homepage Adrian Daerr

Since september 2001, I am "Maître de Conférences" (associate professor) at the Univesity Denis Diderot (Paris VII), which means I both do research and teach students. Until december 2006, our research group was hosted by the laboratory for Physics and Mechanics of Heterogeneous Matter (PMMH) of the ESPCI, and teaching took place on the Jussieu campus. Since january 2007, we have have moved to a newly constructed campus at Tolbiac (13th arrondissement of Paris), and are now part of the laboratory for Complex Matter and Systems (MSC), which is one of three newly founded labs of the Physics department of our university.

With Luc Lebon, Laurent Limat, Bruno Andreotti, Marc Fermigier and David Quéré, I co-organised the European Coating Symposium 2007 in Paris from semptember 12th through 14th, 2007.

Teaching 2009-10

Until september 2007, I was also International exchange coordinator for the Physics Department of Paris 7 (Erasmus, USA, Canada,...). Here is my small (former) informational page. The new contact person is Simona MEI.

Research

Present: Wetting, Contact Line Dynamics, and Meandering

With: Laurent Limat, Nolwenn Le Grand, Emmanuelle Rio, Jacco Snoeijer, Bruno Andreotti, Luc Lebon, ...

See our group's pages for a description of the subjects on which I work currently.

Mass swarming of bacteria

With: Mehdi Banaha, Daria Julkowska, Simone Seror, Barry Holland, ...

Mass swarming: for 10-12 hours, the bacteria only multiply in the bright spot which can be seen in the lower right. But then, within 2-3h, they collectively migrate outwards (branched structures).

Most bacteria are capable of amazing collective behaviour; mass swarming is a way to rapidly colonize a surface, and likely a first step in the formation of biofilms, which are structures adopted by bacteria that allow them to resist antibiotics, the immune system (inside the body), cleaning (sewerage systems, medical instruments) et caetera. The mecanisms involved in mass swarming are still not understood. A key to an explanation might just have been found by a team of genetical biologists around Daria Julkowska, Simone Seror and Barry Holland of the IGM in Orsay: mutants incapable of producing a substance called Surfactine do not migrate any more, except if they are given Surfactine by the biologists.

Surfactine is in particular a powerful surfactant: it reduces the surface tension of water by more than 70%. Could it be that this property, by modifying the forces acting on the bacteria, is an essential ingredient in the mecanisms of mass swarming ?

Together with the biologists from Orsay, chemists, theoretical and numerical physicists, we study the role played by surfactine in the process of mass swarming. Mehdi Banaha and myself try to understand the importance of wetting phenomena: surfactine production by the bacteria reduces the forces which press them against the surface, and creates new forces which could pull the bacteria outwards (the same forces that set the surface of water in motion when a drop of detergent is added). Can these forces explain the dynamics of mass swarming, the successive waves, maybe even the shape of the resulting structures ?

Past (but who kows?): Erosion, Granular Matter, Turbulence

Also see my publications

Formation of erosive structures in a sediment layer

"Map" of the different patterns observed as the sediment layer is pulled out at a given velocity and inclination (bottom: experimental set-up).

with Éric Clément, Florent Maloggi, Peter Lee, José Lanuza, ...

Different erosive patterns observed on the plate pulled from the bath.

Without the erosion by water, the face of our planet would be very distinct from its present look. Erosion is responsible for the shape of mountains, canyons, sea coasts, rivers, and, at a smaller scale, for the patterns which can be observed on the beach at low tide, or on hills deprived of a protective vegetative cover. Despite this ubiquity, our understanding of the physical mecanisms of erosion and granular transport is far from satisfactory.

We built a small experiment in which a sediment layer, initially created under water, is pulled out of a bath of water (from the perspective of the layer, the water flows off). Depending on velocity and inclination, a multitude of patterns can be observed.

The advantage of a laboratory-scale experiment is the precise control over the parameters: water level decrease velocity, inclination, thickness of the erodable layer, size of the grains making up the sediment, ... This allowed us to calculate the forces and to show that the Shield number (ratio of shear and weight) -- well kown to geologists studying river bed dynamics -- determines the threshold at which patterns appear. Furthermore, we were able to show that a multitude of patterns could be observed just by varying two control parameters, and map the corresponding regions in the velocity-inclination space.

