was initially confined between two plates and then wetted on the lower plate (i.e., substrate) until an equilibrium state, after that a uniform electric field in vertical direction was imposed to the system. Due to the electrical force, the droplet started to deform until achieving a new equilibrium state and the dynamic process is recorded. By comparing the equilibrium state under different electric field strength, we found a deformation hysteresis phenomenon, i.e., different deformations were obtained when increasing and decreasing the electric field. To be specific, a large electric field (E = 0.57 V ·nm^-1) is needed to stretch the nano-droplet to touch the upper plate, while a relatively lower field (E = 0.40 V ·nm^-1) is adequate to keep it contacting with the plate. Accompanied by the deformation hysteresis, a distribution hysteresis of the average dipole orientations of water molecules in the nano-droplet is also found. Such a hysteresis phenomenon is caused by the electrohydrodynamic interactions between droplet and plates, and the findings of this study could enhance our understanding of droplet deformation in an electric field.