Effects Of Surface Microstructure On Wetting And Adhesion Of Silicon Substrates By Molecular Dynamics Study

Amazing natural phenomena provide many inspirations to researchers. For example, the lotus effect is found to be as a result of the super-hydrophobic leaf surface. And the special micro/nano protrusions are supposed to have a close relationship with the super-hydrophobic properties. For another example, gecko can walk on the ceiling due to the strong adhesive force on the sole of its feet. And the tiny bristles on the sole are believed related to its special adhesive properties. Wetting and adhesion are becoming unavoidable factors in the manufacturing with the development of nano-machine. So investigation about the effects of surface micro-structure on wetting and adhesion of silicon substrates is helpful to understand the mechanism of wetting and adhesion, and is also important to manufacture nano-machines.Molecular dynamics simulations are performed to establish a model of wetting silicon surface at atomic scale. The spreading direction of water droplets on surfaces with three different micro-structures is discussed, together with the influence of the size of micro-structures on the surface wetting. The results show that there is a critical width value for different micro-structures. A hydrophobic surface is formed when the micro-structure width is smaller than its critical size. Otherwise, it is hydrophilic. When a surface is hydrophobic, a sphere shape of water droplets is approximately formed in a top view after it is stable. When a surface is hydrophilic, the micro-droplet spreads anisotropically. Moreover, different micro-structure shapes lead to different special steady shapes of the micro-dropletMolecular dynamics simulation is also used to study the effects of micro-structures on the surface adhesion. The influence of the size and shape of micro-structure on the surface adhesion is discussed, while the silicon surface is covered with and without a layer of water film. The results show that the maximum adhesive force between the tip and substrate decreases with micro-structure on the substrate surface. The influence of the micro-structure is related to both its size and its shape. And the maximum adhesive force is mainly affected by the size variation of convex-shape micro-structures. When there is a layer of water film on the surface, the maximum adhesive force decreases significantly.The surface wetting and adhesion of micro-structures with different sizes and shapes on the surface is discussed in this study, and the underlying mechanisms are explored. The study provides theoretical basis for improving the wetting and adhesion problem in nano mechanical manufacturing.