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