Dipolophoresis in concentrated suspensions of ideally polarized spheres

Significance 

Unlike dilute and semi-dilute suspensions, little has been done to explore the dynamics of concentrated suspensions under electrokinetic and hydrodynamic interactions. This could be due to poor understanding of the electrokinetics effects on concentrated regimes as a result of the significant multibody interactions. Based on past reviews, polarized particles placed in conducting fluids have been observed to produce particle motions that undergo both induced-charge electrophoresis and dielectrophoresis, a phenomenon commonly known as dipolophoresis. Consequently, highly complex suspension dynamics undergoing induced-charge electrophoresis have been anticipated in both concentrated and semi-concentrated suspensions. They are expected to yield undesirable results even though no attempts have been made to confirm such assumptions.

In an attempt to address the issue, Siamak Mirfendereski (Ph.D. candidate) and Dr. Jae Sung Park from the Department of Mechanical and Materials Engineering at the University of Nebraska-Lincoln investigated the suspension dynamics of ideally polarized spheres undergoing dipolophoresis (a combination of induced-charge electrophoresis and dielectrophoresis) based on large-scale numerical simulations. Thus, particles were assumed to carry zero net charges. The concentrated suspensions were simulated using previous model developed by Park and Saintillan. However, a potential-free algorithm was added to prevent excessive particle overlap. This work is currently published in the Journal of Fluid Mechanics.

The induced-charge electrophoresis dominance over dielectrophoresis resulted in chaotic motion and hydrodynamic diffusion effects. The particle motion gets hindered as the volume fraction increases up to semi-dilute regimes. In the case of concentrated regimes, however, non-trivial behaviors were observed. For instance, the hydrodynamic diffusivity increased gradually with the volume fraction from the minimum mark (about 35%) to the peak maximum (about 50%) before drastically decreasing on approaching the random close packing. A similar scenario was reported for the velocity and number-density fluctuations. This clearly showed the potential role of the fluctuations in enhancing the hydrodynamic diffusion.

The authors went ahead to explain the observed non-trivial behaviors as a consequence of the particle’s contacts related to the dominant mechanism of the particle pairings. For that, the particle contacts were classified into two, by nature of contact: attractive and repulsive. By comparing the radial relative velocity in the transverse and field direction, the former was found to be more dominant with most curves on the repulsive side. Additionally, with most of the contacts coming from the repulsive contacts, a strong repulsive interaction was concluded to be the most dominant mechanism of the particle contact. As a result, the nontrivial behaviors observed in the concentrated suspensions were attributed to these strong, massive and repulsive contacts. It was worth noting that the results agreed well with the change in pair distribution functions.

According to the authors, the presented findings for concentrated suspensions appear, in one way or the other, similar to active suspensions thanks to the quantitatively same far-field fluid disturbances among others. However, the expected difference will majorly rely on the surface velocity magnitude and orientation which defines the relative significance of the attractive and repulsive interactions between the particles. Nonetheless, Dr. Park, in a statement to Advances in Engineering, his study findings will pave way for future investigation of the generality of the non-trivial behavior in concentrated suspensions and provide a concrete comparison with that for inert and active suspensions.

Two movies: One of the most interesting findings in the paper is that particle dynamics at a higher concentration (volume fraction of 50%) are faster than one at a lower concentration (volume fraction of 25%) under dipolophoresis.

About the author

Jae Sung Park
Assistant Professor
Department of Mechanical and Materials Engineering
University of Nebraska-Lincoln
Email:[email protected];
https://engineering.unl.edu/mmfd/

Dr. Jae Sung Park is an assistant professor in the Department of Mechanical and Materials Engineering at the University of Nebraska-Lincoln. He earned a B.S. in Mechanical Engineering at Hanyang University, Seoul, South Korea. He received his M.S. and Ph.D. in Mechanical Engineering from the University of Illinois at Urbana-Champaign in 2008 and 2012, respectively. He was trained as a postdoctoral associate in the Department of Chemical and Biological Engineering at the University of Wisconsin-Madison. His research interests encompass a wide range of fluid mechanics, covering from low to high Reynolds number flows, including complex fluids, electrokinetics, biofluids, and turbulence. He also studies thermo-fluids systems such as heat exchangers.

His research has been funded by the National Science Foundation, NASA, and Nebraska Research Initiatives. He is a member of the American Physics Society (APS), the American Society of Mechanical Engineers (ASME), and the Korean American Scientists and Engineers Association (KSEA).

About the author

Siamak Mirfendereski earned his B.S. and M.S. in mechanical engineering from Isfahan University of technology, Iran, and Amirkabir University of technology, Iran, respectively. During his master, he was dealing with both numerical and experimental studies in the field of fluid dynamics and heat transfer, with main focus on nanofluidic and helical tubes. He joined the University of Nebraska-Lincoln as a Ph.D. student in Mechanical Engineering in August 2017, and has worked as a graduate research assistant.

His current research focuses on investigating a particle suspension undergoing electrokinetics using a large-scale accelerated Stokesian dynamic simulation. He is also developing a numerical model to simulate a blood flow inside a stenotic channel. He is a member of the American Physics Society (APS) and the American Society of Mechanical Engineers (ASME).

Reference

Mirfendereski, S., & Park, J. (2019). Dipolophoresis in concentrated suspensions of ideally polarizable spheres. Journal of Fluid Mechanics, 875, R3.

Go To Journal of Fluid Mechanics

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