Curriculum Vitae - APC - Université Paris Diderot-Paris 7

focused on the structural properties of active suspensions and active gels [14]. Importantly,
despite the intense theoretical activity in this field, no description of the flow in
heterogeneous media has been discussed. Regarding the experimental studies, we shall note
that very few quantitative experiments are available [14]. The reason for this imbalance
between the number of theoretical predictions and of the experimental findings is precisely
the lack of well-controlled physical system. Only for artificial (dry) walkers, a neat minimal
system of vibrated polar disks has been achieved in [15]. The authors obtained the first
experimental evidences of the emergence of long-range collective motion. For active
suspensions, the two most promising options are: (i) the suspensions of robust living
microorganisms such as e-coli bacteria, or Chlamidomonas algii [16] and (ii) solutions of
autophoretic Janus colloids [17]. The main drawback of living microorganisms is the intrinsic
difficulty in identifying their « control parameters », which can merely be varied over a small
range, when identified. Autophoretic colloïds are very appealing when dealing with dilute
suspensions. Conversely, they are not suited for high concentration experiments. As a matter
of fact, the propulsion relies on chemicals reactions that release gaseous byproducts, and
therefore form numerous unwanted bubbles in the solutions. In addition, the need for constant
food/reactant supply makes impossible long experiments in confined geometries.
4.4. First results: Why can we (hope to) solve these questions?
We shall first introduce several preliminary results, which we have already achieved both for
advected and for self-propelled particles. Our motivation is here to stress to stress on the
feasibility of the new and ambitious projects, which we introduce in section 5.
4.4.1. Microfluidic traffic flows: Advected droplets
Figure 2: A- water droplets advected by a hexadecane oil flow in a regular post network. The three pictures
correspond to three different droplet injection-rates. The network invasion occurs between (a) and (b). Length:
9.75 mm. B-Variations of the number of particles in the central lane, N, and in the lateral lanes, Ñ, as a function
of the particle current j. From [8]. C-Total number of particles in the network as a function of time for j=1.2
j*=0.8 Hz. Note that (i) slow increases in N are followed by sharp decreases involving a very large number of
particles.
During the PhD of Nicolas Champagne, we devised efficient microfluidic solutions to study
the transport of emulsions in microfluidic networks [8], Fig. 2A. These tools are based on two
techniques, developed in our group (in collaboration with V. Studer): (i) Microfluidic
Stickers, which enable us to make solvent and mechanically resistant device, in order to
ensure a fast dynamic response and a long-term stability of the microfluidic flows [18]. (ii) A
simple but effective drop on demand device, which makes possible to control independently
the size, the velocity and the distance between microdroplets [19]. Thanks to these two tools,
we carried out the first study on the traffic flow in 2D networks of regular obstacle [8]. We