Deformation and transport of an elastic
fibre in a viscous cellular flow
(Collaboration with Olivia du Roure and Mike
deformation of an elastic filament is
studied in the vicinity of a stagnation point. Its deformation
is shown to modify its transport properties in the cellular flow.
The picture shows a rigid centimetric filament (bottom left)
and a flexible filament (upper right) in the same viscous flow of
counter rotating vortices.
Flow of model suspensions in restrictions
with Olivia du Roure and Schlumberger)
The clogging of restrictions with fiber suspensions is studied as a
function of the properties of the suspended fibers. The pictures show
fiber suspensions approaching a restriction in a microfluidic channel.
The fibers are
directly fabricated inside the microfluidic device.
Mecanisms of adhesive failure
Debonding of a visco-elastic model adhesive
(collaboration with Julia Nase and Mike
We investigate the link between the
viscoelastic properties of model materials and the adhesive performance
using a controlled debonding geoetrie (a TACK-test). The pictures show
the complex fingering patterns observed during debonding of
visco-elastic materials. They are function of the relative importance
of viscous and elastic properties of the adhesive and determine the
Debonding from surfaces with controled
Al Crosby and Costantino Creton)
As a function of the surface topology and
the visco-elasticity of the material, increased or reduced adhesion can
be observed when debonding from rough surfaces.
Flow of dense non-Brownian suspensions
(collaboration with Eric Clement)
We study the flow of dense model suspension
using selected flow geometries, as inclined plane flow or droplet
detachment (see picture).
Active suspensions under flow
We study flow of e-coli suspensions (see
picture) in microfluidic geometries to access properties as their
effective viscosity or their interaction with boundaries.
Elastic flow instabilities
Elastic flow instabilities in a
(collaboration with Rob Poole, Manuel Alves and Sandra Lerouge)
The destabilization of flow of a viscoelastic liquid is studied
in curved flow geometries. The instability onset is
studied as a function of the viscoelastic properties of the fluid and
the flow geometry.
The Saffman-Taylor instability in complex fluids
the Saffman-Taylor instability in complex fluids. More
precisely, we study the relation between rheological properties and
pattern formation in a Hele-Shaw cell. To do so, we use model fluids,
having only one non-Newtonian property at a time.
A rheological study shows that the dominant property of a solution of
the rigid polymer Xanthane is the shear thinning viscosity, that a
solution of the flexible polymer PEO exhibits normal stresses and that
a polymer gel exhibits a yield stress.
For classical fluids, the relative width of the Saffman-Taylor fingers
is determined by the ratio of the viscous forces to the capillary
forces. In the case of a shear thinning fluid, the viscous forces are
altered, leading to a narrowing of the finger. A modification of the
viscous stresses by the existence of a yield stress leads to very
branched patterns with a characteristic finger width, that is a
function of the yield stress. For an elastic fluid, the normal stresses
exert an extra pressure on the finger, which is added to the capillary
forces and leads to finger widening. The knowledge of the influence of
each of this properties separately on the Saffman-Taylor instability,
constitutes a basis for studying the instability in even more complex
fluids. The studied properties are among the most frequently
encountered non-Newtonian properties allowing for a better
understanding of the instability in fluids that exhibit several
non-Newtonian properties simultaneously. (PhD-theses: Pdf-files
Pattern formation in liquid crystals
instabilities observed in
electro-convection in liquid crystals are considered as an archetype of
pattern formatting systems and have been intensively studied. So far
the focus of the investigations has been mostly on the initial
formation of convection rolls. Lately the interest was also drawn to
more complex structures observed at even higher values of the control
parameter. One of the patterns observed in this regime is for example
the so called “chevron-structure”, where the convection rolls order to
form a chevron like structure. This pattern has been known for a long
time, but its origins have so far not yet been understood. Recent
theoretical studies attempted to show that the formation of this kind
of structures is always possible when certain symmetry conditions are
Here we study a specific system showing chevron
formation. More precisely, we performe a linear and weakly non-linear
analysis of the instability leading to the formation of convection
rolls in the dielectric regime of nematic liquid crystals. We develope
the form of the amplitude equations describing this system and performe
an analytic calculation of the coefficients of the latter. The relative
simplicity of the system under study allowes for an understanding of
the basic contributions to the instability and it can be shown that the
symmetry requirements for chevron formation predicted by the
theoretical investigations are fulfilled in this specific system. This
is a first step in the more general understanding of Chevron formation.
The approach used here can in the future also be applied to other
systems showing a second destabilisation at higher control parameters.
This could help to understand other more complex patterns, not yet
understood. (Master Theses: pdf-file (in german))