报告题目：Shaping active matter: Symmetry, confinement, and effective interactions
报告人：Ignacio Pagonabarraga 教授复杂系统研究所主任
University of Barcelona, Spain
报告时间：2025年11月14日（周五）下午3:00-4:00
报告地点：物理科技楼 101室
报告摘要：Active matter comprises systems of self-driven units that consume energy locally to generate motion and mechanical stresses. Unlike passive systems, their behavior lacks detailed balance and is governed by persistent energy dissipation at the microscale. Understanding the self-organization of such systems requires a dynamical framework that accounts for the coupling between self-propulsion, interparticle interactions, and the transport of momentum, orientation, and density.
In this talk, I will examine a class of minimal active matter models where mobile particles interact through alignment rules analogous to those of the Kuramoto model. These models capture the essential features of orientational ordering in active suspensions and allow us to explore how nonequilibrium driving modifies phase behavior and mechanical response. In particular, I will discuss systems with nematic alignment interactions, where polarity is absent. While equilibrium systems with XY symmetry in two dimensions exhibit only quasi-long-range order through a Kosterlitz-Thouless transition, I will show that particle motility can induce true long-range orientational order. The dynamics and interactions of topological defects are also qualitatively altered, leading to broken reciprocity and long-range defect-mediated correlations.
I will also present the impact that chirality, has on active matter. I will address a system where motion emerges collectively from breakdown of mechanical equilibrium due to unbalanced momentum transfer between rotational and translational modes. The resulting imbalance in internal pressure and stress leads to spontaneous density segregation between a low-density phase and a mechanically active dense fluid. This phenomenon demonstrates how sustained angular motion and stress anisotropy can drive nonequilibrium phase transitions in the absence of conventional thermodynamic potentials.
These findings illustrate how self-propulsion, orientational alignment, and dynamical symmetry breaking conspire to produce complex structure and behavior in active matter. They point toward general principles for understanding nonequilibrium assembly and transport in systems ranging from synthetic colloidal swimmers to biological collectives.
References
[1] J. Diaz, I. Pagonabarraga, ‘Activity-driven emulsification of phase-separating binary mixture’, Phys. Rev. Lett. 134, 098301 (2025)
[2] J. Codina, B. Mahault, H. Chat´e, J. Dobnikar, I. Pagonabarraga, X.-Q. Shi, ‘Small obstacle in a large polar flock’, Phys. Rev. Lett. 128, 218001 (2022)
[3] H. Massana-Cid, D. Levis, R.J. Hernandez Hernandez, I. Pagonabarraga, P. Tierno, ‘Arrested phase separation in chiral fluids of colloidal spinners’, Phys. Rev. Res. 3, L042021 (2021)
[4] J. Fries, J. Diaz-Branas, M. Jardat, , I. Pagonabarraga, P. Illien, V. Dahirel, ‘Active droplets controlled by enzymatic reactions’, J R Soc Interface 22, 20240803 (2025)
[5] P. Digregorio, I. Pagonabarraga, F. Vega Reyes, ‘Phase separation in a chiral active fluid of inertial self-spinning disks’ (submitted)
报告人简介：
1996 PhD degree in Physics at University of Barcelona, Spain
1996-1998 Marie-Curie fellow at Institute Amolf, Amsterdam (The Nethelands)
1999-2000 Post doctoral fellow: University Edinburgh, UK
2001-2010 Assistant Professor, University of Barcelona, Spain
2011- Full Professor, Physics Faculty, University of Barcelona, Spain
2017-2022 CECAM Director
2017-2022 Professor, EPFL, Lausanne, Switzerland
2024 - UBICS Director, University of Barcelona, Spain
Ignacio Pagonabarraga studied at the University of Barcelona (UB) where he obtained his PhD in Physics in nonequilibrium statistical physics in 1996. He performed postdoctoral stays at Instituut Amolf (Amsterdam, the Netherlands) and University of Edinburgh (Scotland, UK) and moved back to Barcelona. He is Professor of Physics at the Department of Condensed Matter Physics in UB since 2011. He has been the CECAM Director, with headquarters in the EPFL (Lasuanne, Switzerland) in the period 2017-2022 and is the Director of the Institute of Complex Systems at UB (UBICS) since 2024. His research focuses on the theoretical understanding of soft matter systems out of equilibrium, with particular interest in collective phenomena in active matter, aiming at reaching fundamental new understanding of the mechanics and emerging behavior in such systems. He develops and exploits novel computational methods to study these complex, heterogeneous dynamic systems.
