When physics freezes runaway shapes in time

TDiderot said that everything passes, and the world remains only in front of the Roman ruins painted by Hubert Robert. This melancholy assertion sadly applies to the dramatic times we go through, even if overly optimistic, the sustainability of the world itself seems to be called into question.

However, we physicists tend, for the sake of accuracy, to classify this pessimistic observation, if only because the description of phenomena often works by defining conservation laws. Moreover, systems in evolution always sooner or later reach a state of macroscopic equilibrium, in which nothing passes, precisely, any moment that is forever resembling the previous state.

These states of equilibrium are the Grail to the physicist, because, so to speak, they are the only ones that can be reasonably described. On the other hand, if we deviate significantly from these resting states, the science will be much more difficult. Transient effects, for example, when a modification of an external state forces a system to “migrate” toward a new equilibrium, are often difficult to describe. In some cases, this migration is accompanied by the spontaneous development of staggering fugitive numbers, which unfortunately disappear when the balance returns.

Stop the class dynamic

A simple example of this process is vertebral decomposition. Let’s take a mixture of ethanol (alcohol) and dodecane (oil) in a ratio of 2 / 3-1 / 3. Above 13 ° C, the two types mix and a homogeneous liquid is observed. If the system is suddenly cooled, the species becomes immiscible and “prefers” separation. The dynamics of this separation is amazing: everywhere in the liquid appears an interwoven network of two liquids already separated into very small grains. These patterns grow larger, with ethanol and dodecane-rich regions forming larger, but still overlapping regions. If nothing is done, this “ripening” continues until complete separation, because the system seeks to reduce the contact surface between fluids, just as oil separates from water: it is impossible to take advantage of this unusual but ephemeral state …

In 2005, a European team nonetheless found a way to literally stop time, with this mechanism, and to stabilize these beautiful interlocking structures. The story is more interesting because the prediction of this effect by numerical simulation has already been experimentally confirmed. By adding nanoparticles to the mixture, which had a very high affinity for the interface, they were able to stop the ripening by a remarkable simple mechanism. During this transient time, the interaction between the two species is reduced, and if the nanoparticles are numerous enough, the housing crisis will soon be felt! To further reduce this interface, it will be necessary to eject the nanoparticles from it (towards one or the other of the liquid phases), which does not happen, because the nanoparticles remain firmly attached to the interface. Then everything stops as soon as there is no more space available.

You have 21.96% of this article left to read. The following is for subscribers only.