Suspensions of micro-swimmers : Collective dynamics and rheology
Séminaire G. SUBRAMANIAN
Lundi 27 juin à 10 h 30 Amphithéâtre Nougaro
The last two decades have seen a flurry of research activity into the nature of active matter. Although active matter can, in a general sense, refer to a whole range of materials that are out of equilibrium, and where the input of energy occurs at the level of a single microscopic entity, in this talk, I will focus on a specific realization – suspensions of micron-sized swimmers (bacterial suspensions). Experiments have revealed these suspensions to be quite different from their passive counterparts. In particular, both experiments and simulations have indicated the existence of large-scale collective motion at concentrations corresponding to a hydrodynamic volume fraction of order unity. More recently, experiments have begun to probe the rheology of these low-viscosity fluids, revealing novel stress relaxation features that include a regime of negative apparent viscosities. This has led to the somewhat provocative terminology of ‘bacterial superfluids’.
I will discuss our group’s efforts in identifying, both theoretically and via numerical simulations, the threshold for the onset of collective motion (interpreted as a hydrodynamic instability) in a suspension of micron-sized swimmers. Next, I will focus on a linear response theory that predicts the stress relaxation in a dilute swimmer suspension including the negative viscosity signature. While the theory agrees qualitatively with experimental observations, there are quantitative discrepancies that raise important questions some of which I will discuss. I will then proceed the future directions that we intend to take, highlighting some of the open questions in this area.
G. Subramanian -http://www.jncasr.ac.in/sganesh/publication.php