Promotion of ferrofluid microchannel flows by gradient magnetic fields

Significance 

Ferrofluids have a wide range of engineering applications. They are generally stable colloidal suspensions of surfactant-coated magnetic particles suspended in carried liquid. Although ferrofluids exhibit a strong response to fields, they remain relatively stable and preserve their fluidity even under intense magnetic fields. Due to this property, they have been used for various applications like dampers, seals and accelerometers. Additionally, ferrofluids have attracted attention for potential application in microsystems since they can be effectively controlled by applying magnetic fields.

Microchannel is an important component of most microfluidic devices. Ferrofluids have successfully achieved low to middle -Reynolds number flows in these microchannels, producing intriguing results. Ferrofluid flow involving microchannels have been studied in various fluid flow configuration, especially in the Couette-Poiseuille configuration. This flow is affected by changes in both uniform and time-dependent magnetic fields. For example, the effective viscosity of ferrofluid could become negative when subjected to an alternating magnetic field. This reduces the flow resistance by converting the magnetic energy to quadratic velocity.

There are several methods for generating static magnetic field gradients, such as using permanent magnets of Maxwell coils and their arrays. Similarly, the performance of ferrofluidic seals can be inspected by examining the flow properties by applying field gradients via the influence of asymmetrical magnetic fields in the azimuthal direction. This can be attributed to two main factors: first, ferrofluid seals are held by magnetic fields with nearly constant gradients, and second, the uneven arrangement of magnets enables the application of field gradients. Unfortunately, the ferrofluid channel flow under a static non-uniform magnetic field remains largely underexplored despite its practical implications.

On this account, Professor Wenming Yang, Mr. Boshi Fang and Prof. Beiying Liu from the University of Science and Technology Beijing together with Mr. Zhen Yang from Shanghai Waigaoqiao ShipBuilding Co. investigated the use of gradient magnetic field perpendicular to channel walls to promote ferrofluid microchannel flows. The ferrohydrodynamic problem bounded by the Fokker-Planck magnetization equation in a Couette-Poiseuille configuration was numerically solved under stationary magnetic fields with constant gradient. The effects of the various parameters on the moving plate, including velocity, spin profiles, flow rate, stresses and magnetization, were examined for different field gradients. The work is currently published in the journal, Journal of Non-Newtonian Fluid Mechanics.

The authors showed the feasibility of steady gradient magnetic fields in promoting both pure Couette and Couette-Poiseuille flows in microchannel geometry. Due to this promotion, a velocity increment of 35% in the studied gradient range and a 24% increase in volumetric flow were reported. This represented remarkable progress considering that such phenomenon was previously possible only in ferrofluid flow subjected to either temperature gradient fields or AC magnetic fields. The observed linear vorticity and quadratic velocity fields with larger gradients were attributed to the effects of the gradient magnetic fields. Furthermore, the magnetic fields were able to improve the magnetization relaxation effects and significantly increase stress and reduce the shear stress by comparing with cases subjected to uniform magnetic fields. However, the trade-off between these factors resulted in tangential stress on the moving plate.

In summary, the research team successfully utilized a verified numerical model to promote ferrofluid microchannel flows under magnetic fields with constant gradients. The model is effective in finding steady solutions for velocity and magnetization fields of the ferrofluid microchannel flow. Consequently, the gradient fields could completely suppress the backflow observed in the normal Couette-Poiseuille flows, leading to the disappearance of the separation flow. In a statement to Advances in Engineering, Professor Wenming Yang stated that the proposed model and study findings provided more insights into ferrofluid flow, thereby opening more possibilities for more unique applications in seals and bearings.

Promotion of ferrofluid microchannel flows by gradient magnetic fields - Advances in Engineering Promotion of ferrofluid microchannel flows by gradient magnetic fields - Advances in Engineering Promotion of ferrofluid microchannel flows by gradient magnetic fields - Advances in Engineering

About the author

Dr. Wenming Yang received Ph. D. degree in mechanical engineering from Beijing Jiaotong University, in China in 2012. After that, he was a postdoctoral researcher in the Department of Precision Instrument at Tsinghua University in China. From 2017 to 2018, he worked as an academic research fellow in the Department of Engineering at University of Cambridge. He is currently with the School of Mechanical Engineering, University of Science and Technology Beijing. His research focuses on non-equilibrium ferrohydrodynamics, mechanical behavior of particle suspension systems and precision measurements.

Reference

Yang, W., Fang, B., Liu, B., & Yang, Z. (2022). Promotion of ferrofluid microchannel flows by gradient magnetic fieldsJournal of Non-Newtonian Fluid Mechanics, 300, 104730.

Go To Journal of Non-Newtonian Fluid Mechanics

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