Phase transitions and self assembly in active matter

Lecture by Prof. Ignacio Pagonabarraga, Department of Condensed Matter Physics, University of Barcelona

Date: March 7, 2023, 14:15 h

Abstract: Flocks of birds, schools of fishes, or bacterial colonies constitute examples of living systems that coordinate their motion. In all these systems their constituent elements generate motion due to energy consumption and can exchange information or react sensitively to chemical cues in order to move together or to react collectively to external signals. Artificial systems, such as nanorobots, exploit the  heterogeneous compositions of their surface to displace  as a result of the heterogeneous chemical processes that take place in the presence of appropriate chemical substances. 

All these systems are intrinsically out of equilibrium in the absence of any external driving. Therefore, their  collective properties result as a balance between their direct interactions and the indirect coupling to the medium in which they displace, and a self-consistent  dynamical approach is required to analyze their evolution.The mechanical balance that determines the  states they develop spontaneously  make these systems very versatile  and have a natural tendency to for large scale aggregates. However, an understanding on the basic principles underlying the emergence and self-assembly on active systems poses fundamental challenges.  How do the relevant  entities interact with each other? Can we identify universal, generic principles associated to the main features in the self assembly and emergent behavior of intrinsically out of equilibrium systems?Are there mechanisms that can be shared by  living systems and synthetic, active materials? 

I will consider simple statistical   models to address fundamental questions associated to these systems and will analyze the implications the generic   self-propulsion has in the emergence of structures in suspensions of model self-propelled particles. I will  discuss the potential of schematic models to address fundamental questions that still remain open, such as the connection of  the effective phase diagram and pressure with effective equilibrium concepts. These approaches allow to understand the transformations that characterize these systems as effective phase transitions out of equilibrium.  

In general, the  steady state that characterizes these systems requires a consistent dynamic treatment. I will analyze the role that the medium in which active colloids displace has in generating correlations among active particles them. Iwillconsider different mechanisms, such as swimming or heterogeneous catalysis, that lead to spontaneous self-organization in the absence of external driving for a variety of active suspensions. Such a  comparison will help to discern between specific ingredients and general features determining the emergent properties of active matter.  

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