Research Topic 1: Suspensions and powders
Imbibition
Pascal Hébraud
When one mixes a powder with a liquid, one obtains a triphasic system of particles aggregates, that contain air, in a liquid phase.
In order to obtain a well dispersed phase, one needs to disrupt the aggregates. Among other mechanisms that play a role in the aggregate
disruption is the imbibition by the solvent (i.e. its infiltration under the effect of capillary forces).
But, in order for the imbibition to be complete, the air must exist the aggregate. When the gas solubility is weak,
the imbibition kinetics is the result of the competition between both mechanisms (wetting and exit of the air) and proceeds in three successive steps:
(i) Infiltration of the solvent, due to wetting, during which the air is trapped inside the aggregate. Its pressure increases.
(ii) The infiltration stops when the gas pressure becomes equal to the capillary pressure.
(iii) An outing of the air, at constant pressure, smaller than the capillary pressure, but larger than the atmospheric pressure.
When the difference of pressure between the outside and the inside of the aggregate is large enough, the imbibition front becomes unstable, due to its
macroscopic curvature: its surface decreases and the conservation of mass implies that the front should accelerate. This is the underlying mechanism
of the instability. We have studied the threshold pressure under which the instability develops, and we have experimentally verified that the
larger the front curvature, the less stable the front. As a consequence, cylindrical fronts are more stable than spherical fronts.
Further Reading
A. Debacker, S. Makarchuk, D. Lootens, and P. Hébraud.
Imbibition Kinetics of Spherical Colloidal Aggregates.
Physical Review Letters, 113(2), July 2014
Evolution of the square of the noninfiltrated core radius as a function of time for aggregates of radius 1 mm (black squares), 1.5 mm (dark gray squares), 2 mm (medium gray squares), and 2.5 mm (light gray squares). Inset: Imbibition curves for 2.5 mm radius aggregates when the initial inner gas pressure is equal to 1 bar (light gray squares), 400 mbar (medium gray) and 200 mbar (dark gray).
Image of the aggregate during the infiltration by DIDP during the first regime (left), at the plateau (middle), and at the end of the last infiltration regime (right).