Numerical Simulations Of Nanometer Magnetic Particle Dynamics And Research Of Magnetic Fluid Sealings

Magnetic fluid is stable colloidal function material containing nanometer magnetic particles coated with layers of surfactants and suspended in a liquid carrier. It is one of the few nano-materials in use now, mainly used for magnetic fluid sealing, which use magnetic fluid in the restriction of grade magnetic fields to stop up sealing gaps. The many body kinetic and the microstructure configuration control of magnetic fluids in the present of magnetic fields are the foundation for the development of high performance magnetic fluids and devices.Physical models, microstructure parameters and realization methods of the magnetic fluid materials are systematically summarized at first. The molecular dynamic characteristics of the magnetic nano-particles are studied and verified by evaporation tests of magnetic fluids in vacuum conditions. And then, based on the molecular dynamics, finite element methods and boundary element methods of magnetic fields, the numerical simulation of microstructure magnetization state and many-body kinetic of magnetic fluids in the presence of the magnetic fields are performed. Influence factors, such as, Brownian motions, surface coating layer thickness and nonlinear magnetization properties of magnetic nano-particles are considered in the numerical models. As a typical simulation example, the dynamic simulations of the magnetic fluid in the grade magnetic fields of magnetic fluid sealing gaps are carried out. The properties and shortcomings of the existing magnetic field concentration structures for magnetic fluid sealing are discussed, developed and experimentally verified.Evaporation tests show that magnetic fluids can be gasified in vacuum conditions with the surfactant coated magnetic nano-particle as elementary particles. In comparison, the boundary element method is the optimal method for the magnetization kinetic simulation of magnetic fluid, which can be employed to obtain such useful information as response time, magnetization states, non linear properties of the materials, the dynamic microstructure formation processes and magnetic interactions and so on, even in the complex grade magnetic fields. The simulation results of magnetization states, interaction between particles and dynamic microstructure formation processes of the three-particle systems are in agreement with the corresponding solutions calculated by finite element method software of ANSYS and interaction potential energy theory of dipole moments. On the bases of a normal distribution of dynamics force supposition, the statistic simulation results of the displacement and velocity of the Brownian particles are obtained, which are well consistent with the normal distribution formulas based on the fractional dimensionality theory of Brownian motion. What is more, the simulation results of non linear saturated magnetization properties of the magnetic fluid are also well fit with the testing results and Langevan curves.In the presence of parallel magnetic fields, randomly distributed magnetic particles will undergo short chain formation process and long chain formation and repelling process. During the short chain formation process, the magnetization strength of the magnetic fluids may descend to a low value. The relative long chains possess priority to collect the short chains near their ends and grow even longer; at the same time restrain the shorter chains to grow. The magnetic particles in the same chain connected by strong magnetic fields; different chains are separated by weak magnetic fields and repel each other, and the distances between them increased gradually. The magnetic particles are accelerated during short chain formation process and decelerate during long chain formation and repelling process. So, there exists a maximum value of mean kinetic energy of the magnetic particles. The response time can be determined according to the curves of magnetization vs. time or mean kinetic energy vs. time.The distribution of the magnetic fields in the magnetic fluid sealing gaps is complex and graded, and varied with different concentrating structures. Finite element solutions show that conventional magnetic fluid sealing structures, which possess single side pole teeth, result in notable flux distribution grade along the paths perpendicular to the sealing gaps surface. In comparison, the concentration effects of the structures with opposite pole teeth are very reasonable, distribution of the magnetic fields along the perpendicular direction to the sealing gap surfaces is very consistent, and the grade of the magnetic fields along the parallel direction is about two times that of the structures with single side pole teeth. Moreover opposite pole teeth can overcome the centrifugal forces applied on the magnetic fluid produced by high-speed rotation. Additive concentration structure, which is a new kind of magnetic fluid sealing design developed in this paper, can create two interference magnetic field periodically varying in strength along the sealing path, of which the directions are inverse but the modulus is the same. So the useful magnetic fields are increased and the useless magnetic fields are counteracted along the sealing path. In comparison, additive interference magnetic structure can create the largest magnetic field grade parallel to the sealing path; but it is difficult to create local parallel magnetic fields in the perpendicular direction in the sealing gaps.Dynamic simulations of the magnetic fluids show that, in the presence of complex and graded magnetic fields of the sealing gaps, chains or string aggregations are also formed. The forces applied on to the chains consisted of components perpendicular and parallel to the sealing gap surfaces. The magnetic field grades parallel to the sealing gap surfaces diminish the distance between the chains; the perpendicular component drives the chains move to the high magnetic field grade directions. For the structures with the single side pole teeth, the chains are segregated near the pole teeth areas, and make the opposite areas depleted. So the static magnetic sealing properties of the single side pole teeth are relatively poor. In the structures with opposite pole teeth, segregation near the pole teeth can also take place to some extent, but the chains is dispersed in the whole sealing gap areas, because the magnetic fields in the local areas of sealing gaps are relatively parallel and consistent, and the chains repel each other. Especially the forces applied to the chains by the opposite pole teeth are equilibrium, so they will not move from one side to another side. Accordingly the opposite pole teeth can suffer from high static sealing pressures. Distribution of the local magnetic fields in the sealing gap of additive concentrating structure are peaked along the parallel direction to the sealing path, so the repelling forces between the chains are relatively low, and the chains can not be well dispersed in the sealing gap. Owning to the forces applied from two end of every chain by the pole teeth, serious segregations will occur near the pole teeth areas, meanwhile the middle areas of the sealing gaps are depleted. Therefore, the static sealing properties of additive concentrating structures will be very poor, and further research should be done in the future. Static magnetic fluid sealing tests have been done with these three kinds of concentration structures, and the numerical prediction results are experimentally verified.