Active Matter at Surfaces and in Complex Environments

Scientific report on the workshop
Active Matter at Surfaces and in Complex Environments (Dresden, 19 - 23 June 2023)

Aim and focus: Active matter represents a new class of non-equilibrium soft condensed matter where energy extraction and consumption take place at the level of individual microscopic constituents. Examples include unicellular microorganisms and synthetic active colloids. This workshop was organized in response to the growing interest concerning active matter in complex and inhomogeneous environments. Here, the notion of “complex” applies to either the geometric confinement, the fluid host medium, or the combination of both. The interest in complexity arises from at least two directions: challenges in applications (for instance, bio-fluid viscoelasticity, ambient flows, and fluid/solid interfaces are ubiquitous in vivo), and as a new means to control active matter systems (e.g., with chemically and topographically patterned interfaces). Thus, specific topics considered including confined active nematics, synthetic active colloids at interfaces, biological microswimmers in complex fluids and porous media, and micro-rheology of active suspensions. The underlying common feature in all these examples is that a wealth of length and time scales are introduced by the hierarchical structures of the environment and by the intrinsic structure and dynamics of the active fluids under consideration. Understanding these phenomena requires highly innovative approaches, which synergistically combine experimental, theoretical, and computational tools. Therefore, the organizers sought to balance the program between these three areas. Moreover, age, gender and geographical diversity were given explicit consideration in selection of talks. Out of thirty-nine talks, six were given by female researchers, and six by junior researchers (Ph.D. students, postdocs, and assistant professors / junior group leaders). The conference had participation from the EU, the USA, the UK, Canada, India, China, Singapore, Chile, and Brazil.

Most important participants: Since many leading researchers participated in the workshop as invited speakers, it is difficult to single out specific contributions. However, the workshop revealed some emerging directions in active matter, which can be connected with individual senior speakers. These emerging directions include perception/cognition and sensorial delay (Bechinger, Gompper, Cichos), active unjamming (Dijkstra), and patterned motility landscapes (Stark, DiLeonardo). One highlight was the colloquium address, delivered by Jacques Prost. This talk provided a tour of seminal developments in cell motility and tissue dynamics, culminating in today’s open scientific questions.

Newcomers: The talks and posters from junior researchers were of high quality. It was clarifying to have some important recent articles and pre-prints explained by their first authors. Some specific examples include the talks on “entropons,” a newly identified class of non-equilibrium collective excitations (Caprini); motility and scission of an active fluid droplet on a planar surface (Coelho); and flows driven by interfacially active “carpets” (Guzmán-Lastra).
Several junior researchers commented on the positive career implications of networking with senior researchers, e.g., for securing referee letters for tenure/promotion, or for obtaining postdoctoral positions.

Scientific results in the broader sense: The workshop achieved lively discussion after the talks and in poster sessions. Each talk stimulated at least several questions from the audience, with continued discussion frequently deferred to coffee breaks to keep the program on track. Several participants observed that the program of talks achieved a good balance between theory and experiment, and between various topics in active matter. It was also noted that the meals offered ample time for discussion. According to one of the MPI-PKS staff, there was excellent mixing and networking between participants in the coffee breaks. Expected outcomes of the conference include development of new research collaborations and cross-fertilization of ideas between different subareas of active matter. The workshop also underscored the need for enhanced communication and collaboration between experimentalists, computationalists, and theorists, given the intrinsic complexity and multiscale character of the systems under consideration. Scientific discussions initiated at this workshop are likely to continue at future meetings.