Attraction alone is enough to make droplets chase each other
Within the microscopic interior of living cells, clusters of proteins and molecules have long been understood to organize themselves through a balance of opposing forces — attraction pulling inward, repulsion pushing outward. Physicists at the Max Planck Institute for Dynamics and Self-Organization have now found that this balance may not be necessary at all: attraction alone, it turns out, is sufficient to generate the kind of dynamic, self-propelled movement once thought to require far more complexity. The discovery invites a quieter but profound revision of how we understand life's capacity to organize itself from within.
- Conventional physics predicted that two droplets bound only by mutual attraction would simply collapse into one another and go still — instead, they began to chase each other.
- The unexpected run-and-chase behavior upends decades of assumptions about phase separation, the process by which cells sort and cluster their molecular machinery.
- Researchers built mathematical models varying the size, shape, and chemical activity of the droplets, mapping the precise conditions under which attraction alone generates complex motion.
- The findings reframe cellular self-organization as something that can emerge from simpler rules than previously believed, with living cells as the proof of concept already operating at this edge.
- Scientists now see a path toward engineering artificial molecular machines capable of self-propulsion — with potential applications in drug delivery, synthetic biology, and controlled nanoscale motion.
Inside living cells, proteins and chemicals cluster into dense droplets called condensates — not static structures, but restless ones that shift and rearrange to shape what a cell can do. A team at the Max Planck Institute for Dynamics and Self-Organization recently discovered something that defied expectation: these droplets don't need both attractive and repulsive forces to move. Attraction alone is enough.
Conventional logic would predict otherwise. A system where everything pulls toward everything else should collapse into a single, stable mass and stay there. Lead researcher Jacopo Romano acknowledged as much — that's what the physics seemed to demand. But when his team modeled two condensates bound only by mutual attraction, the droplets didn't merge or go still. They began to chase each other.
The behavior resembled what physicists call a run-and-chase dynamic — a pattern typically associated with nonreciprocal systems involving both attraction and repulsion. The lanternfish offers an intuitive analogy: one fish drawn to another's light, that fish drawn to a third, a chain of pursuit emerging from asymmetry. Here, with only attraction in play, the droplets achieved something similar through variations in size, shape, and chemical activity alone.
Ramin Golestanian, who directs the institute's Department of Living Matter Physics, described the work as evidence that nonequilibrium emulsions — droplet systems held in dynamic states — can produce behaviors that appear complex but arise from surprisingly simple rules. The implications reach in two directions: toward a deeper understanding of how living cells maintain their internal order, and toward the design of artificial molecular machines capable of self-propelled motion without external intervention. Published in Physical Review Letters, the study marks a meaningful shift in how physicists understand the relationship between force, motion, and life.
Inside living cells, work gets done through a process of molecular sorting—certain proteins and chemicals cluster together, forming dense droplets that float within the cellular fluid. These condensates, as they're called, aren't static structures. They shift, merge, and rearrange themselves, and how they interact with one another shapes what the cell can do. A team of physicists at the Max Planck Institute for Dynamics and Self-Organization recently discovered something counterintuitive about these droplets: they don't need both attractive and repulsive forces to chase each other. Attraction alone is enough.
The conventional wisdom would suggest otherwise. If you have a system where everything pulls toward everything else, you'd expect all the material to collapse into a single, stable blob and stay there. Jacopo Romano, the lead researcher on the study, put it plainly: that's what logic would predict. But when his team built a mathematical model of two droplets with mutual attraction and nothing else, the system did something unexpected. Instead of merging or sitting still, the droplets began to move. They chased each other.
The physicists started with the simplest possible setup—two condensates, nothing more. They introduced mutual attraction between them and then watched what happened as they varied the size, shape, and chemical activity of each droplet. Different combinations produced different behaviors. Some of these behaviors resembled a phenomenon already known from other systems: the run-and-chase dynamic. That pattern typically shows up in nonreciprocal systems, ones where both attraction and repulsion are at play, creating an asymmetry that drives motion. The lanternfish offers a useful analogy—imagine a fish drawn to the light of another fish's lantern, while that second fish is itself drawn to a third light, creating a chain of pursuit. Here, with only attraction in the mix, the droplets achieved something similar.
What makes this finding significant is that it challenges a basic assumption about how molecular systems organize themselves. For decades, researchers have understood phase separation—the clustering of like molecules—as a process driven by the interplay of multiple forces. The discovery that attraction alone can generate complex, dynamic behavior opens new questions about how cells actually manage their internal organization. Cells are far from equilibrium; they're constantly burning energy to maintain their structure and function. This model suggests that even in simplified conditions, the physics of attraction can produce the kind of movement and rearrangement that cells rely on.
Ramin Golestanian, who directs the Department of Living Matter Physics at the institute, framed the work as an example of how nonequilibrium emulsions—systems of droplets held in dynamic states—can be engineered to exhibit behaviors that look reciprocal but emerge from simpler rules. The implications ripple outward in two directions. On one side, the findings illuminate how cells organize and maintain themselves, a question central to understanding life at the molecular level. On the other, they suggest a blueprint for designing artificial molecular machines that could move and propel themselves without external intervention. Such self-propelling systems could have applications in drug delivery, synthetic biology, and other fields where controlled molecular motion is valuable. The work was published in Physical Review Letters, and it represents a shift in how physicists think about the relationship between force and motion in biological systems.
Notable Quotes
It's natural to think that a system with only attractive forces would form one large, stationary condensate. However, instead we observed an unexpected emergent property of chasing dynamics resulting in movement and propulsion.— Jacopo Romano, lead author
This is an interesting example of how a nonequilibrium emulsion can be engineered to exhibit nonreciprocal chasing interactions between droplets.— Ramin Golestanian, director of the Department of Living Matter Physics
The Hearth Conversation Another angle on the story
So if these droplets only attract each other, why don't they just merge into one big droplet and stop moving?
That's the intuitive question, and it's what makes the finding surprising. The answer has to do with how the droplets are shaped and sized. When you have two droplets of different sizes or chemical compositions, the attraction between them doesn't pull them together uniformly. Instead, it creates an imbalance that makes them orbit or chase each other.
It sounds like you're saying the droplets are never quite in equilibrium with each other.
Exactly. They're always slightly out of balance. One is always pulling the other in a way that creates motion rather than merger. It's like two dancers where one is always slightly ahead, so the other keeps reaching forward.
Does this happen in actual cells, or is this just a theoretical model?
The model is simplified—just two droplets in a controlled system. But cells contain hundreds of these condensates, all with different sizes and chemical makeups. The principle we've discovered could explain some of the dynamic reorganization that happens inside cells all the time.
What's the practical use of understanding this?
If you can engineer droplets to chase each other using only attraction, you could build molecular machines that move on their own. Imagine a delivery system that propels itself through tissue without needing external energy input. That's the kind of application this opens up.