【科研进展】活性颗粒物质局域振动研究新突破！实验证实弦状局域振动与零群速度非色散关系

01

概况

近日，上海交通大学软物质与活性物质合作研究组（Wilczek量子中心“Matteo Baggioli团队”、自然科学研究院/物理与天文学院“张洁团队”）在活性颗粒物质的局域振动研究中取得重要进展。研究组基于活性布朗粒子体系实验证实了两年前被预测的弦状局域振动及其对应的零群速度色散关系。该研究对理解非晶固体本征振动的奇异性这个上世纪留下的待解之谜有积极意义。相关研究结果以Dispersionless Flat Mode and Vibrational Anomaly in Active Brownian Vibrators Induced by String-like Dynamical Defects为题目在线发表于Physical Review Letters期刊。

论文链接（点击“阅读原文”查看）：

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.133.188302

02

研究背景

自上世纪八九十年代以来，从玻璃到软的胶体以及生物大分子，从实验观测到计算机模拟，在数量庞大的非晶体系中都观测到了与晶体态密度相比大量低频振动态。这种低频振动态显然并不为某种无序体系所独有，而是由无序本身所导致的现象。那无序与低频振动又有怎样的必然联系呢？对这个问题的思考几乎贯穿了整个研究非晶体系振动的历史。大约从十五年前开始，研究这个问题的一大主流观点逐渐认为这种过剩的低频振动态是一种局域振动。随着计算机模拟技术的发展，有人预测这种低频局域振动应该在粒子空间位形上呈现出弦状结构，并且对应于频率不依赖于波矢的非色散关系，即群速度为零。理论研究推测这种弦状局域振动的频率只和弦长度有关系，长度越长频率越低，因此只能贡献低频的振动模式。

03

研究方法及结果

研究团队通过一个均匀驱动的准二维振动平台，实现对自主设计的非极性颗粒的随机驱动。这些颗粒在一定频率和加速度的振动下，单颗粒的平动速度和转动速度分布均符合零均值高斯分布，所以被称为活性布朗颗粒，类似于热浴中的花粉粒子。利用高精度的图像处理与颗粒追踪技术，能够得到颗粒系统的结构和动力学信息。研究团队在动力学结构因子的信号中明显地区分出除声学支线性色散关系之外的另一个零群速非色散关系。然后通过本征模式分析发现在零群速度非色散关系的出现同时伴随着零星分布的弦状局域振动。在我们的观测结果中弦状局域态的长度尺度也较好的符合之前理论所提出的频率与长度尺度依赖关系的规律。

Figure 1: (Left) the experimental identification of the dispersionless flat mode (DFM); (Right) the origin of the DFM in the emergence of string-like dynamical defects.

04

总结

这项研究用全面可靠的实验结果验证了非晶体系振动的近年的理论研究进展，丰富了对活性颗粒体系的玻璃态的表征与研究，对进一步理解无序对惰性与活性系统带来的奇妙现象有积极意义。

研究团队成员包括博士生蒋存源，郑梓涵，陈杨锐博士（现明尼苏达大学博士后研究员），Matteo Baggioli教授与张洁教授。该论文第一作者为博士生蒋存源，通讯作者为Matteo Baggiol教授与张洁教授。该研究得到了国家自然科学基金委与上海市教委科研创新计划的支持。研究团队向国家自然科学基金委，上海市教委，上海交通大学，以及物理与天文学院以及自然科学研究院致以诚挚的感谢！

Overview

Recently, the Soft Matter and Active Matter Collaborative Research Group, led by Prof. Matteo Biggioli at the Wilczek Quantum Center and Prof. Jie Zhang at the Institute of Natural Sciences of Shanghai Jiao Tong University, made significant progress in the study of vibrational dynamics in active granular matter. Based on a vibrating Brownian platform, the research group experimentally confirmed the emergence of string-like dynamical defects as the fundamental origin of vibrational anomalies and the corresponding dispersionless flat mode in amorphous granular systems. This research is significant for understanding the long-standing mystery of vibrational anomalies in disordered systems. The related findings were published in the Journal of Physical Review Letters under "Dispersionless Flat Mode and Vibrational Anomaly in Active Brownian Vibrators Induced by String-like Dynamical Defects."

Research background

Since the 1980s, a large number of unexplained low-frequency vibrational modes have been observed in various disordered systems, ranging from glasses to soft colloids and biological macromolecules, through both experimental observations and computer simulations. These low-frequency vibrational modes are not unique to any specific disordered system and are not of phononic origin as for ordered crystals; instead, they are emergent phenomena resulting from the disorder itself. Consequently, what is the inherent relationship between disorder and low-frequency vibrations? This question has been a central theme throughout research on disordered systems. About fifteen years ago, one of the mainstream views on this issue gradually recognized that these low-frequency vibrational modes are a form of localized vibration. Subsequently, with the advancement of computer simulation technology, some studies predicted that these low-frequency localized vibrations should exhibit a string-like nature and correspond to an emergent mode whose dispersion relation is independent of the wavevector: ''flat''.

Methods and results

The research team can achieve homogeneous and random driving for homemade non-polar particles through a homogeneously driven quasi-two-dimensional vibrating platform. Under vibration of a certain frequency and acceleration, the single particle velocities – both translational and rotational conform to a Gaussian distribution of zero means. Hence, they are referred to as Brownian particles, similar to pollen particles in a thermal bath. The system's structural and dynamic information can be obtained by employing high-precision image processing and particle tracking techniques. The research team distinguished a dispersionless flat mode coexisting with the acoustic phononic branch sound from the dynamical structure factor signal. Subsequently, they found that its frequency corresponds to an anomaly in the vibrational density of states, universally observed in disorder systems and known as the ``boson peak”. Finally, they could associate the vibrational anomaly and the flat mode with randomly distributed string-like dynamical defects, explaining their microscopic origin.

Conclusion

This study validates recent theoretical advances in the study of vibrations of amorphous systems with comprehensive and reliable experimental results. It enriches the characterization and analysis of the glassy state of active granular systems, providing significant insights for the further understanding of the marvelous phenomena brought about by disorder in both passive and active systems.

The research team members include doctoral students Cunyuan Jiang, Zihan Zheng, Dr. Yangrui Chen (Now a postdoc at the University of Minnesota), and Profs. Matteo Baggioli and Jie Zhang. The paper's first author is PhD student Cunyuan Jiang, and the corresponding authors are Prof. Matteo Baggioli and Prof. Jie Zhang. This research was supported by the National Natural Science Foundation of China and the Innovation Program of the Shanghai Municipal Education Commission. The research team sincerely thanks the National Natural Science Foundation of China, the Shanghai Municipal Education Commission, Shanghai Jiao Tong University, the School of Physics and Astronomy, and the Institute of Natural Sciences.

图文编辑：叶丹

责任编辑：叶丹、朱敏

推  荐  阅  读