For more details, please refer to the publication on that subject.

Dynamics of Avalanches

Triangular avalanche in the snow.

The Dynamics of Avalanches was the subject of my PhD thesis (download here), under the supervision of Stéphane Douady at the Laboratory for Statistical Physics of the ENS.

Sequence of snapshots of a triangular avalanche in the lab (triggerd with a stick visible in the upper part). The flowing part (bright) grows laterally as it flows downhill, leaving behind a triangular track (dark).

Granular matter can remain at rest with an inclined free surface, and starts moving spontaneously only above a critical angle of inclination. The amplitude and dynamics of the surface flow which then results is governed by the erosion and deposition of grains at the interface with the static phase. This solid-liquid transition is still not very well understood, despite its great importance not only for the understanding of natural avalanches, but also for industrial processing and transport of granular matter. I studied the transition in two different geometries.

In the first experiment, a layer of grains on a rough plane is put into a metastable equilibrium: in a range of inclinations the layer remains static if unperturbed, but a local perturbation triggers an avalanche. By measuring the minimal amplitude which could trigger an avalanche, I showed that the transition to the moving state is sub-critical.

Once the avalanche has been triggered, its mass and speed increase. With Stéphane Douady, we found two types of avalanches: one in which the layer is mobilized only downhill from the origin of the avalanche (triangular track), and another which eventually sets the whole layer in motion through an uphill-propagating front. We have studied the propagation mecanisms in both cases, finding for example a surprising saturation of the avalanche amplitude, which is linked to the presence of a rigid bottom.

The second experiment consists in studying the transient flow leading to the formation of a conical pile. The most striking finding was that the preparation of the granular packing has a big influence on the observed flow: varying the density by a few percent could double the duration of the flow. We also found a memory effect of anisotropic texture aquired during preparation.

The last part discusses common models for granular surface flows in the light of these results, and develops a new model based on conservation equations and experimental findings.

Injection of energy in turbulent flows

Drawing of the Taylor-Couette cylinders: smooth to the left, and rough (with small ribs) to the right.

Dissipation divided by the theoretical prediction as a function of the Reynolds number. With rough walls (white triangles), the parameter β is constant, which means that dissipation follows Kolmogorov scaling, whereas it doesn't in the case of smooth walls (black triangles and crosses).

I have studied the injection of energy in closed turbulent flows with Yves Couder and Olivier Cadot for only about six month, but the result was so striking that I still remember it :-).

The starting point is the observation that many closed turbulent flows do not dissipate energy as efficiently as they are expected to according to the standard theory for homogeneous isotropic turbulence). It turns out that these flows are not homogeneous enough for the theory to apply: energy is injected into the flow via boundary layers, inside which part of the energy is dissipated. The thickness of these boundary layers decreases with increasing wall velocity. Far from the boundaries, dissipation does follow the predicted scaling (as do certain statistical properties of the turbulence), which means that surprisingly the efficiency of the stirrer to set the fluid into motion does not decrease with increasing velocity. It is only the relative contribution of the dissipation in the boundary layers which diminishes with respect to the dissipation in the bulk.

inertial forcing vs. via boundary layer forcing If the energy injection scale is forced to remain in the inertial range -- by attaching ribs to the walls -- the scaling of energy dissipation according to Kolmogorov's theory can be recovered. See publication for more.

When I was small

DEA in Theoretical Physics (P6,P7,P11,X,ENS) in 1996/97
Maîtrise de Physique at the École Normale Supérieure in 1995/96
University of Heidelberg 1994-95
University of Bonn 1992-94

Miscellaneous

Physics and research

my page with computer programmes, which does currently not feature many interesting things:
- ScionFGAkiz, a programme in C and Java for capturing images or movies on an Apple or PC/Windows computer, through a Scion frame grabber board. With an plug-in for ImageJ.
ImageJ, a great scientific image manipulation and processing tool; powerful, free, fast, extensible, and written in Java (Apple/M$-Windows/Linux/...).
The library of the Jussieu campus
The electronic publication server arXiv

More Internet resources

A comprehensive list of online dictionaries.
The (La)TeX navigator at LORIA.
The End of the Internet...

last modified: November 2nd, 2009