🔬 condensed matter

Active motility and wetting cooperatively regulate liquid-liquid phase separation

By combining experiments on motile *Pseudomonas aeruginosa* with hydrodynamic simulations, this study reveals that the coupling between bacterial activity and interfacial wetting regulates liquid-liquid phase separation by converting self-propulsion into interfacial stresses that dictate complex morphologies and exert a dual kinetic role in either suppressing or accelerating droplet coarsening depending on the phase composition.

Original authors: Dixi Yang, Anheng Wang, Chunming Wang, Hajime Tanaka, Jiaxing Yuan

Published 2026-07-09

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CC BY 4.0

Original authors: Dixi Yang, Anheng Wang, Chunming Wang, Hajime Tanaka, Jiaxing Yuan

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a crowded dance floor where the music is a mix of two liquids that usually don't get along: a thick, sugary syrup (dextran) and a slippery, soapy water (polyethylene glycol). Normally, if you mix these two, they quickly separate into big, distinct blobs to minimize their contact, like oil and vinegar in a salad dressing.

Now, imagine you introduce a swarm of tiny, energetic bacteria (Pseudomonas aeruginosa) into this mix. These aren't just sitting still; they are swimming frantically. The paper you shared reveals that these swimming bacteria don't just float around; they act like a conductor for the liquids, changing the entire dance routine based on how much syrup is in the room.

Here is the story of what happens, broken down into three acts:

Act 1: The Spinning Tops (When there is plenty of syrup)

When the sugary syrup is abundant, the bacteria love to hang out right on the boundary line between the syrup and the soapy water. Because they are so active and love to stick to this boundary, they start pushing against the liquid.

The Analogy: Think of a child spinning a hula hoop. The bacteria are the child, and the liquid droplet is the hoop. Because the bacteria are stuck to the edge and swimming, they make the entire droplet spin like a top.
The Result: These spinning droplets repel each other. Just like two spinning tops that push away when they get too close, these droplets refuse to merge into one giant blob. This stops the "growing up" process (coarsening) of the droplets, keeping the mixture full of many small, spinning balls instead of a few big ones.

Act 2: The Beaded Necklace (When syrup and bacteria are equal)

As the researchers reduced the amount of syrup, there wasn't enough to go around for every bacteria to have its own private party.

The Analogy: Imagine the bacteria are beads and the remaining syrup is a thin thread. Since there isn't enough syrup to make big separate balls, the bacteria pull the little bits of syrup together to form a long, connected chain.
The Result: Instead of separate spinning tops, you get a long, wiggly necklace of droplets. This is a stable, non-equilibrium shape that only exists because the bacteria are active and holding the thread together.

Act 3: The Spiderwebs (When there is very little syrup)

When the syrup becomes very scarce (much less than the number of bacteria), the rules change completely. The bacteria start acting like magnets for each other.

The Analogy: Imagine the bacteria are tiny spiders. Even though they are far apart, they can sense a tiny drop of syrup nearby. They pull that syrup toward themselves, creating a sticky "bridge" of liquid that connects them to their neighbors.
The Result: This creates a rapid, web-like cluster where bacteria grab onto each other over long distances. In this scenario, the bacteria's activity actually speeds up the process of them clumping together, rather than slowing it down.

The Secret Ingredient: "Wetting Glue"

The paper highlights a crucial discovery: the bacteria's ability to swim (activity) only works this way because they have a specific "sticker" on their surface that likes to stick to the liquid boundary (wetting).

The Analogy: Think of the bacteria as a car engine. The engine (swimming) is powerful, but if the car has no tires (wetting), it just spins its wheels in the air and goes nowhere. The "wetting" is the tires that grip the road, allowing the engine's power to actually move the car (the liquid droplets). Without this grip, the swimming bacteria would just swim in circles without affecting the liquid shapes.

The Biological Twist

To prove this isn't just a lab trick, the researchers looked at a specific protein found in real bacterial biofilms (called Bap). They found that this protein acts like a super-sticky "wetting glue." Even in very dilute, watery environments where bacteria usually stay far apart, this protein allows them to grab onto each other and form clusters, just like the experiments predicted.

The Big Picture

The main takeaway is that activity and wetting work together as a team.

Activity provides the energy (the swimming).
Wetting provides the grip (the sticking to the boundary).

Together, they allow bacteria to control the shape of their world, deciding whether to spin in place, form chains, or build webs. This isn't just about bacteria; it's a new physical rulebook for how living things can organize themselves in liquids, turning simple swimming into complex architectural control.

