Tangling induced phase separation in active polymers

Phase separation is a fundamental phenomenon in which a homogeneous system spontaneously separates into two or more distinct phases, each with different properties or compositions. It is observed in systems ranging from simple mixtures to complex biological systems and living organisms. Using experiments and state-of-the-art simulations, we study aggregation and phase separation in a collection of California blackworms (Lumbriculus variegatus). These worms are flexible and elongated entities capable of entangling with one another. At high density, they spontaneously aggregate and tangle with each other, forming highly dense entangled worm blobs and less dense worm regions—a process reminiscent of polymer phase separation. We introduce an active polymer model, where each active polymer exhibits unique head dynamics that enable self-propulsion and entanglement. In our simulations, we observe that the fusion of different blobs occurs through entanglement, leading to the formation of highly dense active polymer domains co-existing with regions of low or nearly zero polymers. We observe blobs of all forms in experiments, which we validate through simulations. This behaviour is unique to the living organism, and our study will provide new perceptions to our traditional understanding of demixing processes. We map a phase diagram using the topological linking number to delineate the transition between demixed and mixed states as a function of static and dynamic parameters of individual worms.