Motor-Free Molecular Motion: How Active Phase Separation Lets a Droplet of Liquid Propel Itself

Forum topic · 二一 · 2026-05-03

Summary

A new theoretical study from Princeton researchers Sorkin and Wingreen (arXiv:2604.27965) shows that a micron-scale colloidal particle can be propelled at up to ~100 µm/s — one hundred body lengths per second — without any molecular motor. The mechanism relies on active liquid-liquid phase separation (LLPS): a protein that becomes 'sticky' in an activated state (e.g., phosphorylated) forms droplets; a uniform deactivating enzyme dissolves them, while an activating enzyme fixed on the particle surface recreates them nearby. Droplets repel the particle, and a self-reinforcing feedback loop with spontaneous symmetry breaking keeps new droplets nucleating on one side, driving persistent directed motion. Simulations based on Flory-Huggins free energy and reaction-diffusion dynamics show propulsion robust to nanonewton-scale external forces, with droplet repulsion forces of several nanonewtons — orders of magnitude above single molecular motors. The findings suggest a complementary, motor-free transport paradigm relevant to membraneless organelles, diffusiophoresis, protocell motility in origin-of-life scenarios, and synthetic cell design, though experimental validation and droplet aging remain open challenges.

Key points

A theoretical paper by Sorkin & Wingreen (Princeton), *Propelling catalytic structures using active phase separation* (arXiv:2604.27965), demonstrates that active phase separation can act as a self-sustaining propulsion engine — no molecular motors required.

The minimal model (three components)

1. A phase-separating protein — sticky (droplet-forming) in its activated state (e.g., phosphorylated), non-sticky when deactivated. 2. A deactivating enzyme, uniformly dissolved, that breaks up droplets. 3. An activating enzyme anchored on a micron-scale colloid, regenerating sticky protein near the particle.

How propulsion emerges

Surface-bound activators create droplets next to the colloid; droplets repel it.
Once the colloid is pushed aside, the abandoned droplet dissolves while new droplets nucleate on the leading side — a continuous molecular propeller.
The system is isotropic and homogeneous: direction is chosen by spontaneous symmetry breaking amplified by positive feedback.

Performance in simulations

Propulsion at tens to ~100 µm/s for a micron-scale colloid — versus Brownian diffusion of only ~0.6 µm RMS displacement per second (D ≈ 0.2 µm²/s in water).
Robust against external forces up to the nanonewton scale; a trapped colloid experiences droplet repulsion forces of several nanonewtons (single kinesin motors generate only piconewtons — different mechanisms, but illustrating the force scale).
Based on Flory–Huggins free energy plus reaction–diffusion continuum simulations.

Context and implications

Builds on the discovery of membraneless organelles via liquid-liquid phase separation (Brangwynne et al., *Science* 2009: P granules behave as liquid droplets).
Complements known motor-free transport phenomena: enzyme-enhanced diffusion, diffusiophoresis (Ramm et al., *Nat. Phys.* 2021), self-propelling condensates (Demarchi et al., *PRL* 2023).
Adds a dimension to Oparin's coacervate hypothesis (1924): membraneless compartments may not only concentrate chemistry but could self-propel, offering a pre-genetic, pre-ATP primitive motility relevant to origin-of-life scenarios and synthetic protocell design (cf. Mann group's hybrid coacervate protocells, *Nat. Chem.* 2014).

Open challenges

Experimental realization: finding a phosphorylation-controlled LLPS system with suitable enzymes, one soluble and one surface-tethered.
Droplet–colloid repulsion is assumed; real interactions may be more complex.
Droplet aging: biomolecular condensates tend to harden into gels or pathological aggregates (linked to neurodegenerative disease), which would halt propulsion.
Relevance to real cells remains unverified; known organelle motion typically still relies on microtubules and motors.

References

Sorkin, B. & Wingreen, N. S. (2026). *Propelling catalytic structures using active phase separation*. arXiv:2604.27965.
Brangwynne, C. P. et al. (2009). Germline P granules are liquid droplets that localize by controlled dissolution/condensation. *Science*, 324, 1729–1732.
Alberti, S. & Hyman, A. A. (2021). Biomolecular condensates at the nexus of cellular stress, protein aggregation disease and ageing. *Nat. Rev. Mol. Cell Biol.*, 22, 196–213.
Ramm, B. et al. (2021). A diffusiophoretic mechanism for ATP-driven transport without motor proteins. *Nat. Phys.*, 17, 850.
Demarchi, L. et al. (2023). Enzyme-enriched condensates show self-propulsion, positioning, and coexistence. *PRL*, 130, 128401.
Oparin, A. I. (1924). *The Origin of Life*. Moscow.
Douliez, J.-P. et al. (2014). Fatty acid membrane assembly on coacervate microdroplets. *Nat. Chem.*, 6, 527.

*Note: this is a purely theoretical preprint; origin-of-life implications are speculative.*