Hydrodynamic interactions in few particle systems
Colloidal particles driven by a constant tangential force along a toroidal laser trap show a characteristic limit cycle. A two-particle cluster catches up with a single particle because the hydrodynamic drag on the cluster is smaller compared to an isolated colloid. Finally, the particles from the surrounding fuid flow, it pushes the front most particle ahead so that the two fron most particles detach from the cluster and the cycle begins again.
When an additional sawtooth potential is applied to the toroidal trap, the particle’s motion alters to a caterpillar-like mode. This behaviour is largely dominated by hydrodynamic interactions. Particulary interesting is the fact, that in contrast to a single particle which is rather tightly trapped inside a potential well, the two-particle clusters still easily surmount the potential barriers.
The experimentally observed caterpillar-like motional behaviour is also observed in numerial calculations which take hydrodynamic interactions into account.
To demonstrate that indeed hydro-dynamic interactions are dominating the caterpillar-like motion we repeated the simulations but now with hydrodynamic interactions turned off. It is clearly seen, that the motional behaviour is entirely different.