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Deformable micro-objects under hydrodynamic forcing

This area concerns the theoretical study of certain physical phenomena occurring at very small micro and nano-scopic scales. The group works on modeling flows within nanotubes. We are particularly interested in dynamic phase transitions in fluids and fluid mixtures. A key aspect is the study of boundary conditions (capillarity problems-molecular statistics, dynamic wetting of fluids on solids, fluid-solid wall interactions). The results can be applied to biological walls. 

Another aspect of this research concerns the numerical simulation of multiphase flows, with phases separated by moving and deformable contact interfaces, a very dynamic field of activity in recent years. Examples range from the flow of immiscible multi-fluid systems, to liquid-solid and liquid-gas phase change. The interfacial dynamics concerned are typically convective transport under the action of various manifestations of surface tension, possibly including heat and mass transfer.
Today, many sectors of activity, mainly related to biology, health and biotechnology, but also process engineering, are raising similar issues, but with interfaces with much richer mechanical behavior. Vesicles and capsules are the subjects of interest, with motivations ranging from individual behavior to suspension rheology. These systems can be described as biomimetic, since they are inspired by the organization of living organisms on a microscopic scale: cells, exchanges and transport. Their study meets objectives as varied as the understanding of living systems, drug vectorization, and the design of microtransporters and microreactors in microfluidic circuits. The general framework of our research, carried out in collaboration with Dr. Leonetti and Dr. Boedec of IRPHE for the experimental and theoretical part, is numerical simulation at very small scales (Stokes regime), for which the complex couplings with other physical, chemical and biological phenomena cannot be ignored.
A vesicle with a characteristic size of 20 microns is a closed lipid membrane whose shape is governed by the bending energy at thermodynamic equilibrium (bending) and the incompressibility stress at the surface, unlike in the case of drops. A non-trivial result is that the shape of the sedimenting drop is unstable and unsteady, whereas that of the vesicle is stationary, with consequences for the stability of emulsions, for example. In the laboratory, we have developed original axisymmetric and 3D models, based on boundary element methods, which allow accurate representation of all vesicle characteristics (G. Boedec, M. Leonetti and M. Jaeger, J.Comp.Phys. 230 (2011) 1020).
Left – a settling vesicle is so soft that a microtether (tubes) emerges at a submicronic scale (numerical simulation). Right – Red Blood Cells (RBCs) change their shapes in blood vessels due to hydrodynamical stresses.
A semi-implicit time discretization combined with an automatic mesh adaptation technique enables the study of vesicles in very large deformation regimes. The computational code is currently being optimized on HPC machines to take into account flows with large numbers of vesicles. In addition, other structures such as cytoskeletons will be taken into account to model more realistically shaped cells.

Contact : Marc Jaeger 

marc.jaeger@centrale-marseille.fr