David Quéré awarded by the APS

The American Physical Society (APS) has just awarded the "Fluid Dynamics" prize to David Quéré, a CNRS physicist at the PMMH laboratory at the school. We met with him in the middle of the summer to discuss his original approach, which was praised by the APS.

Left, David Quéré awarded by the APS ( © W. Parra/ESPCI Paris)
Right, levitation experiment above a hot surface (Ambre Bouillant, David Quéré)
What arouses your interest in the themes that the APS award recognizes (wetting and hydrodynamics at interfaces)?

It is not very easy to know why we work on this rather than that - there is so much to do! But I would say that my starting point was to look around me, with the naked eye (i.e. at intermediate scales, neither very large nor very small), and to select among the things that surround us those that sink and, therefore, seem to escape us. In this game, the more humble the phenomenon, the more attractive it is. A drop is thus an ideal object: more than anodyne, almost invisible, limpid in appearance, it nevertheless reveals a whole world to the one who scrutinizes it: didn’t Newton himself see the globe as an enormous drop? And didn’t Gamow and Bohr imagine the nucleus of the atom as a picodrop?

Our good fortune has been that the physics of liquids at intermediate scales (say, from ten microns to a few centimeters) is of interest to many companies, even because of the triviality of these situations. I did not seek it, but I understand today, thanks to your questions, that going towards the unimportant also led me towards the general. Driving drops, chasing them, gathering them, concerns the glass or concrete industry as well as the detergent or printing industry, and these links with the applied world have become the other major axis of our research. 

 - APS emphasizes the original approach of your contribution in these two fields of fluid dynamics, what does it consist of?  

The simplicity of the subjects (how does a drop hang? how does it bounce? etc.) can be accompanied by sophisticated analyses, linked to the multiplicity of scales involved: from the molecule to the object itself, seven orders of magnitude in size... But we also dream of a more essential analysis that brings together form and substance. This is the language of the scaling laws, which condenses mathematics to an infra-elementary level (multiplication) by a certain manipulation of average quantities. I have been at a very good school to learn this language (Pierre-Gilles de Gennes’ classes at the Collège de France) and I never cease to be amazed by the efficiency of the method: one can both understand classical laws by redesigning them in this abrupt (for some, casual) way, but also, and most importantly, establish new laws in poorly known domains, for which we do not yet know how to write anything analytical. The ultimate advantage of these approaches is the simplicity of the language: working at the interfaces (between disciplines, physics, chemistry and mechanics, between fundamental and applied), having a kind of scientific Esperanto is especially precious.

What remains to be discovered?



My chronic inability (or laziness) to write proposals (of the kind that allows you, maybe, to get a fabulous grant) makes me especially unfit to answer this question. Working on drops and stains, I will tell you however that it is by staining that one becomes a stainer: I know (by experience) that almost everything remains to be colored by our understanding (so I answered you), but that it is the exploration that teaches us what are the colors. The sentence that Pierre Soulages applies to his painting, "What I am doing tells me what I am looking for", is for a good part (and in my case, the only part) what guides researchers.

- You work at the PMMH laboratory, in collaboration with the LADHYX, is it a stimulating environment? 



These are two exceptional labs, mainly because of the variety of talents they host – an ideal amalgam of experimental, numerical and theoretical research. They are like cousins, more than brothers, having been founded by a physicist, Étienne Guyon, and a mechanician, Patrick Huerre, and they still carry, some 30 years later, this genetic imprint that makes them so complementary. At a more individual scale, I have a long-term collaboration with Christophe Clanet, a researcher at LadHyX and former professor of hydrodynamics at ESPCI, and we are reproducing what I said about our laboratories, trying to hybridize physics’ and mechanics’ points of view – that is, to raise a chimaera (a privilege).

- Several of your papers have led to many popularization articles in the French and international press, what does this mean to you? 



The popularization of our work is vaguely flattering, at first sight - especially if one does not perceive that our subjects are actually over-represented, since this type of research that questions everyday life is by definition "within reach" of everyone. Then one reads articles that contain a lot of mistakes, and one gets offended - again in error, since it doesn’t matter: what matters is to say what researchers are working on today and to stimulate everyone’s curiosity. However, one sometimes has the real pleaure of reading a remarkable article of popularization, a very difficult art since it is a question of over-simplifying without betraying a research which is both technical and on which one lacks hindsight. Anglo-Saxon popularization was for a long time far ahead of what could be read in France, and we should be pleased that things have changed, with writers such as David Larousserie, in Le Monde, for example.

– Two results that you found particularly surprising, or that you are particularly happy about?

It’s not exactly a result, although that’s debatable, but I have found particularly surprising the quality of the thirty or so PhD students I have supervised. I owe them about everything and I am infinitely grateful to them. It is a pleasure to find them today in industrial or academic positions where they develop, each in her/his own way, the qualities of curiosity, creativity, rigor, and humor (scaling laws being a kind of scientific joke, humor is essential) that they showed in the lab. I don’t know if this is what you wanted me to say, but probably since it makes me so happy.

Another surprising result was obtained by Ambre Bouillant, now a post-doc at Twente: going back to the basis of the Leidenfrost phenomenon, which is the levitation of a drop of water on a very hot plate, we discovered that water not only levitates but also flees from the place where it is placed. This situation, of extreme simplicity, is truly mysterious: while everything is symmetrical in the experiment, the drop flees along a semi-randomly selected axis. However, for the 250 years that we have been observing the thing, nobody noticed this elementary fact, almost the first one that should be said (a common drop does not run away from where it is deposited). I believe that it is because of the very effect that it was not seen: we naturally trap these drops at the bottom of hot spoons or hold them with a needle, precisely because they run away! This might be a good metaphor of research itself: somewhat levitating above reality, it always escapes in a direction we barely chose...