Harvesting energy from the environment and using it to perform different functions is one of the defining aspects of life. To distinguish “animate” from “inanimate” matter, we typically observe whether a certain body is activated by external forces or moves thanks to its internal energy. In this context, we commonly use the term “metabolism” to speak about how living bodies, through incredibly elaborate biochemical machinery, transform chemical energy into the driving force of all their activities.
Providing artificial bodies with their own “metabolism” isn’t an easy task. This is especially true when working with very small and soft objects, where circuits and engines, cogs and wheels aren’t an option. As we approach the microscale, the propulsion of tiny autonomous robots - known as “microswimmers” in scientific jargon - becomes a technological challenge. Often, the “metabolism” of these tiny “machines” (I’m unsure that the word “machine” can still apply to these systems) is fueled by chemical reactions or by an external driving force like a magnetic field; but these approaches make the robots completely dependent on external human control. Can we build a truly autonomous microswimmer?
The swimming droplet you see in the animation above was prepared in the lab from a simple mixture of three chemical ingredients: an “oily” hydrocarbon, a surfactant (something similar to the soap you use every day), and water. These surprisingly simple liquid robots mimic the flagella of microorganisms, spontaneously protruding elastic “tails” when the temperature of their environment drops below 8 °C. These non-living “organisms” autonomously change shape, shifting from spheres into perfectly symmetrical polyhedra. Although they are exponentially simpler than even the most primitive of living organisms, they mimic the behavior of life with an uncanny resemblance.
The transformation of the oil drops into autonomous swimmers is determined by a phase transition of the surface layer of the oil, which changes from liquid to solid. In this phase transition, some of the liquid oil inside the drop is ejected, creating flexible filaments that propel the robots forward. The robots can keep swimming for up to 12 minutes driven only by their internal energy. Surprisingly, the droplets are more likely to eject either one or two tails depending on the temperature of their environment, something that could be described as a rudimentary “adaptive” behavior. Once their energy store is depleted, they can be recharged easily by simply reheating their environment and then cooling it down again.
Most of our scientific understanding of matter relies on the notion that “things” don’t do much by themselves, and that they are instead animated by external forces that determine their behavior. For instance, the core principles of thermodynamics - the branch of physics that, among other things, tells us how systems transition from order to disorder - were developed with “passive” particles in mind. But what are we missing - both scientifically and culturally - by considering passivity as the general rule of matter and activity as the unlikely exception?
Many real-world material systems are actually composed of bodies with autonomous capacity for movement: from swarms of insects and flocks of birds to cars in trafficked city streets and bacterial colonies. Today, all of these vastly different systems are defined as “active matter”. The rapidly expanding field of active matter is studying how matter behaves on different scales when it is composed of autonomous “agents” rather than passive particles pushed around by external forces. The study of “active matter” isn’t simply theoretical: it’s also changing our approach to technology, pushing us to develop increasingly autonomous systems like the swimming robots I showed you today.
When I speak and write about materials science, I’m often asked why I choose to use words like “intelligent”, “perception”, “memory” and “self”, instead of simply talking about the mechanistic forces behind the behaviors I’m observing. My answer is that frequently, treating bodies as active rather than passive is a matter of cultural perspective. We (almost always) treat humans as “agents” because, culturally and practically, it makes sense to us; but, in theory, nothing forbids us to look at our “agency” as the result of a collection of mechanistic forces. As philosopher Jane Bennet has brilliantly explained in her book Vibrant Matter, the true nature of “agency” is largely a mystery: it appears to be more of a cultural and political category than a rigorously scientific one. “If we do not know just how it is that human agency operates”, Bennet asks, “how can we be so sure that the processes through which nonhumans make their mark are qualitatively different?”
References
Cholakova, D., Lisicki, M., Smoukov, S.K. et al. Rechargeable self-assembled droplet microswimmers driven by surface phase transitions. Nat. Phys. 17, 1050–1055 (2021).
Jane Bennett. Vibrant Matter. A Political Ecology of Things, Duke University Press, Durham, 2010.