Videos

Self-generated flow-fields of micro-swimmers in confinement

Animation showing the theoretical flow-field for a 3-forces swimmer (like the micro-alga Chlamydomonas reinhardtii) vertically confined between two solid walls as the swimmer rotate along the axis of swimming (swimming axis parallel to the 2 boundaries). The topology of the flow field greatly depends on the arrangement of the 3 forces with respect to the walls. Here finite-size effects of the swimmer are neglected.

De-mixing of active-passive mixtures in confined microfluidic systems

When placing weakly Brownian beads (10-micrometer diameter) together with motile micro-algae in straight quasi-2D channels, the colloids tend to accumulate along the walls because of the complex interactions of the micro-swimmers with these solid boundaries.

Microfluidic de-mixing experiment, where narrow side channels let 6-micrometers colloid enter while retaining the micro-algae in the main chamber. Because of the activity of the micro-swimmers in the main chamber and their non-trivial interactions with the walls, the side channels get slowly filled with beads: the system gets effectively de-mixed. The total duration of the movie is 2h45m.

Particle diffusion in active baths of micro-algae

Dynamics of a 1-micrometer colloid in a quasi-2D bath of the micro-algae Chlamydomonas reinhardtii that can be seen swimming around. From time to time an alga entrains the colloid in its path for a short time, bringing it far away from its previous position. Such entrainment events (see high-speed movie below) highly increase the effective diffusion of the particle.

Movie showing a 1-micrometer colloid being entrained by a swimming micro-alga Chlamydomonas reinhardtii, as it gets temporally trapped close to the no-slip surface of the micro-alga. Movie slowed down 17x.

Dynamical reversible/irreversible transition in semi-dilute quasi-2D emulsions

A microfluidic oil-in-water emulsion being sinusoidally flown to probe the reversibility of the droplets dynamics expected from the ultra-low Reynolds nature of the system (where viscous dissipation dominates over inertia). See here for a great video by G.I. Taylor illustrating what kinematic reversibility of low Reynolds number flows means precisely. The droplets (made of Hexadecane) are all 26 micrometers in diameter while the chamber thickness is 27 micrometers. The droplets were created in situ using a flow-focusing generator. Shown on this real-time movie are about 3 000 droplets among ~400 000 in the chip.

Strobed dynamics of sinusoidally advected droplets exhibiting a globally reversible dynamics. Although they move slightly, the particles come back close to their previous position and keep the same neighbors. You can also distinguish small patches where the droplets are moving about more: the particle configuration here changes and the dynamic is locally irreversible. In fact these patches correspond to a local emulsion structure different from the surrounding reversible sea: the system is phase separated. Increasing the driving amplitude by only 2% compared to this run leads to a completely different dynamics as shown on the video above.

Increasing by only 2% the driving amplitude, the situation changes drastically as the particles do not return to their previous position: the dynamics has become irreversible. In such case the emulsion is not structurally heterogeneous as above.