The aim of the CECAM workshop, where communities of liquid, solid, and bio-active thin films will meet and interact, is twofold. First, we expect the tools developed in one field to help solving problems which arise in other fields. Second, we wish to initiate bridges between the different domains, which may both foster the emergence of new directions of research, and allow for a deeper general understanding. More specifically, we have identified three questions to be addressed during the workshop.

The first focus is on the role of fluctuations. As an example, the edge of spreading films (whether of a fluid, solid, or biological nature) undergoes kinetic roughening. Can we infer information about the underlying dynamics from this type of disorder in the morphology? For example, are violations of the fluctuation-dissipation theorem in active systems reflected in the edge morphology via a characteristic set of roughening exponents? Also, fluctuations are often controlled by the discreteness of the elementary components of the film material. Can we use the same strategies to collectively describe elementary units in the various problems (from micron-size biological cells, down to Angström-size atoms), in order to derive the fluctuations at play in their corresponding macroscopic limits?

The second focus is the effective non-equilibrium boundary condition at the film edge, the so-called triple-line. Much work has been devoted to this question for liquids in the past decades. Indeed, due to the divergence of viscous dissipation at the triple line, dynamics are controlled by a small-scale cutoff (e.g. a slip length, or the thickness of a precursor film). However, there is very little work on this question in the case of solid films and biological films.

The third focus relates to the formation of patterns. For example, is the similarity between branching patterns observed in solid or liquid-state dewetting, and in bacterial colonies fortuitous? What are the implications of the different morphologies for the underlying dynamics in each case? Instabilities can also appear away from the film edge due to elastic effects in solid films, and in bacterial colonies. But do elastic deformations of the film and/or of the substrate, play similar roles in these different contexts?

Furthermore, when focusing on these questions, the workshop attendees will crucially confront their computational strategies. What are the best numerical schemes to implement the lubrication equations, with and without fluctuations? When is the hydrodynamic description more efficient than particle-based computations, such as lattice Boltzmann or molecular dynamics? And when should we use hybrid numerical strategies which solve the film dynamics with continuum models, and local dynamics (at interfaces or at the triple line) via molecular dynamics or discrete (agent-based) cell models? Could hybrid schemes also help to describe the influence of two-dimensional gas-like dilute phases (composed e.g. of isolated cells or of adatoms on solid substrates), interacting with the film edge?

In summary, we expect our workshop to help find innovative solution and new bridges in the non-equilibrium dynamics of thin films in liquid, solids, of bio-active matter, thus starting a more comprehensive understanding of these challenging systems.