Curriculum Vitae - APC - Université Paris Diderot-Paris 7

showed that the traffic dynamics is a non-linear process: the particle current does not scale
with the particle density even in the dilute limit where no particle collision occurs, Fig. 2B.
We have demonstrated that this non-linear behaviour stems from long-range hydrodynamic
interactions. Importantly, we have also established that there exists a maximal current, j*,
above which no stationary particle flow can be sustained, Fig. 2B. For higher current values,
traffic jams forms thereby inducing correlated ejections of the particles out of their the initial
path and the subsequent invasion of the network. We proved that the reason for this invasion
transition is akin to the formation of bona fide jams in vehicle-traffic flows, which display
current-density relations qualitatively comparable to the one observed in our minimal fluidic
setup. We also performed preliminary experiments above the network invasion threshold, and
demonstrated that the traffic dynamics is strongly intermittent in this regime. The traffic jams
form slowly and quickly break along the initially preferred path, which results in an
avalanche-like dynamics as illustrated in Fig 2. C. This first set of experiments revealed that
the traffic dynamics above j* yield non-Gaussian density fluctuations in the network, [8]. The
first axis of our project will be directly motivated by these first experimental findings.
4.4.2. Self-propelled colloids: Quincke rotators
Over the last four months, with Antoine Bricard (PhD), we have devised a new experimental
set-up to create, to manipulate, and to observe a new kind of self-propelled colloids in
microfluidic devices. In brief, we have taken advantage of an overlooked electro-rotation
phenomena discovered by Quincke more than a century ago [18-19]. The so-called Quincke
effect is an electrohydrodynamic instability, which arises when an insulating particle is
immersed in a weakly conducting fluid and subject to an homogeneous DC electric field, E.
Above a critical field amplitude Ec, the induced charge distribution around the particle
becomes unstable (supercritical), thereby inducing a net torque on the particle, which then
rotates at a constant angular velocity. The rotation vector is normal to the electric field, Fig.
3A. Our idea was basically to exploit this phenomenon, to build “colloidal rollers”. Indeed,
when a Quincke rotor lies on a solid surface, it should start rolling on it for electric fields
normal to the solid surface. This is precisely what we observed with commercial polystyrene
colloids in alcane oils. In Fig. 3B, we show the trajectories of an ensemble of 5 microns
colloids rolling on the surface of a microfluidic chamber made of a microfluidic sticker
(height 50 microns). As the induced polarization of the colloids spontaneously breaks the
rotational symmetry, the particles are prone to strong rotational diffusion in the directions
normal to the electric field. In addition, the angular velocity increases as (E-Ec) 1/2 .
Consequently, varying the electric field amplitude enable us to fine-tune the persistence
length of these self-propelled colloids, see in Fig. 3C.
The three major advantages of this novel system are: (i) Its lifetime is not limited by a finite
fuel reservoir. The particles stop only when the electric field is shut down. (ii) It is effective in
a wide range of surface concentration: from very dilute to close-packed colloids. (iii) As
opposed to biological systems for instance, the Quincke rollers, have a very limited number of
control parameters, which are well identified and that can be tuned over decades. These
unique properties open the way to a quantitative program dedicated to the collective dynamics
of motile particles in simple geometries and in random networks.