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DFG Priority Programme SPP1726 "Microswimmers"

Swimming of deformable microcapsules

Jan Kierfeld
students: Horst-Holger Boltz, Hendrik Ender

Elastic capsules and shells

Elastic shells are thin (quasi two-dimensional) elastic solids in a curved geometry. Of particular interest in soft matter theory and applications are closed spherical elastic shells, which can also be termed elastic capsules and which enclose a liquid of given volume or pressure. On the microscale, capsules have received a lot of attention as delivery systems and as biologically relevant model systems. Prominent examples of biological microcapsules are red blood cells or virus capsules; more generally, all biological cells where a thin layer of elastic cytoskeleton is surrounding the liquid cytosol and participating in deformations can be viewed as elastic capsules. If a coupling to the cytoskeleton is absent, a description of the membrane as a quasi two-dimensional liquid is more appropriate, and we obtain a vesicle rather than a microcapsule. Artificial capsules can be fabricated by various methods, for example, by interfacial polymerization at liquid droplets or by multilayer deposition of polyelectrolytes, and have numerous applications as delivery systems.

Swimming of deformable microcapsules

This research project is focused on the theoretical investigation of self-propelling deformable microcapsule swimmers, which serve as simple model systems for biological cells. Two problems, which occur generically by the interplay of self-propulsion and the resulting hydrodynamic forces and of deformability of such elastic microcapsules will be addressed:

  1. Because of hydrodynamic forces, a self-propelled microcapsule will deform during swimming, which changes in turn the velocity field of the surrounding fluid and, thus, the resulting swimming velocity. We will develop theoretical and simulation methods in order to calculate the effect of elastic microcapsule deformation on swimming shape and velocity for simple self-propulsion mechanisms by solving the coupled hydrodynamic equations of the fluid and the elastic capsule shape equations simultaneously.
  2. A deformable microcapsule can use cyclic shape changes as a mechanical swimming mechanism. For spatially asymmetric microcapsules, cyclic swelling and shrinking can give rise to such cyclic shape changes. This swimming mechanism will be investigated theoretically (i) based on hydrodynamic and shape equations and (ii) by using simplified discrete model systems for capsules coupled to a fluid, which is simulated using Multiple Particle Collision Dynamics (MPCD).