| Aiming at the technique issue of regulating and controlling ferrofluids with magnetic field, research works on two major problems are carried out:(Ⅰ) The influence of the magnetic and non-magnetic interaction on the structure formation and evolution in the ferrofluids.(Ⅱ) The influence of the size and the orientation of the columns in the magneto-flow coupled field on the viscous of the ferrofluids are investigated.For the first problem, some work as follows are carried out.The structure formation mechanism of the equilibrium state of the ferrofluids is studied. Configuration and diameter distribution of the magnetic columns under uniform magnetic field are observed with optical microscope. It is found that the diameter distribution of the columns satisfies the bi-normal distribution. The calculation model for the numerical relationship between the two average column diameters and the magnetic field is constructed. Based on the calculated results, it is proposed that the formation of the magnetic columns is dominated by two means:one is the growth of the chains/columns and the other is the lateral aggregation of chains/columns.By using of light transmission experiments and optical microscope observations with a longitudinal gradient magnetic field configuration, the relationship between the behavior of the transmitted light relaxation and the microstructure evolution in the central region of an axisymmetric field is investigated. Under a low-gradient magnetic field, there are two types of relaxation process. One is the formation of the magnetic columns and the other is the motion of the magnetic columns. It is found that a low-gradient magnetic field has little influence on the column formation dynamic, and magnetic transport and separation resulted from magnetophoresis under high-gradient fields change the formation dynamics of the local columns and have influence on the final state of the column system.The microstructure model of the ferrofluids after removing the magnetic field is established, and the dissociation dynamic of the microstructure is uncovered. The model describes that the microstructure evolution after removing the magnetic field has three stages which satisfies with the experiments well.The magnetic interaction (including the attractive interaction because of column’s flexibility) among the columns is reconstructed, a new model is established and dynamic simulation for the quasi-3D column system is carried out. By analyzing the simulation results, the conditions for the formation of ordered hexagon structure are uncovered, the convergence effect of the gradient field only strengthening their lateral aggregation limitedly, scale effect of the column system under gradient magnetic field are uncovered.Eight types of ferrofluids are prepared for comparative researches. All of the samples are characterized and the important parameters such as their diameter distribution, saturation magnetization and the magnetic dipolar interaction parameter are obtained. Light transmission experiments for different types of ferrofluid samples are carried out and the experiment results are compared and analyzed. It is found:magnetic dipolar interaction parameter is the dominate factor for the chain/column formation in ferrofluid; the applied magnetic field and the volume fraction are correlated positively to the strength and the speed of the chain/column formation, the average particle diameter and its dispersity have influence on the formation and motion of the chain/column; particle surface actions, which play a decisive role in the secondary lateral aggregation of the chain/column; the particle saturation magnetization has influence on the magnetic moment of the chain/colun, thus has influence on their translation motion and geometric sensitive parameters.As far as the second problem is concerned, in this thesis, the magnetic chain as the structure unit is regarded as an ellipsoid, and the magnetoviscosity is considered to be resulted majorly from the rotation of the structure unit in the carrier. Taking Brownian torque, magnetic torque and hydrodynamic torque into account, the differential equation about orientation is established. Using Galerkin method, magnetization relaxation of the non-interaction particle system under permanent field is studied, and Shliomis’s magnetization relaxation is evaluated. Using Sanchez’s method, a method of Brownian rotation dynamic simulation, chain orientation is calculated. Substituting the calculated results into the tension tensor including both symmetric and asymmetric parts, the intrinsic viscosities can be obtained. Based on the results, magnetoviscous effects under shear and with temperature are analyzed respectively. Following conclusions can be reached:(i) Growth of the chain structures is the major reason for the fold increase of the magnetoviscosity of ferrofluids, and the orders of the intrinsic viscosities of the average2,4and8-particle chain systems are100,101and102respectively;(Ⅱ) Shear flow has limit on the chain size, and it is estimated that the shear rate of the order102-103s－1may has restriction on the chain size;(Ⅲ) The order of the viscosity is decided by that of the carrier;(Ⅳ) Decrease of temperature has benefit on the chain growth, and leads to the fold increase of the viscous;(Ⅴ) Decrease of temperature has benefit on the decease of intrinsic viscosity when the chain is stable. The intrinsic viscosities of the average24, and8-particle chain systems are below the order of102at230K.The researches in this thesis are of important significance to quantify and expand the application of ferrofluids. |
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