Soft Matter Seminar: "Order in Disorder: Hidden Geometries and Exotic Phases in Cellular Structures"

The Department of Physics Soft Matter Group is pleased to welcome Dapeng Max Bi, associate professor of physics at Northeastern University, for his talk titled, "Order in Disorder: Hidden Geometries and Exotic Phases in Cellular Structures."

Abstract: Biological tissues are often framed simply as fluids or solids, creating a binary that overlooks rich intermediate structures. In this talk, I present two theoretical advances revealing how tissues utilize activity and mechanics to access exotic geometric orders. First, I discuss the emergence of the hexatic phase—orientational order without translational order. We show that cell division, typically a disordering force, drives hexatic order when coupled with motility. This occurs via a "defect healing" mechanism where motility actively binds disclinations into dislocations. Second, I examine hyperuniformity—a hidden global order suppressing density fluctuations. We demonstrate that hyperuniformity exists across a spectrum of tissue states, distinct from mechanical rigidity, and introduce the "Hyperuniform Poisson Ensemble" (HyPE) as a thermodynamic lower bound. These findings highlight how tissues decouple rigidity from structure, offering blueprints for active metamaterials.