Study On Force Field And Molecular Dynamics Simulations Of Water Nanodroplets Diffusion On 2D Materials' Surface

Due to the excellent biocompatibility,graphene and phosphorene have exciting applications in biological detection,tumor treatment and drug carrier.In order to achieve further biomedical applications of graphene/phosphorene,it is important to understand their interaction with biomolecules/water.In this paper,the water-graphene system with mature force field is firstly studied.The molecular dynamics(MD)simulations is used to study the diffusion of water on the graphene with double-vacancy defects.Then,we explored the immature force field for water-phosphene system,and developed a new force field for the actual situation through the improvement of Dreiding force field.Finally,a further study to the diffusion of water nanodroplets on the phosphorene under external physical field is investigated.The main work done in this paper is as follows:(1)The diffusion behavior of water nanodroplets on the graphene with double-vacancy defects was analyzed by molecular dynamics(MD)simulations.We found that four equal-distance defects form a ‘quad-like defect' in which water droplets are completely restricted and cannot escape;when the defects are arranged equidistantly in a straight line,the water droplets cannot pass through the defect wall that composed by defects.We call these two limited modes as completely restricted diffusion and partially restricted diffusion,respectively.What's more interesting is that for partially restricted diffusion,a larger water droplet(including 810 water molecules)will be confined to a certain area after a period of time.However,a smaller water droplet(including 252 water molecules)will continue to spread.After that,the effects of the defect spacing and the size of water droplet on the limitations were studied.It was found that a wider spacing results a weaker limitation,in conjunction with a smaller water droplet.Then,the effects of tensile strain,temperature,and initial release height of the water nanodroplets on the limitation are discussed in terms of two different limited modes.Finally,a theory is used to explains why there are two different partial restricted diffusion modes on basis of different sizes of water droplet.This research will contribute to the development of water confinement and provide ideas for structural design.(2)We present an improved Dreiding force field for single layer black phosphorus(SLBP)obtained by first-principle calculations in conjunction with the particle swarm optimization algorithm and molecular dynamics(MD)simulations.The proposed Dreiding force field can describe material properties of the SLBP very well in comparison with first-principle calculations and the Stillinger–Weber potential,including Young's modulus,Poisson's ratio,shear modulus,bending stiffness and phonon spectrum.Through the improved Dreiding force field,the wetting of a water nanodroplet and the adsorption of a villin headpiece on SLBP under compressive deformation are also studied by MD simulations.The simulation results show that the microscopic contact angle increases with the level of compressive strain on the SLBP.Meanwhile,the compressive strain reduces disruption caused by SLBP to the structure of the villin headpiece.The proposed Dreiding force field shows great potential to describe the interaction between SLBP and water molecules.It can be further used to simulate the transport of water on SLBP,especially under mechanical deformation,and interactions between SLBP and biological systems.(3)Diffusion of water across surfaces generally involves motion on an uneven and vibrating but otherwise stationary substrate.Based on molecular dynamics simulations,we study the diffusion behavior of water nanodroplets on the compressed phosphorene at different temperatures.The compression is applied in three different ways: equal biaxial compression,uniaxial compression along armchair and zigzag directions.As the compressive strain increases,the diffusion coefficients are first reduced and then increased in all three cases at T = 300 K.When the compressive strain exceeds 1.6%,the diffusion coefficient grows the fastest for the case of equal biaxial compression.As the temperature increases,two different modes are observed at the lower strain level(?< 1.4%)and relatively higher strain level(? >1.4%).In the regime of lower level of strain,the diffusion coefficient exhibits an oscillating trend as the temperature increases.In the regime of higher level of strain,the diffusion coefficient increases linearly as the temperature increases.The fastest diffusion occurs under the equal biaxial compressive loading at the strain level of 2:4% and temperature T = 340 K.The different diffusion behaviors of water nanodroplets are found to be related to the surface morphologies of phosphorene under compression,as well as friction coefficient and diffusion energy barrier of water molecules.Our results show that compressive deformation of phosphorene and temperature are important to control the dynamics of water molecules on the phosphorene.The phenomena reported here enrich the knowledge of molecular mechanisms for nanofluidic systems,and may inspire more applications with phosphorene and other 2D materials.