Hierarchical and Cascading Cooperative Rearrangements in Dense Colloidal Suspensions

Arising from particle-level interactions—including surface potentials, entanglements, and excluded volume effects—cooperative dynamics (CD) are ubiquitous in soft matter systems such as colloidal suspensions, gels, and polymers. Understanding CD is essential, as it plays a critical role in transient phenomena such as yielding, phase transitions, and jamming, which fundamentally govern the rheological properties of disordered materials. However, conventional approaches that assume uniform and continuous dynamics often overlook CD, leading to misinterpretation of data obtained from X-ray Photon Correlation Spectroscopy (XPCS) experiments. To address this limitation, we present an AI-assisted analysis workflow that automatically detects and tracks CD, enabling quantitative extraction of key dynamic features. Validated through both theoretical modeling and experimental measurements, this approach reveals that cooperative dynamics are inherently intermittent and spatially heterogeneous. Our results further uncover a hierarchical, cascading nature of CD, where small-scale fluctuations in particle motion can trigger bursts of cooperative rearrangements throughout the relaxation process. As next-generation synchrotron sources enable access to finer spatiotemporal details, this work offers a robust and effective tool for probing subtle CD across a range of rheological processes. By facilitating the extraction and interpretation of these dynamics, it paves the way for linking microscopic cooperative rearrangements to transient phenomena and predicting macroscopic behavior in soft matter systems.