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Low Reynolds Number Pumps


23 avril 2010

At the microscale, the absence of inertia causes any reciprocal sequence of motions to produce no net thrust. As a consequence, the two most standard propulsion strategies used by microorganisms are the oscillation of flexible flagella and the rotation of rigid helices.

(1) First, I will focus on an intermediate mechanism, where a long flexible filament is rotated in a viscous fluid with a tilt from the rotation axis. The competition between viscous stresses and elastic forces bends the filament into a chiral shape, and this induced helicity generates a propulsive force along the axis of rotation. A non trivial dynamic shape transition will be discussed.

(2) I will then present new results in the beating dynamics of artificial microcilia. The cilia are soft magnetic micro-rods made of a single self-assembled superparamagnetic colloid chain. We introduce a new microfluidic assay that allows us (i) to control the spatial ordering of the micro-rods, (ii) to tune their aspect ratio and (iii) to stop or to accurately control the flow of the surrounding fluid . These assays consist in a magnetic template embedded in a microfluidic circuit comprising monolithic pneumatic valves. The artificial cilia are actuated homogeneously and in three dimensions with a set of 3 Helmholtz coils. We first analyse the precesion of a single soft rod, and characterize its linear and non-linear response to AC actuation. We then focus on the collective dynamics of doublets and of regular arrays.We reveal that, due to strong hydrodynamic interactions, the beating trajectories undergo a discontinuous "structural" transition (see figure below). A minimal theoretical model conveys a simple but insightful picture to understand the observed phenomena.

Figure : Superposition of snapshots of the filaments taken over time (top view). The magnetic actuation field rotates along a cone. From left to right : as the rotation speed increases, the trajectory of the filaments become elliptical and align with the main axis of the array. The pitch between two filaments is 30µm. Filament length : 100µm.

Figure - 19.5 ko