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22 May 2024
Research Insights: Modelling Chiral Living Matter with Grass Seed
The structures of biological materials, commonly called “living matter”, such as DNA, proteins, bacterial flagella, and flower petals to name a few, are predominately chiral. However, the origin of the prevalence of one type of handedness over the other in this part of nature  remains a mystery, despite knowledge of its existence for hundreds or thousands of years. When living matter self-propel, they also exhibit handedness, a notable example of which is the tendency of bacteria to swim in a circle near a surface. Therefore, understanding how self-propelling or active chiral living matter self-organize, navigate, and transport in complex environments will help us decipher the mechanisms of living systems. Moreover, identifying and sorting different cells or microorganisms according to their degree of activity and chirality holds great promise for applications such as diagnostics, omics, and drug delivery. However, the interactions between these chiral active matter and their environment are notoriously difficult to examine, which impedes their further study.

Prof. Rui Zhang and Prof. Yilong Han and members of their research groups in the HKUST Department of Physics, Chung Wing Chan, Daihui Wu, Kaiyao Qiao, and Kin Long Fong, explored the intriguing behavior exhibited by chiral active matter using a simple platform with the help of Prof. Zhiyu Yang of the same department. Prof. Han  discovered that  Echinochloa crus-galli grass seed has chiral surface ridges that  give rise to  circular or linear motion when it is propelled under the application of external vibrations. Careful studies of the seed particle motion on a vibrating stage performed by Dr. Wu, Mr. Qiao, and Mr. Chan under the guidance of Prof. Han and Prof. Yang revealed that an active Brownian dynamics model can capture the seed dynamics well.

Mr. Fong and Mr. Chan developed a simulation model with Prof. Zhang’s guidance, which they used to examine the dynamics of chiral active particles in an obstacle array that mimics the complex environment of the experiment. Similar to opotaxis of active particles or cells that describes the tendency to move to regions with less dense obstacles, chiral active particles can also migrate according to the obstacle density. Surprisingly, it was  found in this work that at certain conditions a particle tends to be trapped in a triangular obstacle lattice but exhibits fast diffusion in a square obstacle lattice with the same obstacle density. Furthermore, this tendency can be reversed by changing the particle chirality. The remarkable ability to differentiate obstacle lattice structure exhibited by chiral particles is not observed for achiral active particles. Interestingly, further studies also revealed that left-handed and right-handed particles exhibit different effective diffusivities in a parallelogram lattice with broken mirror symmetry. Such a difference can be understood by a pure geometric quantity, which was defined to characterize how far a lattice deviates from mirror symmetry. The key simulation findings were confirmed by experiments.

This work demonstrates that the chiral motions of active particles can sense lattice configurations. Therefore, the work facilitates applications of active matter in chirality-based separation and therapeutic delivery, and also paves the way toward novel applications such as using chiral active matter as environmental sensors.

The work has been recently published in Nature Communications 15, 1406 (2024) and highlighted by the Editor in the “Applied physics and mathematics” section. This work is financially supported by the Research Grants Council via no. 26302320 (R.Z.) and no. 16305822 (Y.H.).

This research also highlights research opportunities for outstanding undergraduate (UG) students in the Department of Physics. Mr. Kin Long Fong, who initiated the work, was a student in the elite International Research Enrichment (IRE) undergraduate program at HKUST between 2017 – 2021. He was involved in research projects under Prof. Rui Zhang’s supervision during his final year of UG study and he is now working towards his Ph.D. degree at the Technical University of Berlin. Mr. Chung Wing Chan was the lead author of publication about the work. He initiated the project during his fourth year as a UG student in the Department of Physics and completed the work during his M.Phil. studies in Prof. Zhang’s group. Mr. Chan is now pursuing his Ph.D. degree in the Department of Physics at Kyoto University, where he is focusing on soft condensed matter physics.