Technical Summary: Active Motility and Wetting Cooperatively Regulate Liquid–Liquid Phase Separation

Problem Statement
Liquid–liquid phase separation (LLPS) in aqueous two-phase systems (ATPS) is a fundamental phenomenon in physical and biological sciences, governing processes from intracellular compartment formation to industrial biotechnology. While the dynamics of passive phase separation and domain coarsening are well-established, the regulation of these processes by active agents, such as motile bacteria, remains largely unexplored. Previous research has focused on hydrodynamic interactions (HIs) generated by self-propelled bacteria or the partitioning of passive colloids. However, a systematic understanding of how the coupling between bacterial activity and interfacial wetting steers phase separation, dictates pattern morphology, and controls coarsening kinetics is missing. Specifically, the biological implications of this coupling for organizing bacterial communities in dilute suspensions have not been investigated.

Methodology
The authors employ a combined approach of experimental observation and computational simulation to investigate Pseudomonas aeruginosa in a dextran (DEX)–polyethylene glycol (PEG) mixture.

Experiments: The study utilizes spinning-disk confocal microscopy to visualize 3D phase separation patterns in real-time. Contact angle measurements are performed to quantify the wettability of P. aeruginosa at DEX–PEG and water/PEG/DEX–air interfaces. The volume fraction of the DEX phase ( $\phi_{dex}$ ) is systematically varied relative to the bacterial volume fraction ( $\phi_{bac}$ ) to isolate competition between coarsening, wetting, and activity.
Simulations: A coarse-grained particle–field hybrid model is developed using Fluid Particle Dynamics (FPD). Bacteria are modeled as pusher-type swimmers (head, tail, virtual particle) with an activity force dipole. The binary fluid mixture is described by a Ginzburg–Landau free-energy functional. Crucially, bacteria–fluid wetting is incorporated via surface-coupling terms with affinity coefficients for the head and tail, encoding the symmetry breaking required for activity–wetting coupling.

Key Contributions and Results
The study identifies interfacial wetting as the symmetry-breaking element that renders bacterial activity mechanically effective, allowing active stresses to couple to LLPS. By varying $\phi_{dex}$ , the authors uncover three distinct non-equilibrium morphological regimes:

Self-Spinning Droplets ( $\phi_{dex} > \phi_{bac}$ ):
- Mechanism: Bacteria pinned at the DEX-rich droplet interface break fore–aft symmetry. This converts force-free activity into a polar interfacial stress, generating a coherent torque that drives spontaneous droplet rotation.
- Dynamics: This rotation induces effective hydrodynamic repulsion between droplets at low Reynolds numbers. Consequently, activity suppresses domain coarsening, resulting in significantly smaller droplet sizes compared to systems with non-motile bacteria or bacteria-free controls. This represents a kinetic frustration where coalescence is arrested.
Elongated Droplet Chains ( $\phi_{dex} \approx \phi_{bac}$ ):
- Mechanism: As the DEX phase becomes limited, droplets connect to share the available minority phase among interfacial bacteria.
- Dynamics: Interfacial pinning and activity cooperatively stabilize this hierarchical, non-equilibrium state, preventing further coarsening and enabling long-lived droplet chains.
Branched Capillary-like Clusters ( $\phi_{dex} < \phi_{bac}$ ):
- Mechanism: In this regime, the role of activity reverses. Wetting-mediated attraction dominates. Chemical-potential gradients drive solvent fluxes toward the inter-bacterial region, forming capillary bridges.
- Dynamics: Unlike the droplet regime, activity accelerates coarsening here. Motility enhances aggregation, allowing bacteria to capture neighbors over micrometer distances—far exceeding the range of depletion attraction—leading to rapid cluster growth.

Biological Validation
To validate the mechanism in a biological context, the authors demonstrate that biofilm-associated proteins (Bap) act as an interfacial "wetting glue." Even in dilute suspensions, the presence of Bap promotes bacterial clustering via the wetting-mediated mechanism, confirming that endogenous proteins can trigger assembly through this physical principle.

Significance and Claims
The paper establishes activity–wetting coupling, rather than activity or wetting alone, as the governing physical principle for regulating pattern morphology and coarsening dynamics in active matter.

Dual Kinetic Role: The authors show that activity has a dual effect on coarsening: it suppresses coarsening via flow-induced repulsion in the droplet regime but accelerates it via wetting-mediated attraction in the capillary regime.
General Framework: The findings provide a general framework for bacterial community formation, suggesting that interfacial wetting is the key element that activates bacterial motility to mechanically couple with phase separation.
Practical Implications: The work suggests strategies for stabilizing bio-emulsions and controlling aggregation by introducing a minor wetting phase, bridging microbiology and soft matter physics.

The authors conclude that their simulation framework is broadly applicable to modeling active matter in ATPS, offering a unified explanation for how active motility and wetting cooperatively engineer non-equilibrium states.