Microrheology versus macrorheology of active microswimmers suspensions – MicMacSwim

In nature, organisms that can propel themselves in a fluid medium are ubiquitous. While larger organisms, such as fish, use inertia in their motion, microorganisms like spermatozoa, microalgae or bacteria move at low Reynolds number, where viscous forces dominate over the effects of inertia. A recent and currently unresolved issue involves understanding the hydrodynamics associated with the individual or collective motion of such microswimmers through their fluid-mediated interactions. These self-propelled particles create a force multipole either at the front of the body, in which case they are called pullers, or at the back, in which case, they are pushers. The flow induced by the force multipole is responsible for hydrodynamic interactions between swimmers that are expected to have a dramatic effect on their dynamics and particularly on the rheology of the suspension. As an example, microswimmer suspensions have been shown to lead to complex dynamics such as the so-called weak turbulence or bioconvection phenomenon.

Here we propose a fundamental study – experimental and numerical – of flow properties and of the rheology of this new kind of complex fluid: active suspensions, i.e. suspensions of microswimmers. Our proposal focuses on the development of active microrheology of such an active suspension and on the comparison of the obtained results with our previously acquired knowledge of the macrorheology of the system. Besides fundamental understanding of the relationship between micro and macrorheology data, our aim is to develop a new tool for the study of the rheology of small quantities of active fluids. This should lead to numerous applications, for instance in fertility analysis.