Complex fluids

 

We seek to understand dynamical processes that govern stability and flow of complex fluids, which include amorphous systems, foams, disordered materials, yield stress fluids, etc. The goal that underlines our experiments is to explain the macroscopic response of these materials from the microscopic structures and dynamics.

  

 

Members:

Barentin, Catherine
Biance, Anne-Laure
Jorgensen, Loren
Le Merrer, Marie

Leocmach, Mathieu

 

 

Highlights

 

Wrinkling yoghurt

We study the wrinkling of a thin protein gel film (basically yoghurt) between two glass plates. pH-controlled contraction and swelling cause wrinkling. We show that the wavelength of the wrinkles is controlled dynamically either by the viscosity of the whey or by the porosity of the gel, two properties that have been overlooked so far in theoretical explanations of wrinkling. The results could be helpful to understand the physical forces that help shape the embryo.

Hierarchical wrinkling in a confined permeable biogel, Leocmach et al., Science Advances (2015).

 

Dynamics of pattern formation. Top: transmitted light, colours added, top view. Bottom: 3D reconstruction from confocal microscopy.

 

Capillarity of yield stress fluids

We use yield stress fluids like microgels to perform different kinds of capillarity experiments : capillary rise, bridge tensiometry, wetting... These situations provide an original way to probe the interplay between equilibrium properties like surface tension and dynamical arrest due to the fluid solid-like behavior.

Capillary rise of yield-stress fluids, Géraud et al., Europhys. Letters (2014). Yield stress and elasticity influence on surface tension measurements, Jørgensen et al., Soft Matter (2015).

 

Capillary bridge of carbopol gel and capillary rise of several fluids.

 

Confined flows of microgels

Microgels are complex fluids widely that are widely used in applications such as cosmetics or oil recovery. We use microfluidic devices to investigate how their flow is modified under confinement, a situation arising for instance in porous media. Such small scales indeed enhance confinement effects or wall slip.

Confined flows of a polymer microgel,  Géraud et al, EPJE (2013).

 

   Glass microfluidic channel used for microgel flow under confinement and velocity profiles.

 

Shearing of cellular aggregates

To understand cellular motility and morphogenetic processes, we study mechanical properties of aggregates made of hundred of cells. Using a blade tensiometer, we have found an elasto-plastic behavior at short times (<10 min) and a viscous behavior at long times (hours). The properties at short times are also probed  by a rheometer. A model based on the soft glass rheology allow to rationalize the main results observed with the rheometer.

Multicellular aggregates: a model system for tissue rheology, Stirbat et al, EPJE (2013).

 

A) Aggregate compressed between two walls. B) Two-photon microscopy image of a type F9 WT aggregate C) Hundreds of aggregates sheared in the rheometer.

 

 

Elementary processes in foams

Whenever a foam flows, drains or break, the bubbles making the foam move with respect to one another. Using experiments on model film assemblies, we investigate the structure and dynamics of such rearrangements. In particular, we seek to understand the role played by the surfactant. Moreover, we try to connect the rearrangement phenomena with macroscopic properties  of the foam such as stability and rheology.

How topological rearrangements and liquid fraction control liquid foam stability, Biance et al., Phys. Rev. Lett. (2011).

 

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(Left) Soap film geometry that allow a topological rearrangement and observation of the newly created film. (Right) Interference pattern on a newly formed film of width 1 mm.

 

 

 

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