Hydrodynamics of nanomachines in biology

Abstract

In the hydrodynamic environment of biological microorganisms inertia is irrelevant and all motion is dominated by friction. As a consequence, concepts like hydrodynamic interactions and the need for non-reciprocal motion for generating propulsion become important. These effects are mostly unknown on the human scale and require a different approach towards the problem how a micro-swimmer achieves locomotion. The present works aims to shed some light on the relevance of the hydrodynamic environment that caused the evolution of the marvelously designed swimming apparatuses employed by many bacteria, such as E. coli, paramecium and also spermatozoa. To achieve this task, computer simulutions are performed based on a model filament which resembles inherently active biological filaments but is much simpler and experimentally realisable at the same time. Therefore, a numerical model of the superparamagnetic filament used by Dreyfus et al. for building the first man-made micro-swimmer (Nature 437, 2005) is adapted. Motived by this success, the one-armed swimmer of Dreyfus et al. consisting of a viscous load attached to a biomimetic flagellar tail is studied. Besides an excellent agreement between experimental and simulation data, the latter lead to a profound and comprehensive understanding of the dynamics and suitable operating modes of such a system. Furthermore, a second system with a model cilium attached to a wall is investigated. The dynamics of this system is heavily influenced by the complicated effects of hydrody- namic interactions close to a bounding wall, which are considered to a good approximation in our simulations. We study two ways of generating an asymmetric beating cycle with a magnetic actuation technique. Both are successful in that they show that surrounding fluid is indeed transported along the wall by the beating of the model cilium.