Fernando Peruani
Professor at the Laboratory of Theoretical Physics and Modeling (LPTM), CY Cergy Paris University (CYU)

“Challenges in Active Matter”

Active systems are studied in large number of disciplines and include, on the one hand, artificial out-of-equilibrium systems such as synthetic swimmers, phoretic colloids, and active rollers, and on the other hand, biological systems at various scales: e.g. suspensions of bacteria, insect swarms, and sheep herds, among many other examples. In particular, active matter theory emerges as a novel, powerful, theoretical branch of non-equilibrium statistical physics that will help to elucidate complex processes of key importance not only in physics, but also in biology and medicine, for instance in relation to wound healing, tumor growth, and embryogenesis.

Our current theoretical understanding of active matter is based on two paradigmatic mechanisms. (i) The so-called velocity alignment mechanism that exploits the analogy to spin systems to explain self-organized patterns in active systems and constitutes the cornerstone of Vectorial Active Matter. And (ii), motility-induced phase separation, a central element in Scalar Active Matter, that makes use of the analogy between classical and active phase separation by assuming an effective coupling between local density and local particle speed. It is worth noting that these two mechanisms are the results of theoretical speculations — exploiting analogies with classical physical models — formulated before the realization of specifically designed, quantitative, active-matter experiments. Several fundamental questions need to be addressed. Are these mechanisms robust? What are the limitations of descriptions based on these concepts? And finally, are there alternative mechanisms that produce similar macroscopic patterns? In this talk, we will visit the foundation of Active Matter Theory and address these issues.