Research

Here are my main research interests, the currently most active being in red and the cold cases in blue. They combine fluid mechanics, acoustics, and soft matter physics through objects like liquid films, foams and bubbles. Most of them are fundamental, curiosity driven, and many come not from formal research projects, but from casual meetings and friendly collaborations. I try to do theory and experiments. The number refer to my articles on this subject, which you can find in the Publications page; the main articles, or the closest to my heart, are in bold.

• • rheology of foams in two and three dimensions [2,3,4,5,6,8,9,11,12,15,18,24,29,31,39,50,51,54,56,89,97].

Liquid foams are complex fluids, showing elastic, plastic and viscous properties. Their elasticity originates from surface tension and bubble deformability. Their plasticity arises from bubble rearrangements, which change the foam network topology. Their viscous properties come from various dissipative mechanisms in the liquid phase. While the classical tool to study complex fluid properties is rheometry, I specialised in studying foam rheology by image analysis at the bubble scale.

• • ultrasound contrast agent microbubbles and bubble acoustics [10,14,16,19,26,27,28,30,32,35,36,45,66,68,69,70,71,77,81,82,87,91,96].

Bubbles are loud, owing to the compressible nature of gases. This is used in medical imaging, where coated microbubbles are used as ultrasound contrast agents to increase the contrast between blood vessels and the surrounding tissues. I was largely involved in the study of the physical properties of such coated bubbles under acousting forcing, especially using ultrahigh-speed imaging. In another context, sap conduits of trees under drought can cavitate leading to embolism, and I was involved in the theoretical study of such cavitation events in confined space.

• • bubbles and droplets in microfluidics [13,33,36,74].

Microfluidics is an exquisite tool in generating controlled droplets and bubbles. I studied the dynamics of bubble creation in flow-focusing devices and coflow, and clarified the role of channel geometry in this process. I also studied the dynamics of droplets under microfluidic confinement.

• • dynamics of soap films [17,21,22,23,37,38,44,47,57,98].

Soap films are the elementary building blocks of foams. Despite their beauty and apparent simplicity, their dynamics remains poorly understood, because of the intricate coupling between surfactant transport and free-surface hydrodynamics, I tried to clarify some of their dynamical aspects in different situations: flow of lamella through tubes, coating, or extension of freely supported films.

• • fracture in foams [41,46].

Liquid foams being somehow intermediate between elastic solids and viscoplastic fluids, we showed that when placed under tensile stress, they can either show brittle fracture and be shattered in pieces like glass along narrow cracks of bursting films, or show ductile fracture with accumulation of plasticity in crack tip without film bursting.

• • foam flows in porous media [43,49,58,63].

Liquid foams are used in enhanced oil recovery and in soil remediation, because they are thought to invade low-permeability zones in porous media efficiently. I contributed to this area by focusing on two-dimensional model systems where the flow of foam can be precisely studied by image analysis, and revealed subtle coupling btween the structure, bubble size, and flow properties of foams in porous media.

• • acoustic and shock waves in foams [25,40,48,52,55,59,61,78,85].

Liquid foams are extremely efficient in damping mechanical energy and mitigating pressure waves. When I started studying this topic with my colleagues, the underlying dissipative mechanisms were largely elusive. Through joint experimental and theoretical efforts, we have proposed a coherent framework to explain and predict the foam acoustical properties, and revealed that they can behave as acoustic metamaterials. We have brought this new knowledge to better understand the propagation and damping of shock waves through foams.

• • convective-diffusive mass transfer: nanobubbles and evaporation [20,62,64,67,72,80,83,86,90,93,95].

I got involved on mass transfer problems in different context, like the intriguing stability of surface nanobubbles, or the influence of natural convection of the evaporation rates of horizontal liquid surfaces or vertical films at the scale of a few centimetres. Recently, we have developed an intensive research activity on the biomimetic drying of leaves.

• • sloshing [60,65,76].

Sloshing is an important industrial concern, affecting the stability of liquid carriers. It is thus important to control and optimise the damping of sloshing waves. In this context, I studied various sources of damping, such as confinement and interfacial effects related to wetting films and triple line dynamics.

• • ageing of foams: coarsening and coalescence [73,75,88,92,99].

Liquid foams are metastable materials which evolve by drainage, coarsening and, ultimately, coalescence. Some aspects of these ageing mechanisms are still under investigation; using advanced image analysis and modelling, I contributed to a better understanding of coalescence statistics and the disordering of initially ordered foams. I also recently studied the transfer of gases of different solubilities in foams, with potential applications to carbon dioxide capture.

• • miscellaneous [1,7,42,53,79,84,94].

Collaborations

Most of the joy in research comes from meeting great colleagues from next door or from all around the world, with whom to work and to have great fun. I was very lucky to work with the following scientists:

Present:

• • Francesco Viola and François Gallaire, LFMI, EPFL (Switzerland)

• • François Boulogne and Emmanuelle Rio, Laboratoire de Physique des Solides

  • Detlef Lohse, Guillaume Lajoinie and Michel Versluis, Physics of Fluids, Twente University (The Netherlands)

• • Charles Baroud, Institut Pasteur and Laboratoire d'Hydrodynamique de l'Ecole polytechnique

  • Valeria Garbin, University of Delft (The Netherlands)

• • Christophe Raufaste (LPMC, Université de Nice-Sophia Antipolis), Stéphane Santucci (Laboratoire de Physique, ENS Lyon) and Rajmund Mokso (MAX-IV, Lund University, Sweden)

• • Philippe Marmottant, Laboratoire Interdisciplinaire de Physique, Université Grenoble Alpes

• • Elise Lorenceau, Laboratoire Interdisciplinaire de Physique, Université Grenoble Alpes

  • Kaare Jensen , DTU (Denmark)

Past (to be revived):

• • Mauro Sbragaglia, University of Rome Tor Vergata (Italy)

  • Howard A. Stone, Princeton University (USA)

  • Simon J. Cox, Aberystwyth University (UK)

  • Valentin Leroy and Florence Elias, Matière et Systèmes Complexes

  • Isabelle Cantat and Arnaud Saint-Jalmes, Institut de Physique de Rennes, Université Rennes 1

  • Yves Méheust, Géosciences Rennes, Université Rennes 1