Understanding active drops
Not all droplets are the same. In fact, some are active while others are passive. “Whereas passive droplets are in or near equilibrium, active droplets are inherently unstable,” explains Job Boekhoven(opens in new window), an associate professor at the Technical University of Munich(opens in new window) (TUM). This means that, unlike passive droplets, active droplets form in response to fuel and decay in its absence. In other words, the molecules that form active droplets must be produced via fuel-driven reactions, otherwise they will constantly fall apart. “None of this behaviour is what we are used to seeing with ‘normal’ droplets, such as those in your vinaigrette or milk,” adds Boekhoven. Helping to better understand the bizarre behaviour of active droplets is the EU-funded ActiDrops project.
The unexpected behaviour of active droplets
Coordinated by TUM and supported by the European Research Council(opens in new window), the ActiDrops project created model systems that allow researchers to study the unique phenomena and behaviours of active drops. One of the most interesting findings was that these droplets can regulate their size when active. According to Boekhoven, this behaviour is completely the opposite of what one finds when droplets are in equilibrium. “If you look at vinaigrette, you’ll see that the oil droplets are not stable but continue to grow,” he explains. Researchers also found a new and entirely unexpected behaviour: under certain conditions, active droplets can transform from a droplet into a shell of droplet material. “This is the equivalent of a solid bowling ball transforming into a hollow football,” notes Boekhoven. Boekhoven says that, from a thermodynamic point of view, this makes no sense as the new structure has a larger surface area, which is unwanted. “What we discovered is that the droplets behave this way not because of thermodynamics, but due to the kinetics of activation and deactivation,” he adds. “Figuring out why and how droplets do this was a very gratifying moment.”
From lab to life – active droplets are everywhere
The project’s findings are already having a real-world impact. “Active droplets are not some weird matter that can only be found in our lab, they are found everywhere in biology,” remarks Boekhoven. “The more we learn about them, the more we see how essential they are to a healthy life.” For example, having discovered that active droplets play a key role in disease, there’s the potential that they could be used to develop new materials for drug delivery and synthetic biology applications. However, doing so demands that researchers be able to study the biological version of active droplets. “The bottom-up model developed by the ActiDrops project makes such research possible,” says Boekhoven. Boekhoven and his team are already using these models to synthesise life. “Synthetic life would completely revolutionise biotechnology, enabling the production of medicine and the degradation of unwanted plastics and other contaminants,” he concludes.
