🔬 condensed matter

Active Reinforcement of Jammed Emulsions by Living Microswimmers

This study demonstrates that motile *Chlamydomonas reinhardtii* microswimmers mechanically reinforce jammed castor-oil-in-water emulsions by doubling their yield stress through a feedback loop where droplet confinement amplifies swimmer propulsion forces, thereby stiffening the microstructure via activity-induced attractive interactions.

Original authors: Marie Corpart, Hugo Le Roux, Daniel Bonn, Antoine Deblais

Published 2026-08-24

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CC BY 4.0

Original authors: Marie Corpart, Hugo Le Roux, Daniel Bonn, Antoine Deblais

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

Soft matter physics explores the strange, in-between world of materials that are neither solid nor liquid, but something fluid yet structured. Think of mayonnaise, toothpaste, or a thick paint: these substances flow when you push them hard enough, but hold their shape when left alone. This behavior often arises from "jamming," a state where tiny particles or droplets are packed so tightly together that they lock into place, forming a rigid network. Scientists have long understood how this happens with simple, passive ingredients, but a new frontier is opening up by adding life to the mix. The question is no longer just how particles jam, but what happens when those particles are crowded with tiny, living swimmers that constantly push and pull against their neighbors. Understanding this interaction could one day allow us to design materials that change their strength or texture simply by turning on or off the activity of the microscopic life inside them.

In a recent study, researchers at the University of Amsterdam investigated exactly this scenario by creating a thick, oily mixture and filling it with living algae. They started with an emulsion of castor oil droplets suspended in water, a system that naturally becomes a rigid, jammed solid when the oil takes up enough space. To this mixture, they added Chlamydomonas reinhardtii, a single-celled alga that swims by beating two tiny tails. The team created three different versions of this mixture to test what was really driving the changes in the material's strength. The first version contained only the oil and water. The second contained the same oil and water, but with algae that had been killed and immobilized, so they sat still like tiny rocks. The third version contained the living, swimming algae. By carefully measuring how much force was required to make each mixture flow, the researchers discovered that the living swimmers made the material significantly stronger, doubling its resistance to flow in some cases. The dead algae, sitting in the exact same spots, made almost no difference at all. This proved that the extra strength came not from the algae simply taking up space, but from the active energy they expended while swimming.

The key to this reinforcement lies in the tight squeeze the algae experience. As the researchers increased the amount of oil in the mixture, the droplets packed closer together, leaving smaller and smaller gaps for the algae to swim through. The team used a high-powered microscope to track the movement of individual algae and found that as the gaps shrank, the algae were forced into smaller and smaller cages formed by the surrounding oil droplets. In these tight spaces, the algae had to push harder against the walls of their cage to move. This confinement amplified the force they exerted on the surrounding network. The researchers found that the stronger the confinement, the greater the force the algae applied, which in turn made the entire network of oil droplets stiffer and harder to break apart. It is a self-reinforcing loop: the crowded environment traps the swimmers, the trapping makes them push harder, and that extra push locks the material together more tightly.

This discovery challenges the idea that adding living things to a mixture always makes it more fluid. While some swimming bacteria are known to thin out thick liquids, these algae, which pull on the water with their tails, acted to stiffen the jammed mixture. The study suggests that the algae create a kind of invisible glue between the oil droplets, effectively pulling them together and making the whole structure more resistant to breaking. The researchers were able to estimate the forces involved by looking at how much the algae were confined, and their calculations matched the extra strength they measured in the lab. This work identifies a feedback mechanism where the structure of the material controls the behavior of the living swimmers, and the swimmers, in turn, control the strength of the material. It opens the door to a new way of thinking about soft materials, where the mechanical properties of a substance could be programmed or tuned simply by managing the activity of the microscopic life within it.

