Diffusion of water across surfaces generally involves motion on an uneven or defective but otherwise stationary substrate. Studying the diffusion of water nanodroplets on the surface of such a substrate is useful for the development and progress of nano-fluidic systems. In this study, the diffusion of water nanodroplets on double-vacancy graphene is investigated based on molecular dynamics simulations. We find that defects have a constraining effect on water nanodroplets. According to the defect distribution in conjunction with the range of water nanodroplet diffusion, the constraining effects can be differentiated as fully constrained or partially constrained. Subsequently, the impact on the constraining effects from water nanodroplet size and defect distance is studied. Tensile deformation and temperature are considered as two possible ways to break the constraining effects. The results show that the constraining effects from defects on water nanodroplets can be broken when biaxial tensile strain reaches $0.6\%$ for both cases, but temperature has no obvious effect for partially constrained conditions. The differently constrained behaviors of water nanodroplets are found to be related to the vibration amplitude of the carbon atoms. Our results reveal that tensile deformation of graphene is more helpful for controlling the diffusion of water nanodroplets on double-vacancy graphene. The phenomena reported here can enrich the knowledge of molecular mechanisms for nanofluidic systems, and may inspire more applications with structural design for graphene and other 2D materials.

水在表面上的扩散通常涉及在不平坦或有缺陷但静止的基材上的运动。研究水纳米液滴在这种基底表面上的扩散对于纳米流体系统的发展和进步是有用的。在这项研究中，基于分子动力学模拟研究了水纳米液滴在双空位石墨烯上的扩散。我们发现缺陷对水纳米液滴具有约束作用。根据缺陷分布结合水纳米液滴扩散的范围，约束效应可分为完全约束或部分约束。随后，研究了水纳米液滴尺寸和缺陷距离对约束效果的影响。拉伸变形和温度被认为是打破约束效应的两种可能方式。结果表明，当双轴拉伸应变达到$0.6\%$对于这两种情况，但温度对部分约束条件没有明显影响。发现水纳米液滴的不同约束行为与碳原子的振动幅度有关。我们的结果表明，石墨烯的拉伸变形更有利于控制水纳米液滴在双空位石墨烯上的扩散。这里报道的现象可以丰富纳米流体系统分子机制的知识，并可能通过石墨烯和其他二维材料的结构设计激发更多应用。