Soft matter: a physical state you probably don’t know you know

A car jour­ney in the rain. The drops run down the wind­shield, some­times stop­ping, as if they were stick­ing to it. What form do they take then? What allows them to cling on when water is nor­mally so flu­id? The answer is not as simple as it may seem¹… 

Moreover, the same water, when it starts to flow, is curi­ously slow, if one com­pares it to that of rivers or tor­rents – even though they are less steep than a car wind­screen. Des­pite their appar­ent sim­pli­city, these unex­pec­ted obser­va­tions actu­ally raise pro­found ques­tions that sci­ence is try­ing to explain: soft mat­ter. 

Chris­tophe Josserand, a research­er at Lad­HyX (IP Par­is), believes that the first dis­cov­er­ies in soft mat­ter date back to the begin­ning of the 20^th Cen­tury, with liquid crys­tals and poly­mers – two typ­ic­al examples of mater­i­als that lie on the bor­der­line between sol­id and liquid. But it was not until later that the term came into use. “It can­not be dis­so­ci­ated from the name of Pierre-Gilles de Gennes, a great French phys­i­cist, who, when he received the Nobel Prize for Phys­ics in 1991, entitled his speech ‘Soft Mat­ter’².”

Pierre-Gilles de Gennes, a great French phys­i­cist titled his speech ‘Soft Mat­ter’ when he received the Nobel Prize for Phys­ics in 1991.

Pierre-Gilles de Gennes is the lead­ing fig­ure in this field. Dav­id Quéré, a research asso­ci­ate at Lad­Hyx, worked under his dir­ec­tion dur­ing his time at the Collège de France, and Chris­tophe Josserand even describes him as ‘a spir­itu­al heir’ to the Nobel Prize win­ner. Author of a book co-authored with de Gennes³, the spe­cial­ist in the field con­firms the weight of this “major, ultra-inter­na­tion­al” figure.

“It was on the day of the Nobel Prize that this ill-defined field became a sci­ence,” says Dav­id Quéré. Pierre-Gilles de Gennes uni­fied a whole group of dis­par­ate prob­lems, often involving liquids in spe­cial situ­ations: either liquids that are com­plex in essence, or sim­pler liquids in a com­plex state. With this speech and the recog­ni­tion of his work, de Gennes gave cre­dence to a sci­ence “with inter­me­di­ate scales, often linked to every­day life, but full of unex­pec­ted elements.” 

Most often, the objects stud­ied in soft mat­ter are com­pos­ites – mean­ing het­ero­gen­eous mix­tures of at least two con­stitu­ents whose mix­ing con­fers a para­dox­ic­al or ambigu­ous char­ac­ter. Sand, for example, is one such object: “Depend­ing on the situ­ation in which the sand is found, it can sink as well as take on a sol­id form, with slopes,” says Dav­id Quéré. How­ever, it remains a two-phase mater­i­al: the grains that make it up, which are sol­id, are dis­tinct from the air, the flu­id that sur­rounds them. It is this in-between state that is import­ant to under­stand, based on the simplest pos­sible rules of inter­ac­tion between grains.

Soft, ambigu­ous, or even intel­li­gent, there are many peri­phrases to describe this type of mat­ter, almost as many as there are examples. From may­on­naise to foam, from mud to con­crete, this state sur­rounds us, and it is often very use­ful. “Let’s take shav­ing foam,” con­tin­ues Dav­id Quéré, “which is primar­ily made up of a mix­ture of water and gas: two ele­ments that flow eas­ily but which, sud­denly, on my cheek, make a quasi-sol­id; and a sol­id that can be manip­u­lated since I was able to shape this foam before­hand and spread it out. It’s a kind of mir­acle, isn’t it?”

Chris­tophe Josserand adds: “To whisk the white of an egg, you have to beat it: the move­ment of the whisk incor­por­ates air into a het­ero­gen­eous mix­ture made up of water, oil and, if we sim­pli­fy things, molecules sim­il­ar to soap,” he explains. “Again, the liquid takes on a frothy form that will solid­i­fy when I heat it in the oven: this is meringue!” He con­cludes: “Look­ing around, I see extraordin­ary prob­lems that are often of imme­di­ate prac­tic­al interest.” 

There is thus an essen­tial link between soft mat­ter and applic­a­tions. The emer­gence of mod­ern soft mat­ter can be dated back to 1973, the year of the first oil crisis. At that time, the oil com­pany Exxon – the par­ent com­pany of the Esso brand – decided to look at the 50% of oil left in a well after extrac­tion. In this highly charged eco­nom­ic cli­mate, the Amer­ic­an industry could no longer afford to aban­don half of its black gold resources.

“This is an exem­plary soft mat­ter prob­lem on sev­er­al levels,” insists Dav­id Quéré. “The pre­cious liquid is a vis­cous oil, which is com­plic­ated in itself. But to extract it, we push it with water that is often soapy, which cre­ates emul­sions with­in a rock with tor­tu­ous pores: phys­ic­al chem­istry, flu­id mech­an­ics, con­fined spaces, the prob­lem is extremely dif­fi­cult and only a more fun­da­ment­al approach will make it pos­sible to pri­or­it­ise and sim­pli­fy it – invent­ing a num­ber of new and excit­ing prob­lems in the process.”

Defin­ing this field of research remains dif­fi­cult, par­tic­u­larly because of its tend­ency not to focus on a single type of object. It is almost easi­er to identi­fy it by the style of its research than by its sub­ject. For Chris­tophe Josserand, “the rich­ness of this sci­ence comes from its abil­ity to com­bine very dif­fer­ent fields, from phys­ics to bio­logy, from applied math­em­at­ics to chem­istry.” The research is there­fore com­ple­ment­ary. “The hydro­dynam­ics labor­at­ory at École Poly­tech­nique (Lad­HyX) is a good example of these inter­sect­ing inter­ac­tions, with a com­munity that touches on both non-lin­ear phys­ics and flu­id mech­an­ics and which, moreover, cre­ates strong links with the oth­er sci­ences, par­tic­u­larly through the research car­ried out in bio-mech­an­ics. We can there­fore con­sider this to be a soft mat­ter com­munity in the broad­est sense,” he adds.

Dav­id Quéré con­firms this: “This dis­cip­line is at the bor­der between many dif­fer­ent sci­entif­ic fields,” he says. “That’s anoth­er reas­on why we keep going round in circles: blurred bound­ar­ies lead to blurred defin­i­tions. But, for all that, its dis­ciples recog­nise them­selves in their way of pro­ceed­ing, between fun­da­ment­al and applied sci­ence, and through their neces­sary work on the sim­pli­fic­a­tion of a real­ity that is by nature complex.”

Pablo Andres