This workshop is a satellite of the Joint 12th ESBA 10th ICBP-IUPAP Biophysics Congress held in Madrid (20-24 July 2019). We therefore subscribe to the statement on free circulation of scientists in addition to CECAM's own policies.
Introduction to Soft-Matter and Biophysics in Complex Media
Recent years have seen an upsurge in the study of active matter, which deals with collections of particles that can transform (internal) sources of energy into directed motion . Readily available realisations of such systems are suspensions of motile microorganisms, like bacteria or algae, and a significant research effort has been devoted to understanding the mechanisms of their propulsion, mechanical properties of their suspensions, and collective motion exhibited by some of these organisms . Most of this work was performed in simple fluids, like water, and this has lead to greatly improved understanding of the impact of Newtonian hydrodynamics on the motion of swimmers and other (living) active components.
However, there is a growing body of experimental work on self-propulsion of biological organisms and synthetic swimmers in viscoelastic, shear-thinning, and other complex fluids, which shows that the obtained insights cannot be readily transferred to fluids with non-Newtonian response . These experiments further demonstrate a plethora of behaviours at seemingly similar conditions. Complex fluids encountering out-of-equilibrium particles is a ubiquitous scenario in rheological and engineering settings. Moreover, many fluids inside living organisms have a nonlinear rheological response, for example: mucus, blood, vitreous humor, and cytoplasm. Their rheology plays a crucial role in defending the body from invading organisms, but also has an impact on fertility and transport. Finally, living components are increasingly integrated into industrial processes, and it is, therefore, crucial to understand how the rheological response of the suspending fluid impacts the behaviour of microswimmers, and driven particles.
Presently, there exists no unified theoretical framework to understand this phenomenology, despite concentrated effort to establish such a description . Due to the strongly non-linear nature of the fluids involved, theoretical progress can only be achieved through the use of and comparison to large-scale numerical simulations. Several efficient methods to simulate motile objects in such fluids have seen recent development, including the Finite Element Method (FEM) , the Lattice-Boltzmann (LB) algorithm , and Multi-Particle Collision Dynamics (MPCD) . However, there remain many open questions concerning the strengths and weaknesses of different methods, for example: their numerical stability, the range of application of continuum and particle-based (explicit) descriptions, the types of non-Newtonian response that is captured and how to relate this to real-world systems, and the ways to verify these methods against theoretical and experiment results.
Aim of the Workshop
The aim of this workshop is to bring together experimentalists, analytical theorists, and computational scientists to formulate a community-wide research program to address the open questions in theoretical and numerical modeling of out-of-equilibrium systems in viscoelastic media. The workshop will specifically address the following:
- Recently, experimental techniques have substantially improved and now allow for accurate analysis of the effects of the medium’s rheology on the behavior of immersed microorganisms . We have invited experimental world leaders in studying out-of-equilibrium systems in complex fluids to discuss their latest research and give their views on future development in this field. This will provide a background against which ideas can be developed during the meeting. In addition, we will identify open problems where input from theory and simulation is necessary to make progress. Exciting topics include: (i) The coupling between rheology and motion, i.e., how does a microorganism modify its motion to account for changes to its environment ? (ii) Accurate modeling of the coupling between translation and rotation for spherical objects at low Weissenberg numbers  and the consequences for particle-boundary and particle-particle interactions.
- There are various numerical approaches for the study of out-of-equilibrium systems in non-Newtonian fluids. We have invited experts of the three main branches: FEM, MPCD, and LB, to provide an overview of the state of the art in these respective approaches, and their limitations. This will be the starting point for discussion on their respective strengths and weaknesses, as well as methods to facilitate comparison between the algorithms. Specifically: (i) What are the limitations of continuum stress-strain relations when describing systems, where the length scales of the moving elements becomes comparable to the size of the, e.g., polymers that induce viscoelasticity? (ii) How do the explicit models link up to the continuum descriptions? (iii) What are the performance bottlenecks of the various algorithms?
- We have also invited top researchers involved in analytic theory for out-of-equilibrium systems in complex fluids. Their input will prove invaluable to the discussion on benchmarking the various numerical methods available, as well as how experimental problems can be targeted by a combined analytic and numerical effort. Of strong interest in this regard is the issue of verifying numerical techniques far from the linear regime, where analytic approaches can make only limited progress.