Physics Colloquia: Active Matter: The Physics of Self-Organization

Birds flock, bees swarm and fish school. These are just some of the remarkable examples of collective behavior found in nature. Physicists have been able to capture some of this behavior by modeling organisms as “flying spins” that align with their neighbors according to simple but noisy rules. Successes like these have spawned a field devoted to the physics of active matter – matter made not of atom and molecules but of entities that consume energy to generate their own motion and forces. Through interactions, collectives of such active particles organize in emergent structures on scales much larger than that of the individuals. The paradigm of such behavior are living systems: motor proteins orchestrate the organization of genetic material inside cells, swarming bacteria self-organize into biofilms, cells migrate collectively to fill in wounds and to coordinate the shape and function of organs and organisms. But the remarkable behaviors of living matter have also been mimicked in the lab through engineering a variety of “active particles” that self-assemble to form smart materials. These synthetic analogues provide the opportunity for controlled studies and test of theoretical efforts.

 

In this lecture I will introduce the field of active matter and highlight ongoing efforts by physicists, biologists, engineers and mathematicians to model the complex behavior of these systems, with the goal of identifying universal principles. I will specifically focus on two examples of active behavior. The first highlights how active particles bypass the laws of equilibrium thermodynamics and spontaneously aggregate in the absence of any attractive interactions. The second describes the interplay of flow and topological defects in controlling dynamics and structure of active phases with liquid crystalline order, with relevance on scales from subcellular to entire organisms.

                                       

Cristina Marchetti is a Professor of Physics at the University of California Santa Barbara, where she joined the faculty in 2018, after about thirty years on the faculty at Syracuse University. She is a theoretical physicist who has worked on a broad range of problems in condensed matter and biological physics, including supercooled fluids, transport in disordered media, superconductors, cell mechanics, and out-of-equilibrium systems. Most recently, Marchetti has played a leading role in the development of the field of active matter. The active matter paradigm is relevant to phenomena on many scales, from the control of human crowds to the collective migration of epithelial cells in wound healing. It has additionally paved the way to the development of synthetic analogues that may serve as the base for the development of materials with life-like functions.

 

Marchetti is a Fellow of the American Physical Society and the American Association for the Advancement of Science, a member of the American Academy of Arts and Sciences and the US National Academy of Sciences.  She has served on elected positions within the American Physical Society, including chair of GSNP and of DSOFT, and as member-at-large of DCMP. She is currently co-lead editor of the Annual Reviews of Condensed Matter Physics and previously served as co-Lead Editor of Physical Review X. She was awarded a Rotschild-Mayent Fellowship at the Institut Curie, a Simons Fellowship in Theoretical Physics, and the inaugural 2019 Leo P. Kadanoff Prize by the American Physical Society for ``original contributions to equilibrium and non-equilibrium statistical mechanics, including profound work on equilibrium and driven vortex systems, and fundamental research and leadership in the growing field of active matter.”