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DFG Priority Programme SPP1726 "Microswimmers"

Swimming of deformable microcapsules

Jan Kierfeld
students: Horst-Holger Boltz, Hendrik Ender, Christian Wischnewski

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 or sedimenting 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 hysteretic shape changes (e.g. snapping or buckling) as a mechanical swimming mechanism. For elastic swimmers with hysteretic shape transitions, completely periodic parameter  changes (swelling and shrinking, volume increase and decrease) can give rise to non-time-reversible shape changes. This novel swimming cncept mechanism will be investigated theoretically.

Moreover, capsules can be used as containers to store "fuel". We investigated surfactant-filled capsules which are very effective Marangoni swimmers or surfers at the liquid-air interface. Even if they are spherically symmetric, advection can give rise to spontaneous symmetry breaking and swimming.



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Location & approach

The campus of TU Dort­mund University is located close to interstate junction Dort­mund West, where the Sauerlandlinie A 45 (Frankfurt-Dort­mund) crosses the Ruhrschnellweg B 1 / A 40. The best interstate exit to take from A 45 is “Dort­mund-Eichlinghofen” (closer to South Campus), and from B 1 / A 40 “Dort­mund-Dorstfeld” (closer to North Campus). Signs for the uni­ver­si­ty are located at both exits. Also, there is a new exit before you pass over the B 1-bridge leading into Dort­mund.

To get from North Campus to South Campus by car, there is the connection via Vogelpothsweg/Baroper Straße. We recommend you leave your car on one of the parking lots at North Campus and use the H-Bahn (suspended monorail system), which conveniently connects the two campuses.

TU Dort­mund University has its own train station (“Dort­mund Uni­ver­si­tät”). From there, suburban trains (S-Bahn) leave for Dort­mund main station (“Dort­mund Hauptbahnhof”) and Düsseldorf main station via the “Düsseldorf Airport Train Station” (take S-Bahn number 1, which leaves every 20 or 30 minutes). The uni­ver­si­ty is easily reached from Bochum, Essen, Mülheim an der Ruhr and Duisburg.

You can also take the bus or subway train from Dort­mund city to the uni­ver­si­ty: From Dort­mund main station, you can take any train bound for the Station “Stadtgarten”, usually lines U41, U45, U 47 and U49. At “Stadtgarten” you switch trains and get on line U42 towards “Hombruch”. Look out for the Station “An der Palmweide”. From the bus stop just across the road, busses bound for TU Dort­mund University leave every ten minutes (445, 447 and 462). Another option is to take the subway routes U41, U45, U47 and U49 from Dort­mund main station to the stop “Dort­mund Kampstraße”. From there, take U43 or U44 to the stop “Dort­mund Wittener Straße”. Switch to bus line 447 and get off at “Dort­mund Uni­ver­si­tät S”.

The AirportExpress is a fast and convenient means of transport from Dortmund Airport (DTM) to Dortmund Central Station, taking you there in little more than 20 minutes. From Dortmund Central Station, you can continue to the university campus by interurban railway (S-Bahn). A larger range of international flight connections is offered at Düsseldorf Airport (DUS), which is about 60 kilometres away and can be directly reached by S-Bahn from the university station.

The H-Bahn is one of the hallmarks of TU Dort­mund University. There are two stations on North Campus. One (“Dort­mund Uni­ver­si­tät S”) is directly located at the suburban train stop, which connects the uni­ver­si­ty directly with the city of Dort­mund and the rest of the Ruhr Area. Also from this station, there are connections to the “Technologiepark” and (via South Campus) Eichlinghofen. The other station is located at the dining hall at North Campus and offers a direct connection to South Campus every five minutes.

The facilities of TU Dortmund University are spread over two campuses, the larger Campus North and the smaller Campus South. Additionally, some areas of the university are located in the adjacent “Technologiepark”.

Site Map of TU Dortmund University (Second Page in English).