Aggregation kinetics of carbonyl iron based magnetic suspensions in 2D

Hierarchical assemblies of superparamagnetic colloids in time-varying magnetic fields

Magnetically-guided colloidal assembly has proven to be a versatile method for building hierarchical particle assemblies. This review describes the dipolar interactions that govern superparamagnetic colloids in time-varying magnetic fields, and how such interactions have guided colloidal assembly into materials with increasing complexity that display novel dynamics. The assembly process is driven by magnetic dipole-dipole interactions, whose strength can be tuned to be attractive or repulsive. Generally, these interactions are directional in static external magnetic fields. More recently, time-varying magnetic fields have been utilized to generate dipolar interactions that vary in both time and space, allowing particle interactions to be tuned from anisotropic to isotropic. These interactions guide the dynamics of hierarchical assemblies of 1-D chains, 2-D networks, and 2-D clusters in both static and time-varying fields. Specifically, unlinked and chemically-linked colloidal chains exhibit complex dynamics, such as fragmentation, buckling, coiling, and wagging phenomena. 2-D networks exhibit controlled porosity and interesting coarsening dynamics. Finally, 2-D clusters have shown to be an ideal model system for exploring phenomena related to statistical thermodynamics. This review provides recent advances in this fast-growing field with a focus on its scientific potential.

The squeeze strengthening effect on the rheological and microstructured behaviors of magnetorheological fluids: a molecular dynamics study

Systematic molecular dynamics simulations are conducted on magnetorheological (MR) fluids under steady state, squeeze flows and shear flows. The present study concerns the squeeze-assisted MR fluid strengthening and correlates the suspensions' macroscopic rheological properties to their microstructure evolution in terms of the aggregation kinetics. Simulation results demonstrate that the squeeze strengthening effect on the rheological properties of MR fluids is enhanced with the increasing magnetic field and becomes more prominent for dilute suspensions, but weakened with the increasing squeeze rate after the critical squeeze rate is surpassed. By microscopic inspection, it is found that the rheological properties of MR fluids under squeeze flows are consistent with the microstructured behaviors of MR suspensions in terms of the particle distribution, cluster kinetics, particle connectivity and magnetic energy. This study provides a microstructural insight into the squeeze-assisted MR fluid strengthening, which helps to attain an elegant design of MR devices with high shear performance requirements.

Magnetorheology: a review

Magnetic Soft Matter is a rapidly evolving discipline with fundamental and practical interest. This is due to the fact that its physical properties can be easily controlled through external magnetic fields. In this review paper, we revisit the most recent progress in the field (since 2010) emphasizing the rheological properties of these fascinating materials. New formulations and flow kinematics are discussed. Also, new members are integrated into the long-lived magnetorheology family and suggestions are provided for future development.

Simulation of a bidisperse magnetorheological fluid using the combination of a two-component lattice Boltzmann method and a discrete element approach

In this study, a two-component lattice Boltzmann method (TLBM) in combination with a discrete element approach (DEA) based on a representative volume element (RVE) with respect to a bidisperse magnetorheological fluid (MRF) was performed to simulate the behavior and microstructure formation of magnetic particles in a carrier liquid. The bidisperse MRF reported herein is a suspension dispersed by soft micron carbonyl iron (CI) particles and Fe₃O₄ nanoparticles dual-coated with gelatin and multi-walled carbon nanotubes (MWCNTs) in the carrier liquid. In the TLBM of two-component liquid and particle simulation, the inter-particle interactions were completely ignored; on the other hand, the DEA was just employed for calculating the dynamic behavior of particles. Therefore, due to the explicit nature of the combination of TLBM and DEA, it is a more efficient and accurate strategy for the simulation and calculation of complex MRF systems. Moreover, the immersed boundary method (IBM) was used to describe the interactions between the magnetic particles and the carrier liquid. The RVE of the bidisperse MRF was selected to eliminate specific operational conditions and reduce the calculation costs. New numerical results were obtained for the motion velocities, microstructure evolution and kinetic energy of bidisperse magnetic particles (BMPs) under the effect of a magnetic field; when the mass fraction of the nanoparticles was 0.1, the bidisperse MRF exhibited an improved response time in terms of the particle motion and microstructure formation. In addition, the effects of the Reynolds (Re) and Hartmann (Ha) numbers on the fluid-particle interaction and flow velocity were illustrated.

Simulation of the optimal diameter and wall thickness of hollow Fe3O4 microspheres in magnetorheological fluids

This work reported a simulation study on the optimal diameter (D) and wall thickness (H) of hollow Fe3O4 microspheres to improve the magnetorheological (MR) effect. Modified formulae for the magnetic dipolar force, van der Waals force, and hydrodynamic drag force were employed in the simulation model. Typical evolution of shear stress and microstructures in steady shear flow was obtained. The shear stress-strain curve was divided into linear, fluctuant, and homeostasis regions, which were related to the inclination of particle chains and the lateral aggregation. For hollow Fe3O4 microspheres with different diameters and wall thicknesses, the shear stress curves collapsed onto a quadratic master curve. The best wall thickness was H = 0.39D for a 20 wt% MR fluid and H = 0.35D for a 40 wt% MR fluid, while the optimal diameter was D = 1000 nm and D = 100 nm, respectively. The maximum shear stress of the 40 wt% MR fluid was twice that of the 20 wt% MR fluid. The change of shear stress was due to the competition that results among the magnetic interaction, number of neighbors, tightness, and orientation of the particle chains. Simulated dimensionless viscosity data as a function of Mason number for various diameters, wall thicknesses, and weight fractions collapsed onto a single master curve. The simulated shear stress under both a magnetic field and shear rate sweep matched well with experiments.