Transport in microscale flows

In small scale flows, fluid motion is dominated by viscosity, and so are the fluid forces experienced by colloid particles, bubbles, droplets and biological micro-organisms, which profoundly impacts their passive or active transport. Such processes present many fundamental biological and engineering challenges, e.g. to understand the mechanical interaction of living systems with their fluid environment or to control the motion and deformation of artificial systems, for the textile industry or to design new active fluids with controllable properties.

Individual and collective motion of active microparticles

Sébastien Michelin

By setting the fluid around them into motion, biological and artificial microswimmers alike modify the effective macroscopic properties of their environment (e.g. viscosity, mixing). An example of such ``active’’ fluids are catalytic particles, which convert the physico-chemical energy of a fuel present in their environment into mechanical energy. Our work focuses on the modeling and analysis of the complex interplay between hydrodynamics and physical chemistry, at the heart of the self-propulsion and of interaction of these particles. Our goal is to characterize and control the macroscopic properties of such active suspensions.

Symmetry-breaking and propulsion of a spherical catalytic particle in a self-generated gradient of chemical solute. The solute is released isotropically with a constant flux from the particle’s surface. When its advection by the flow field prevails over molecular diffusion, an asymmetry of the chemical field can be created by this non-linear hydro-chemical coupling, which maintains the self-propulsion of the particle.

Transport of flexible fibers

Camille Duprat

The transport in viscous flows of rigid or flexible elongated objects such as fibers is important for a variety of small-scale applications, from swimming micro-organisms and biofilm streamers in blood capillaries to fabrication of non-woven fibrous media. We consider the transport of elongated objects in pressure-driven flows in microfluidic Hele-Shaw cells and in gravity-driven sedimentation, and study the effect of confinement, shape and flexibility on the trajectories.

Chronophotographies of (left) a flexible fiber settling in a viscous fluid and (right) a rigid fiber transported in a microchannel exhibiting oscillations between the walls of the channel (flow from top to bottom).

Bacteria swimming in external flows

Gabriel Amselem

Marine microorganisms are permanently subject to external flows that interfere  with their natural propulsion strategies. In particular, it has been shown that the interaction between a shear and the chiral geometry of bacterial flagella induced a bacterial drift perpendicularly to the shear direction. Here, we use microfluidic devices to encapsulate motile bacteria and subject them to shear. We study the dynamics of the bacterial response to shear, as well as the stationary distribution of bacteria as a function of the external forcing.

Motile bacteria are encapsulated in two droplets held immobile in a microfluidic device. At t=3 seconds, an external flow (from right to left) is imposed, which induces a recirculation in both droplets. As a result, bacteria in the bottom (resp.) droplet move towards (resp. away) the field of view.