Wetting Behaviors Of Sessile Water Nano-Droplets Using Molecular Dynamics Simulation

The cooling problems of micro/nano electromechanical systems (MEMS/NEMS) are related to wetting properties of micro/nano droplets. Inspired by the super hydrophobic in nature, the influence of rough nano-structure to water droplet wetting properties has become study hot in Bionics, MEMS, Surface Chemistry, Atomic Physics and Nanometer Materials and so on. Theoretically, the wetting properties of micro-nano droplets can use experimental method and computer simulation method to determine. But for nano-droplets, the wetting properties are difficult to study with experience method. With the rapid development of computer technology, it becomes possible to calculate the wetting properties of nano-droplets with the molecular dynamics simulation. In this article, Molecular Dynamics simulation is applied to research wetting behaviors of water nano-droplets on smooth surfaces and different rough surfaces. In addition, we also researched contact angel hysteresis preliminary.Simulation results show that with surface potential energy decreases, the contact angles increase linearly. When the surface potential energy is 1.674kJ/mol, the contact angel is about 90°.With temperatures increase, the vapor-liquid interface thicknesses increase gradually. The contact angles increase with temperatures increase for the hydrophobic surfaces, the contact angels have no obvious changes with temperatures increase for the neutral surfaces, and the contact angles decrease with temperatures increase for the hydrophilic surfaces. When the surfaces are in the same roughness and phase area fraction, the water droplet contact angels have little change in fencing form surfaces and square column matrix form surfaces. For hydrophobic surface, with the increase of roughness, when θt＞θw,θt＞θo, the droplets are in the Cassie contact state; whenθc=θw,θt=θo, the droplets are in the critical contact states; when θt＜θw,θt＜θo, the droplets are in a stable state of Cassie contact. With the increase of phase area fraction, droplets changed from Wenzel contact state to Cassie contact state gradually.For the neutral surfaces, droplet contact angles and contact states have little change.For hydrophilic surfaces the surfaces become more hydrophilic with the increase of roughness and phase area fraction. What’s more, Simulation results show that the droplet Contact Angle Hysteresis increases gradually with the increase of surfaces potential energy. For the rough surfaces, the droplet Contact Angle Hysteresis increases gradually with the increase of surfaces roughness. And the square array form surfaces have a bigger fixation force to water droplets than fencing form surfaces. As a result, it has a stronger contact angel hysteresis.