Problem and Context
Jamming is a fundamental transition in soft matter where disordered packings of particles acquire rigidity once their packing fraction ( $\phi$ ) exceeds a critical threshold ( $\phi_c$ ). In athermal, purely repulsive systems like emulsions, this occurs at random close packing (RCP), where the material transitions from a shear-thinning fluid to a solid with a finite yield stress ( $\sigma_y$ ). While attractive interactions (e.g., via depletion) can induce solid-like behavior below $\phi_c$ , the role of active inclusions in displacing this rigid-fluid boundary remains unclear. Previous numerical studies and experiments on active suspensions have yielded conflicting results: pusher-type swimmers often reduce viscosity and promote unjamming, while puller-type swimmers (like Chlamydomonas reinhardtii) can increase viscosity. However, the specific regime where active particles are size-comparable to passive constituents, embedded within, and caged by a jammed soft solid, has not been experimentally addressed. The central question is how the mechanical coupling between confined microswimmers and a jammed droplet network affects macroscopic rheology.

Methodology
The authors investigated castor-oil-in-water emulsions stabilized by the biocompatible surfactant Pluronic F-127. The system was prepared with a mean droplet radius of $\approx 5$ $\mu$ m, comparable to the size of the microalga Chlamydomonas reinhardtii. The study compared three distinct states across a broad range of oil mass fractions ( $\phi_{oil}$ from 0.40 to 0.80):

Passive: Emulsions without algae.
Static Inclusions: Emulsions with algae immobilized by Lugol's solution (suppressing motility while maintaining cell presence).
Active: Emulsions with motile, living algae.

The experimental approach combined:

Oscillatory Rheometry: Stress sweeps were performed to measure storage ( $G'$ ) and loss ( $G''$ ) moduli, from which the yield stress ( $\sigma_y$ ) was extracted using the tangent method.
Confocal Microscopy: Nile Red labeled oil droplets and chlorophyll autofluorescence allowed for the visualization of the droplet network and the tracking of individual algal positions.
Single-Cell Tracking: Mean-square displacements of algae were analyzed to quantify the confinement length ( $\xi$ ) within the interstitial pores of the droplet network.

Key Results

Reinforcement by Activity: Oscillatory rheology revealed that motile algae systematically increase the yield stress of the emulsion by up to a factor of two compared to algae-free samples. This enhancement was observed across the entire range of oil fractions, including below the classical RCP threshold.
Activity vs. Steric Effects: Emulsions containing immobilized algae showed yield stresses essentially identical to those without algae. This confirms that the reinforcement originates from the activity (propulsion) of the swimmers rather than from the mere steric presence of inclusions.
Confinement-Enhanced Propulsion: Single-cell tracking demonstrated that as the oil fraction increases, the droplet network shrinks the interstitial pores, confining the algae. The confinement length $\xi$ scales as $\phi_{oil}^{-2}$ .
Quantitative Agreement: The authors estimated the active stress contribution ( $\sigma^a_\phi$ ) based on the known relationship between confinement and propulsion force for C. reinhardtii (where force increases exponentially under strong confinement). Using the measured confinement scaling, a simple model predicted an active stress increase that matched the magnitude and evolution of the measured excess yield stress.

Mechanism and Interpretation
The paper proposes a feedback loop mechanism:

The dense microstructure of the jammed emulsion confines the swimmers in shrinking pores.
This confinement amplifies the propulsion forces exerted by the algae (a phenomenon previously established in bulk confinement studies).
These amplified active forces generate effective, depletion-like attractions between the passive droplets, strengthening the contact network and increasing the macroscopic yield stress.

Significance and Claims
The authors identify active emulsions as a model platform for programming the mechanics of disordered soft solids through activity. The work demonstrates that biological activity can serve as a tunable control parameter to enhance the rigidity of dense emulsions beyond what is achievable with passive inclusions. Crucially, the study highlights a coupling between microscopic activity and macroscopic mechanics mediated by confinement. The authors suggest that the sign of the active contribution to rheology (fluidization vs. rigidification) depends jointly on the swimming mechanism (puller vs. pusher) and the state of the passive matrix (jammed vs. fluid). This provides a route to designing active materials with programmable mechanical properties by exploiting the interplay between confinement and active stress generation.

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