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## Fluid-Structure Interactions in Soft-Matter Systems: From the Mesoscale to the Macroscale

#### Location: Monash University Prato Centre, Italy, see http://www.ita.monash.edu/

#### Organisers

Overview:

Essentially every non-trivial problem in hydrodynamics involves either boundary conditions, or driving by external or internal forces, or both. From the computational point of view this means that one needs not only a flow solver, but also some method to couple the flow to the boundaries and/or forces. The conceptually simplest case is a non-moving rigid boundary that limits the flow of a Newtonian fluid, and referred to as "fluid-structure interactions" in the engineering literature. More complicated cases arise when the boundary can move and evolves in its geometry, and/or the solid is elastic, and/or the fluid is non-Newtonian. Furthermore, the fluid may undergo phase separation (multiphase flow), and at small length scales thermal fluctuations need to be taken into account - this latter regime is the typical Soft Matter domain. Blood flow, to just mention one well-known example, exhibits many of these complexities.

Many methods have been developed to deal with the resulting computational problems, using quite different approaches. Partly these choices have been dictated by the underlying physics, partly by researchers' experiences and preferences. In the field of Soft Matter the host of available methods is particularly broad. The intended meeting aims at bringing together researchers with different and complementary backgrounds (concerning the physical and computational problems outlined above), who are experts in their respective fields. Apart from physicists, this will involve also chemical engineers and applied mathematicians. The format is planned as a two-day tutorial, followed by a three-day workshop. The workshop part is intended as a forum to exchange experiences and results, with the hope that such a synoptic view will foster the creation of new methods and ideas, and help researchers in solving their problems. The tutorial part will comprise summer-school style lectures which provide an overview of existing methods and the pertaining theoretical background. The main target group for the tutorial are of course young researchers (PhD students, postdocs); however, the tutorial lectures may be also useful for researchers, in order to enable them to follow workshop talks that are somewhat remote from their own expertise.

As a venue we have chosen the Monash University Prato Centre, Italy (see http://www.ita.monash.edu/). Prato is located close to Florence in Tuscany. We hope that, apart from the program itself, this location will make the meeting even more attractive to high-calibre scientists. Beyond this obvious motivation, the choice also offers certain advantages concerning funding, since Monash University has agreed upon sponsoring the use of the facilities.

Essentially every non-trivial problem in hydrodynamics involves either boundary conditions, or driving by external or internal forces, or both. From the computational point of view this means that one needs not only a flow solver, but also some method to couple the flow to the boundaries and/or forces. The conceptually simplest case is a non-moving rigid boundary that limits the flow of a Newtonian fluid, and referred to as "fluid-structure interactions" in the engineering literature. More complicated cases arise when the boundary can move and evolves in its geometry, and/or the solid is elastic, and/or the fluid is non-Newtonian. Furthermore, the fluid may undergo phase separation (multiphase flow), and at small length scales thermal fluctuations need to be taken into account - this latter regime is the typical Soft Matter domain. Blood flow, to just mention one well-known example, exhibits many of these complexities.

Many methods have been developed to deal with the resulting computational problems, using quite different approaches. Partly these choices have been dictated by the underlying physics, partly by researchers' experiences and preferences. In the field of Soft Matter the host of available methods is particularly broad. The intended meeting aims at bringing together researchers with different and complementary backgrounds (concerning the physical and computational problems outlined above), who are experts in their respective fields. Apart from physicists, this will involve also chemical engineers and applied mathematicians. The format is planned as a two-day tutorial, followed by a three-day workshop. The workshop part is intended as a forum to exchange experiences and results, with the hope that such a synoptic view will foster the creation of new methods and ideas, and help researchers in solving their problems. The tutorial part will comprise summer-school style lectures which provide an overview of existing methods and the pertaining theoretical background. The main target group for the tutorial are of course young researchers (PhD students, postdocs); however, the tutorial lectures may be also useful for researchers, in order to enable them to follow workshop talks that are somewhat remote from their own expertise.

Some more details:

In the Soft Matter (or "mesoscopic") domain, there are several important methodologies to treat flow problems or the problem of particles moving in flow: Solving the Navier-Stokes equation directly, Lattice Boltzmann, Multi-Particle Collision Dynamics, Dissipative Particle Dynamics, and Brownian Dynamics.

Some creative solutions for fluid-structure coupling have been found by applying concepts from another set of problems (or even sub-field) to the problem at hand. For example, well-known tricks are the replacement of a boundary condition by an equivalent force or considering a rigid particle as a fluid droplet with high viscosity. Other implementations consider the momentum transfer from fluid to solid rather directly (e.g. the famous bounce-back rule in Lattice Boltzmann), or "smear out" the boundary in position space or in the time domain (by means of a dissipative coupling to the surroundings). Slip lengths can be adjusted by simulating the friction between fluid and wall directly. Elasticity of an immersed body such as a red blood cell is typically taken into account by a particle model. Multiphase flows are often simulated in terms of phase-field models, which in the LB community are typically realized in terms of the Shan-Chen or the Yeomans model.

Further details and registration: Please visit the home page of the meeting,

## References

**Australia**

Ravi Jagadeeshan (Monash University, Melbourne) - Organiser

**Germany**

Burkhard Duenweg (Max Planck Institute for Polymer Research) - Organiser