From chaos to ‘intelligent’ soft matter

Disorder can make sense, and understanding its logic can be genuinely useful. A study by a research group at the University of Trento shows how a minimal chemical difference can turn chaotic, disordered motion into a highly coordinated and reproducible structure. The results open the way to designing a new generation of smart materials able to adapt to their environment, repair themselves or assemble autonomously: from tissues that regenerate in prosthetics, to soft robotics, to mechanical components and sensors capable of repairing micro-fractures on their own.

The study, carried out together with researchers from Sorbonne University in Paris and the University of Copenhagen, was published today in the journal npj Soft Matter. The results show that minimal microscopic variations in the chemical composition of synthetic agents do not cause random changes. Instead, they steer the system towards well-defined forms of self-organisation and collective behaviour.

The experiment: droplets with a chemical ‘motor’
The study focuses on the class of deformable or fluid materials (such as emulsions, gels, colloidal suspensions or biological tissues) known as ‘soft matter’. In particular, the work addresses ‘active’ soft matter, made up of units that are out of thermodynamic equilibrium and able to absorb energy from the environment or from an internal chemical reserve – in effect, a built-in ‘motor’ – and convert it directly into autonomous motion.

The research group analysed the behaviour of 50 oil droplets placed on water. The droplets are active because they release a substance that makes them move on their own. The authors of the study explain how it works: Roberto Menichetti and Raffaello Potestio of the Department of Physics, University of Trento, and of INFN-TIFPA; Martin Hanczyc and Silvia Holler of the Department of Cellular, Computational and Integrative Biology (CIBIO), University of Trento; first author Matteo Scandola (Sorbonne University, formerly a researcher at the University of Trento); and Richard Löffler of the University of Copenhagen.

“At the start, the droplets move at high speed like frenzied flies, colliding and bouncing off one another. Over the course of the experiment, however, they tend to aggregate into an ordered structure. Compared with similar past studies, the real breakthrough was introducing a small variation: we dyed 25 droplets blue and 25 red. The dye was not just a way to label and tell them apart: although it is an infinitesimal chemical change, it slightly altered the droplets’ surface, affecting how fast they consumed their ‘fuel’ and, consequently, the structure that forms.”

The researchers then describe the stages of the self-assembly process: “We found that the system evolves through five reproducible phases. At first, the droplets collide chaotic. As they consume fuel, the blue droplets slow down first and begin to stick together. The red droplets, still fast and active, end up trapping the blue ones, pushing them towards the centre and confining them there. Once the fuel runs out, activity stops. The result is a perfectly ordered, motionless ‘raft’, with a core of blue droplets enclosed by an outer ring of red droplets.

“It is a perfect example of an ‘emergent phenomenon’, just as happens in nature with flocks of birds: the individual has no overview and follows local rules, yet the group moves as a single coordinated organism,” they explain. “In particular, had the droplets been equally active, they would have mixed randomly without ever forming a concentric structure.” By changing only a nuance in activity, the researchers were thus able to ‘program’ the final organisation of matter. “The coexistence of extreme sensitivity to microscopic variations and strict reproducibility at the macroscopic level is one of the most remarkable aspects of this system.”

Potential applications
“Our long-term goal falls within the field of so-called ‘bio-inspired systems’, where we want to understand how the information contained in individual components can control the whole system, laying the foundations for the materials of the future: dynamic, responsive and able to adapt to their environment,” the researchers explain.

More specifically, the research provides new tools to guide the autonomous assembly of matter simply by acting on the speed and activity of its constituent elements, with no need for electronic parts or physical sensors. This opens potentially transformative prospects in the field of active soft matter: from smart materials able to adapt to their environment, self-repair or self-assemble (such as tissues that regenerate in biomedical prostheses), to liquid robotics devices.