[ -- Internship Proposals -- ]

Dmytro Kushnir, Pascal Hébraud

The viscosity of a particules suspension in a Newtonian fluid increases with its volume fraction.
At weak volume fractions, solid particles act as short-circuit for the transport of momentum and the viscosity
increases linearly with the volume fraction. However, at higher concentrations, or when the suspension is non-Newtonian,
the viscosity is no longer Newtonian.
We study the flow properties under these conditions.

(i) when the continuous phase is non-Newtonian, we have observed that the suspension obeys
the same flow law as the suspending medium and that one may superimpose the flow curves (viscosity as a function of the shear rate) of all suspensions,
at all volume fractions. One may thus define a scaling coefficient for the viscosity and one other for the shear rate. These coefficients are not independent
one another, but are linked by a universal relationship that does not depend on the nature of the suspending medium.
**We seek to understand the microscopic origin (at the scale of the inter-particle distance) of this relationship (viscosity and shear rate)**.

(ii) When the suspension is Newtonian, the law flow is no longer Newtonian at high shear rates: the viscosity decreases at low shear rate
(the suspension shear thins) and increases at high shear rates (the suspension shear thickens) up to the point where the suspension jams under shear.
We study this last regime, in particular the role of contacts in the transport of momentum.

(iii) Finally, when the particles are attractive, the suspension forms a viscoelastic gel. We perform diffusing wave spectroscopy (DWS) measurements
in order to study the dynamics of the particles during gel formation:

(a) if the suspension is at rest during the gelification phase, then, an isostatic state may form. Any subsequent new
floculation of a particle with the already formed network increases the number of constraints supported by the network above its number of
degrees of freedom and puts it under tension. This tension may lead to rearrangements of the network.
We study the dynamics of these rearrangements using Diffusing Wave Spectroscopy.

(b) If one shears the suspension during its gelification, the suspension becomes partially fluidized and explores configurations
of higher cohesiveness and of higher elastic modulus. The evolution of the elastic modulus with time may, as a consequence, no longer be monotonous.