Researchers from the School of Physics, Beihang University, recently published a paper titled "Anomalous Transport of Elongated Particles in Oscillatory Vortical Flows" in Physical Review Letters, which was selected as the cover article. This work, led by Associate Professor Hu Shiyuan in collaboration with partners from the Courant Institute of Mathematical Sciences at New York University and the Institute of Physics, Chinese Academy of Sciences, systematically investigated the transport behavior of elongated particles in oscillatory vortical flows. The study reveals a matching effect between particle length and the dynamic structure of the flow field, which plays a decisive role in particle diffusion.

In microscale fluid environments, the random walks of suspended particles are prevalent in processes such as biological transport, microfluidic sorting, chemical reactions, and pollutant dispersion. For a long time, it has been commonly believed that in vortical flow fields, longer particles are more likely to cross streamlines, leading to faster spatial diffusion. However, this study discovered that the particle transport rate does not increase monotonically with length. At specific oscillation frequencies, particles of intermediate length exhibit the slowest diffusion, displaying pronounced subdiffusive behavior. This finding challenges conventional understanding and reveals a complex and delicate coupling mechanism between unsteady flow fields and particle geometry.

The research team constructed a quasi-two-dimensional periodic vortex array in a shallow electrolyte solution using electromagnetic driving. By periodically moving the magnet array, the entire vortical flow field was set into oscillation. The Reynolds number of the flow ranged between 1 and 10, placing it within the low-Reynolds-number dynamics regime. Upon oscillation, the originally regular streamline structure was disrupted, forming a mixed phase space structure where chaotic regions and regular elliptic regions coexisted. These elliptic regions act as transport barriers for fluid particles. Through further experiments and numerical simulations, the team found that when the particle length matches the spatial scale of the elliptic regions, particles become trapped within these regions for extended periods, exhibiting robust subdiffusive behavior. Remarkably, this subdiffusive behavior persists even in the presence of random noise.

This research not only deepens the understanding of transport phenomena in unsteady flow fields and reveals new physical mechanisms underlying the random motion of suspended particles in low-Reynolds-number flows but also offers new insights for engineering applications. By tuning the dynamic structural scale of the flow field, it is possible to achieve selective trapping or directed transport of particles of different lengths. This provides new ideas for practical applications such as particle sorting in microfluidic chips and the migration control of polymers or biological macromolecules in complex flow fields.

Associate Professor Hu Shiyuan, Professor Man Xingkun from the School of Physics, Beihang University, and Professor Zhang Jun from New York University are the co-corresponding authors of the paper. Associate Professor Hu Shiyuan and Ph.D. student Yang Xiuyuan are the co-first authors. The School of Physics, Beihang University, is the primary affiliation and corresponding affiliation. This research was supported by the National Natural Science Foundation of China and the Fundamental Research Funds for the Central Universities of Beihang University.

Link to the article: https://doi.org/10.1103/cj7w-f9vx

Editor: Liu Tingting