Theoretical Study On Self-assembly And Dynamic Properties Of Magnetic Colloidal Particles

Magnetic viscous fluids are new potential chemical materials,which have significant applications in sewage treatment,magnetic seal in chemical equipment,lubrication,mineral separation,oil-water separation,and other fields.A deep understanding of the physicochemical and rheological properties of the magnetic viscous fluid can make this material more efficient in industrial production.In fundamental research,a magnetic viscous fluid is usually modeled as a magnetic colloid system,which is used to explain the unexplained phenomena in magnetic viscous fluid experiments.At the same time,the model system research also provides theoretical guidance for the new function development of magnetic viscous fluid.In this thesis,superparamagnetic colloidal particles are used as the research subject.Langevin dynamics simulations combined with simulated annealing algorithm and parallel tempering algorithm are used to study the rheological properties of magnetic viscous fluids on the interface,the principle of selecting the system size and long-range correction of interaction.This thesis also proposed a new theoretical model of attached polymers in the inner surface of a tube.The contents mainly include:1.Two-dimensional magnetic networks'microrheological properties are studied by the motion of non-magnetic and magnetic tracer particles in a two-dimensional magnetic network under different external forces.The results show that with the increase of the external force,the non-magnetic tracer particle will gradually destroy the network structure and form a channel-like structure.The magnetic tracer particle has the same tendency,but the required external force is much larger than the non-magnetic tracer particle.For the two kinds of tracer particles,with the increase of external force,the viscosity of the whole system can be divided into three regions:the stable region under smaller external force(viscosity is a constant value),the linear change region under medium external force(viscosity decrease during the increase of pulling force),and the stable region under larger external force(viscosity does not change with the increase of pulling force).With the increase of external force,the diffusion behavior of tracer particles gradually changes from sub-diffusion behavior to super-diffusion behavior,while the diffusion behavior in the direction perpendicular to the pulling force direction presents sub-diffusion behavior and diffusion behavior in the whole range of external force.This work can provide guidance for a better understanding of the rheological behavior of magnetic viscous fluid on the interface and provide references for the new function development of magnetic viscous fluid.2.Langevin dynamics simulations are used to simulate the two-dimensional model system or the three-dimensional model system composed of superparamagnetic colloids in order to deeply understanding magnetic dipole-dipole interactions in the magnetic colloidal system.The simulation results of finite size system(not considered long-range correction of magnetic dipole-dipole interaction)and bulk phase system(considering long-range correction of magnetic dipole-dipole interaction)under different cutoffs are systematically compared.The results show that the long-range correction of magnetic dipole-dipole interaction is very important for the superparamagnetic magnetic colloidal model system.The behavior of the body phase system can be simulated by using a very small simulation system after considering long-range correction.If a finite size system is used to simulate a bulk phase behavior,the dipole-dipole interaction cutoff must be greater than some threshold value.But it is not suitable to study the thermodynamic properties.This work provides a theoretical foundation for better and more efficient use of the magnetic colloidal system.This work also provides some theoretical support for more efficient use of the modeling magnetic colloid system to explain the unexplained phenomena in the magnetic viscous fluid experiments.3.The magnetic polymer brush model is build as a model of attached polymer filaments in the inner surface of a tube.The model is constructed by the orderly arrangement of the magnetic filaments composed of superparamagnetic colloidal particles and springs on a substrate board.The impurities in the polymer solution are described by dispersing the non-magnetic colloidal particles in the model.The results show that the magnetic polymer brush model can self-assemble into multi-layer-wall-like structure,cottage-like structure,house-like structure,plate-like structure and so on under different external magnetic field strengths and filament rigidity coefficients.For the total energy of the self-assembly structures of the model system,the magnetic energy usually contributes the most part,followed by the bond angle energy of the magnetic filament,and finally the bond energy of the magnetic filament.For the self-assembe structure of the model system,the larger external magnetic field strength,the easier to form a board-like structure.While the system will be more easiler to form a cottage-like structure or house-like structure with larger magnetic filament rigidity coefficient.At this time,the non-magnetic colloidal particles in the model system gathered in the empty space of the substrate and the inner space of self-assemble structures which also plays a role in stabilizing the self-assemble structure of the model system.These results show that the properties of polymer filaments and impurities in polymer solution affect the aggregation behavior of polymer in the inner surface of a tube,which provides a theoretical basis for better understanding of the deposition and scaling phenomena of polymers in a tube.