Simulations On The Mechanical Properties And Mechanism Of Magnetic Fluids

Magnetic fluid is a kind of magneto-sensitive smart material fabricated by highly dispersing microscale or nanoscale magnetic particles into non-magnetic carrier fluids.After applying the external magnetic field,magnetic particles attract each other and rapidly form chain-like or plate-like microstructures.Those specific aggregations hinder the flow of carrier fluid and increase the shear viscosity and yield stress by several orders of magnitude within a few milliseconds,which is called the magnetorheological(MR)effect.Based on the controllable and reversible mechanical properties,magnetic fluid has been widely applied in engineering,such as dampers,vibration absorbers,and polishing,and the biomedical area,such as magnetic hyperthermia and magnetic resonance imaging.In the past decade,modifications in the morphology,inner architecture,and surface roughness of magnetic particles were sufficiently developed to improve the MR effect and overcome sedimentation.A lot of novel magnetic fluids based on nanostructured magnetic particles have been reported.However,the influences of particle shape,inner architecture,and surface roughness on the MR effect are still unclear.The enhancement mechanism in the MR effect of novel magnetic particles requires further elaboration.In this work,experiments and simulations were combined to systematically study the mechanical properties and MR mechanism of novel magnetic fluids.Firstly,the existing particle-level dynamic simulation method was improved with higher precision and a wider range of applications.Then,the novel Fe3O4 hollow microspheres which possessed a low density and excellent MR effect were synthesized.The optimal size and hollow ratio were determined by using particle-level dynamic simulations.Furthermore,Fe3O4 chains composed of monodisperse hollow spheres were prepared by combining the chain morphology and hollow inner architecture.Rheological tests revealed the merits and possible applications of this novel magnetic fluid compared to the above Fe3O4 hollow sphere magnetic fluid.Simulations were employed to investigate the mechanism of unique rheological behaviors of Fe3O4 hollow chains.Finally,the influence of interparticle friction forces on the MR effect was sufficiently discussed,in which the best friction coefficient was obtained.This work will open a promising avenue in the development of high-performance magnetic particles and magnetic fluid.Detailed contents are as follows:1.Investigation of the particle-level dynamic simulation method and programming.A novel particle-level dynamic simulation method that was applicable for spherical and chain morphology,as well as solid,core-shell,and hollow inner architecture was developed based on the existing theoretical model.The accuracy of simulations was improved by developing the model of van der Waals force and introducing correctional factors in the magnetic dipolar force and viscous drag force.The efficiency of the program was increased by adopting reasonable approximations in which the buoyancy and Brownian motion were neglected.This numerical method was applicable for coarse magnetic particles due to the consideration of elastic normal force and tangential friction force between particles.2.Study on the optimal size and hollow ratio of Fe3O4 hollow spheres in magnetic fluid.Micron-sized Fe3O4 hollow spheres with low density and excellent MR effect in magnetic fluid were synthesized.Shear rheological properties under different diameters,wall thicknesses,and weight fractions were discussed.The optimal hollow inner architecture was determined from simulations.Shear stress exhibited a quadratic dependence on the wall thickness,the change in shear stress was due to the competition among the strength of interparticle forces,particle number density,the compactness and orientation of microstructures.Simulations could not only explain the experimental phenomenon but also guide the preparation of materials.3.Preparation and MR mechanism of magnetic fluid based on Fe3O4 hollow chains.To further improve the MR effect while maintaining the settlement stability,Fe3O4 chains composed of monodisperse hollow spheres were developed.Rheological tests showed magnetic fluid based on hollow chains exhibited superior MR effect than those based on hollow spheres under a weak external field(B?100 mT),which made these novel magnetic particles be widely applied for small and low power devices.Simulations revealed the evolution of microstructures was dominated by the torque equilibrium.Hollow chains aggregated into inclined plate-like structures under a weak field,while vertical columns were formed under a strong field.Chain morphology enlarged the average dip angle of microstructures and generated stronger shear stress.4.Influence of interparticle friction force on the MR effect.Finally,the focus of research on MR mechanism transferred from magnetic dipolar forces to non-magnetic friction forces.The object of the study became general coarse magnetic microspheres instead of any specific particles.The trend of shear stress versus friction coefficient was systematically discussed.The optimal friction coefficient was determined in simulations by considering the saturation magnetizations,external fields,shear rates,and particle concentrations.Moderate friction forces(0.2???0.5)hardly influenced the mechanical properties of magnetic fluid.Under a large friction coefficient(1.0<??2.5),the shear stress could be improved by 102%.However,the MR effect was weakened if the friction forces further enhanced.A possible mechanism was developed to evaluate the influence of friction interaction on the MR effect.This work was helpful to guide the preparation of high-performance magnetic particles and magnetic fluid.