laboratoire de physique statistique
 
 
laboratoire de physique statistique

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PHYSICAL REVIEW FLUIDS 


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2016
Destabilization of a flow focused suspension of magnetotactic bacteria - Waisbord, Nicolas and Lefevre, Christopher T. and Bocquet, Lyderic and Ybert, Christophe and Cottin-Bizonne, Cecile
PHYSICAL REVIEW FLUIDS 1 (2016)

Abstract : Active matter is a new class of intrinsically out-of equilibrium material with intriguing properties. The recent upsurge of studies in this field has mostly focused on the spontaneous behavior of these systems. Yet, many systems evolve under external constraints and driving forces, being subjected to both flow and various taxis. We present a new experimental system based on the directional control of magnetotactic bacteria which enables quantitative investigations to complement the challenging theoretical description of such systems. We explore the behavior of self-propelled magnetotactic bacteria as a particularly rich and versatile class of driven matter, whose behavior can be studied under a range of hydrodynamic and magnetic field stimuli. In particular we demonstrate that the competition between cell orientation toward a magnetic field and hydrodynamic flow lead not only to jetting, but to a new pearling instability. This model system illustrates new structuring capabilities of driven active matter.
Large-scale instabilities of helical flows - Cameron, Alexandre and Alexakis, Alexandros and Brachet, Marc-Etienne
PHYSICAL REVIEW FLUIDS 1 (2016)

Abstract : Large-scale hydrodynamic instabilities of periodic helical flows of a given wave number K are investigated using three-dimensional Floquet numerical computations. In the Floquet formalism the unstable field is expanded in modes of different spacial periodicity. This allows us (i) to clearly distinguish large from small scale instabilities and (ii) to study modes of wave number q of arbitrarily large-scale separation q << K. Different flows are examined including flows that exhibit small-scale turbulence. The growth rate sigma of the most unstable mode is measured as a function of the scale separation q/K << 1 and the Reynolds number Re. It is shown that the growth rate follows the scaling s. q if an AKA effect [Frisch et al., Physica D: Nonlinear Phenomena 28, 382 (1987)] is present or a negative eddy viscosity scaling sigma alpha q(2) in its absence. This holds both for the Re << 1 regime where previously derived asymptotic results are verified but also for Re = O(1) that is beyond their range of validity. Furthermore, for values of Re above a critical value Re-S(c) beyond which small-scale instabilities are present, the growth rate becomes independent of q and the energy of the perturbation at large scales decreases with scale separation. The nonlinear behavior of these large-scale instabilities is also examined in the nonlinear regime where the largest scales of the system are found to be the most dominant energetically. These results are interpreted by low-order models.