Aggregated chain morphological variation analysis of magnetorheological fluid in squeeze mode

Being a non-Newtonian liquid, magnetorheological fluid is widely used in numerous industrial applications owing to their high yield stress index. In particular, it provides very high shear yield stress in squeeze mode due to squeeze strengthening and sealing effects. When the squeeze force is applied to aggregated chains, the chains undergo complex morphological variations majorly thickening and fracture. This is attributed to the synthetic effects of various forces acting on the particles: hydraulic, magnetic, repulsion and friction forces. The fracture, being the most abrupt microscopic variation, has exhibited a potential influence on the macroscopic yield of magnetorheological fluid. This necessitates the need for alternative approaches for investigating the fracture of single chains in the squeeze process for a better understanding of the microscopic mechanism of squeeze yield.

To this note, scientists at Dalian University of Technology: Professor Yongqing Wang, Qi Luo (PhD candidate), Professor Haibo Liu, Dr. Jiakun Wu, Dr. Meng Lian and Professor Te Li investigated the fracture phenomenon of aggregated chains in the magnetorheological fluid.  The magnetic force between the particles was modeled. A fracture criterion based on the magnetic force model was proposed to analyze the fracture angle and position of the chains. Consequently, they examined the driving effects of hydraulic forces and the restoring effects of the magnetic forces. Finally, the fracture mechanism was validated by comparing the experimental observation results and theoretical analysis. The work is currently published in the journal, Smart Materials and Structures.

The fracture feature was experimentally observed and noted to be as a result of the interactions between the hydraulic forces and magnetic forces. Even though many forces are considered to theoretically influence the fracture of the squeezed chains, it is important to note that only hydraulic and magnetic forces were considered while others were rendered negligible due to their balancing effects. The authors successfully analyzed the fracture mechanism by answering two fundamental questions: what are the fracture factors of the aggregated chains and how they influence the fracture features. The hydraulic force was the driving factor behind the morphological variation while the magnetic force was the factor preventing morphological variation. As such, the driving effects made the chains bend while the restoring effects prevented the chains from breaking.

The fracture phenomenon happed when the effects of the hydraulic force on fracture were ignored and the chains bent to an extent that the horizontal component of the magnetic force turned to zero. Considering the material performance, the fracture was noted to have a significant influence on the yield strain and yield stress. This macroscopic squeeze yield stress/stain is a key consideration in engineering applications due to its remarkable influence on the rigidity and stability properties of the magnetorheological fluid used in such applications. Thus, in a statement to Advances in Engineering, Professor Haibo Liu explained that their fracture criterion presented here is a promising tool for future modeling of the macroscopic squeeze yield stress/strain.