Unexpected ordered phases in active matter

Michael Hagan, Associate Professor of Physics and chair of the Biological Physics program at Brandeis, will show that order emerges from this chaos, in the form of heretofore unknown broken-symmetry phases in which the topological defects themselves undergo orientational ordering.

Active matter describes systems whose constituent elements consume energy to generate motion, and are thus intrinsically far-from equilibrium. I will describe computer simulations of two recently developed active matter systems. (1) Self-propelled colloids with repulsive interactions and no aligning interactions are a minimal model active matter system. We and others have shown that, even when particles experience only repulsive interactions, this system undergoes a phase coexistence that mimics the equilibrium phase separation of attractive colloids. I will present a simple kinetic theory that describes the dynamics of phase separation, resulting in a framework analogous to equilibrium classical nucleation theory. (2) Active nematics are liquid crystals which are driven out of equilibrium by energy-dissipating active stresses.  The ordered nematic state is unstable to the proliferation of topological defects, which undergo birth, streaming dynamics, and annihilation to yield a seemingly chaotic dynamical steady-state.  In this talk, I will show that order emerges from this chaos, in the form of heretofore unknown broken-symmetry phases in which the topological defects themselves undergo orientational ordering. Time permitting, I will also show that active nematics are remarkable insensitive to topological constraints even under high confinement.

Professor Michael Hagan

Michael Hagan

Michael Hagan is Associate Professor of Physics and chair of the Biological Physics program at Brandeis. He received a BSE and PhD in Chemical Engineering respectively from the University of Connecticut and University of California at Berkeley. Michael’s lab uses computational modeling and theory to understand the physical principles that control assembly and dynamical organization in biological, biomimetic, and other soft condensed matter systems. Because assembling structures can be orders of magnitude larger than the individual components that comprise them, the lab develops and apply computational and theoretical methods that bridge disparate length and time scales. Applications include understanding the assembly of viral capsids and other large protein complexes, discovering the mechanisms by which proteins interconvert between different conformations, and understanding emergent behaviors in active matter systems (materials whose constituent elements consume energy to generate motion, such as the internal components of a cell).