Active matter turns physics upside down
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Have you ever seen mayonnaise rise by itself, waves appear on a windless sea or even paint spread by itself on a wall? Normally not because these different fluids are inert and need an external force to start moving. Physicist Zvonimir Dogic and his collaborators (University of Santa Barbara, California) have achieved these feats by adding an “active” liquid to an inert fluid capable of producing by itself the force necessary for its flow. Their work has been publishedThursday, August 11, in the American magazine Science.
The physics of active matter is an emerging field of research, appeared in 1995 following an article by the Hungarian Tamas Vicsek, recalls the physicist Olivier Dauchot, from the Higher School of Industrial Physics and Chemistry of the City of Paris (ESPCI). “He was interested in the collective movements of birds [comme les nuées d’étourneaux] and wondered how a coordinated movement could emerge in these communities where we know that there is no coordination between individuals. » These behaviors have also been observed in other species, in schools of fish or herds of bison, and at other scales, for example inside our cells.
These natural systems have inspired scientists to design new “active” materials. They are in fact non-equilibrium systems, made up of elementary objects which draw energy from around them, expend it and produce movement. These objects then exert a force on their environment and organize their independent movements into a collective movement.
A non-electric current
For several years, the Zvonimir Dogic team has been developing a material whose activity emerges from the work of molecular motors: proteins that move by consuming energy at the molecular level. These nanomotors activate microtubules, which are long filaments present in our cells, and thus create a current – non-electric – within the fluid where they are located.
In this active liquid, the American team recently dispersed an “inert” fluid, known not to mix with the first (just as one disperses drops of oil in water to make mayonnaise). The flow produced by a small number of motors pushes the drops of passive liquid to merge: the “mayonnaise” falls more quickly. But with a large number of motors, the turbulence generated makes it possible to maintain the passive liquid drops at a constant size. These drops continuously join and divide, and the mayonnaise emulsifies from the inside.
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