Researchers at the University of Vienna have uncovered a surprising phenomenon: polymer chains with segments that simply fluctuate at different intensities can spontaneously develop directional, persistent motion when densely packed – even though nothing in the system points them in any particular direction. This "entropic tug of war," driven by fundamental physical constraints, could help explain how DNA organizes and moves inside living cells, and may lead to new materials. The study was currently published in Physical Review X.

"Think of a chain threaded through a dense forest of trees, which represent obstacles posed by the other chains in the system. One end of the chain is being shaken much more vigorously than the other," explains lead author Jan Smrek from the Faculty of Physics at the University of Vienna. "You might expect it to just wiggle randomly in place. But we found that because the chain has to find its way by going in-between the trees, the difference in shaking intensity creates an imbalance that actually propels the entire chain forward through the forest."

This refers to a polymer, a large molecule consisting of many units linked together in a long chain, such as DNA. The Viennese research team – Adam Höfler, Iurii Chubak, Christos Likos and Jan Smrek – used computer simulations and analytical theory to show that this directed motion arises purely from topological constraints. When polymer chains are entangled and cannot pass through each other, segments with stronger fluctuations generate larger entropic forces (See Figure.1 for explanation). This creates an imbalance that pushes the entire chain forward along its own contour, with the stronger fluctuating part acting as the “head of the snake” moving through the forest of obstacles.

Unlike previous active polymer models that build upon directional forces, this mechanism requires only a difference in fluctuation magnitude between segments. The finding has direct relevance to chromatin – the complex of DNA and proteins in cell nuclei. Various cellular processes like transcription and DNA repair create localized regions of enhanced activity along the chromatin fiber. The researchers' work suggests these activity differences alone could drive the coherent chromatin motions observed in living cells.

The study also reveals how the dynamics depend on the degree of chain entanglement. At higher densities, the directed motion becomes faster and more pronounced. The researchers found that individual segments can exhibit superdiffusive motion – moving faster than random diffusion would predict – on intermediate timescales.

"This work bridges materials science and biology," says Smrek. "We're showing that the same physics that governs synthetic polymers can explain behaviors in living systems. And it suggests we could design new materials that spontaneously develop directed transport properties," adds Smrek.

The findings open new avenues for creating functional active materials and provide a framework for interpreting chromatin dynamics experiments. They could further investigate how these effects combine with other active processes in biological systems and explore applications in smart materials that could transport cargo or heal themselves.

The research was supported by the European Union through the QLUSTER project. This project builds on Adam Höfler's Master's thesis under supervision of Jan Smrek.

Summary:

• Polymer chains with segments that fluctuate at different magnitudes spontaneously develop persistent, directed motion when densely packed
• The mechanism arises from an imbalance in entropic forces at chain ends due to topological constraints – chains cannot cross each other
• No built-in directional forces are needed; the difference in fluctuation magnitude alone drives the effect
• The findings help explain chromatin dynamics in living cells and could enable new self-propelling materials
• Individual segments exhibit superdiffusive motion, moving faster than random diffusion on intermediate timescales

About the University of Vienna:

At the University of Vienna, curiosity has been the core principle of academic life for more than 650 years. For over 650 years the University of Vienna has stood for education, research and innovation. Today, it is ranked among the top 100 and thus the top four per cent of all universities worldwide and is globally connected. With degree programmes covering over 180 disciplines, and more than 10,000 employees we are one of the largest academic institutions in Europe. Here, people from a broad spectrum of disciplines come together to carry out research at the highest level and develop solutions for current and future challenges. Its students and graduates develop reflected and sustainable solutions to complex challenges using innovative spirit and curiosity.