Study On Molecular Dynamics Simulation Of Nanoparticle Preparation And Microflow

Molecular dynamics is the science of simulating the motions of a system of particles. The essential elements for a molecular dynamics simulation are knowledge of (1) the interaction potential (i.e., potential energy) for the particles, from which the forces can be calculated, and (2) the equations of motion governing the dynamics of the particles. The algorithm numerically integrates the equations of motion to get instantaneous position and velocity vectors of each particle, producing a simulated trajectory in phase space. At last we extract the macroscopic parameters of concern by statistical average.Microfluidics is a new technology developing very fast since the 1990's. As one new crossed science, it consists of analytical chemistry, MEMS, computer science, electronics, material science, biology, medicine and so on. Aiming to the problems of nanoparticle sinter and collision lying in fabrication process of nickel electrode on microfluidic chip and the heat transfer process through microflow, we took Molecular Dynamics method to simulate.Firstly we review main techniques of molecular dynamics method and computer realization of them partly. Secondly the frame of basic molecular dynamics simulation is constructed and verified by one simple case. Then we concentrate on the two applications of molecular dynamics method: research on preparation, sinter and collision of Ni nanoparticle; research on fluid flow and heat transfer aiming at different channel wall conditions.Molecular dynamics simulation of Ni nanoparticle's preparation, sinter and collision contains the following research contents and conclusions: 1. Simulation of Ni nanoparticle preparation:We simulated relaxation process of Ni nanoparticle with different sizes and analyzed the influence of particle size and relaxation temperature on radial distribution function (RDF), atom-potential distribution and atom diffusion process. After relaxation process, the structure of Ni nanoparticle is no longer regular and its RDF has no difference in shape but the smaller the particle is, the higher the peak is. To the same size of Ni nanoparticle, RDF has the lower peak and wider range and the extent of atom diffusion is higher after relaxation under the temperature of 800K than 300K. We also verified the percentage of high energy atom which measures the reaction activity is higher to the smaller nanoparticle.2. Simulation of Ni nanoparticle sinter:We discussed the variation of energy, temperature, and atom-potential distribution during sinter process and we also investigated the influence of nanoparticles'size and sinter temperature on sinter extent and rate. Combining with nanoparticle structure graph and mass-center distance figure, we concluded: sinter extent becomes higher and sinter rate becomes faster as the sinter temperature becomes higher; particle size has no influence on sinter extent and sinter rate doesn't become higher as the size becomes smaller. At the same time we verified the simulation result of the past and watched mechanical rotation of particle during sinter process.3. Simulation of Ni nanoparticle collision:We studied the variation of energy, temperature, mass-center distance, atom-potential distribution and particle structure during the collision process with different sizes and at different rates. And we got the following conclusions: (1) The final structure of Ni nanoparticle has close relationship with collision velocity: when the velocity is lower than 4?/ps, the two particles only combine with each other and atom permeation is little; when the velocity is equal to 4?/ps, the two particles forms one sphere particle and it takes long time to end the process of atom permeation. (2) The process of collision makes atom-potential distribution continuous and the stable temperature of system after collision decreases and the time taken to arrive at the stable temperature increases as the particle size increases.Molecular dynamics simulation of fluid flow and heat transfer contains the following research contents and conclusions:1. Simulation of thermal reflection wall model:We took thermal reflection wall model to investigate the process of heat transfer with different wall temperatures and the influence of flow density on heat transfer. As the flow density increases the influence of heat transfer on density distribution becomes smaller and the heat transfer ability of flow is enhanced.2. Simulation of real wall model on interface wettabilityWe took real wall model to investigate the influence of interface wettability on flow distribution. When wettability coefficient c is equal to 2.0, the interface presents relatively strong hydrophilicity and the phenomenon of several fluid layers next to wall as if it was solid is apparent; when c is equal to 0.2, the interface presents relatively strong hydrophobicity and the layer phenomenon is no longer apparent.3. Simulation of real wall model on triple-phase coexistence phenomenon and capillary condensation process:We took real wall model to investigate triple-phase coexistence phenomenon and capillary condensation process by figures of fluid density distribution and structure graphs of nanochannel fluid and discussed the influence of fluid density and temperature on them.4. Simulation of real wall model on density distribution under different heat transfer styles:We took real wall model to investigate the fluid density distribution under different heat transfer styles. When the two walls have the same temperature, the distribution curve has no difference with it under no heat transfer; when the temperatures of walls are different, the density peak next to low-temperature wall is higher than it next to high-temperature wall and the middle of density distribution curve is inclined. This tendency becomes more apparent as the temperature difference of two walls increases.5. Simulation of real wall model on boundary velocity slip and density distribution of Poiseuille flow.We took real wall model to discuss the influence of the four parameters-interface wettability, driving force, fluid initial density and heat transfer-on boundary velocity slip and density distribution. Changed one of them and kept others, we got the following conclusions: the possibility of boundary velocity slip becomes higher when interface wettability and fluid initial density decreases and when driving force increases; with wall temperatures different the possibility of velocity slip next to the higher temperature wall is higher than it next to lower temperature wall.