The goal of this colloquium is to bring together two communities that are both interested in the complex flows of dense suspensions of objects: drops and bubbles for one community and cells, lipid vesicles and capsules for the other. The aim is at a better understanding the behavior of dense flows, that qualitatively departs from dilute flows and second, the development of active manipulation of these soft objects within microfluidic devices, for lab-on-chip applications.

Biological conditions (such as red blood cells in blood circulation) or efficiency requirements (in a droplet/bubble generator) indeed imply to consider concentration levels at which particular interactions between objects have to be considered. Interactions between objects modify their trajectories and cause shear-induced diffusion. Additional interaction with confining walls or adhesion controls the spatial organization of the suspensions. This often results in non-Newtonian rheological behavior, up to jamming/clogging when concentration increases.

The recent advances in the handling or the simulation of isolated soft objects have brought a clear insight into their dynamics under flow. This knowledge can now be used as a basis for a bottom-up approach for a better understanding the dense flows of these objects in mildly confining geometries up to channel networks of complex geometry, such as blood capillary networks, porous media, and microfluidic chips.

On the other hand, the multi-body and long ranged interactions, coupled with the deformability of the interfaces, have raised new challenges for numerical methods developers aiming at describing quantitatively the complexity of these flows. The consequent cooperative flow behavior requires the development of new theoretical concepts taking into account this multiscale rheology.