Curriculum Vitae - APC - Université Paris Diderot-Paris 7

Figure 5. A- and B-Sketch of Quincke rollers in a Hele-Shaw microfluidic cell. Top views. A-Dilute system,
uncorrelated velocities. B- Concentrated system: Collective directed motion? C-Tentative phase diagram.
The first series of experiment will focus on homogenous channel geometries, namely wide
isotropic shallow cells. The following question will be addressed first: can one observe a
spontaneous collective and directed motion, viz. can Quincke rollers form swarms? An
experimental fact, strongly suggests than a spontaneous symmetry breaking should occur in
this system at sufficiently high concentration. As a matter of fact, it has been shown, that
when an external macroscopic shear is applied to a suspension of Quincke colloids, they all
rotate coherently, with the imposed fluid vorticity. This symmetry breaking enforced by the
external drive results in an electrorheological shear thinning [19]. When rolling on a solid
substrate, the colloids induce a net flow over distances comparable, at least, to their diameter.
This induced flow is therefore expected to orient the rotation of the surrounding particle and
yield correlated motion. At sufficiently high concentration, this local coupling is expected to
propagate up to the system size. Practically, the height of the microfluidic channels containing
the colloids will be chosen much larger that the colloid diameter to maximize the
hydrodynamic couplings (height~100 microns, for micron size colloids). To check our
prediction, we will measure the average particle velocity for increasing concentrations and
electric field amplitudes. To distinguish between a crossover regime and a spontaneous
symmetry-breaking scenario, we will measure the velocity-velocity correlation length and the
lifetime of the coherent clusters as a function of the electric field amplitude for various
concentrations. The magnitude of the E field is the parameter, which we control with the
better accuracy. We shall notice that, this phenomenology is not opposed to the instability the
polar phases predicted by the current models for active suspensions [14]. This theoretical
prediction indeed applies only to anisotropic particles, having a velocity slave to the particle
orientation. In our experiments, the translation speed of our colloids is not set by any
permanent shape asymmetry; the direction of the roller velocity is set by its surface charge
distribution, which is only weakly slaved to the geometrical orientation of the particle. In
addition, the hydrodynamic coupling between two Quincke rollers is likely to differ from the
conventional force-dipole picture. Therefore, our system will not process the two features
responsible for the destabilization of the splay/bend modes of the active polar phase [14].
To rationalize quantitatively our experimental findings, we will pursue our ongoing work on the
modeling of active suspensions in collaboration with Eric Lauga UCSD. In line with the remark
made above, we will need first to construct a realistic model for the roller-roller scattering
process. These models will be built upon the experimental characterization of the roller-roller
collision rules, which we will extract from direct image analysis in dilute systems.
4.5.2.2. Traffic dynamics along colloidal roads.
As a first step toward the traffic in more complex geometries, we will investigate, the
collective dynamic of Quincke colloids along a 1D track, see Fig. 6. The colloidal tracks will
be made by direct lithography on a conducting glass slides (ITO coating). The colloids will be