Membraneless Active Droplets

Updated 11 November 2025

Membraneless active droplets are dynamic liquid domains that lack a stabilizing membrane yet maintain persistent compartmentalization through phase separation, chemical turnover, and active agent incorporation.
They are investigated using state-of-the-art experimental methods and simulations such as coarse-grained molecular dynamics and phase-field models, revealing phenomena like subdiffusion, viscoelastic memory, and chemical feedback.
Their unique behaviors enable practical applications in synthetic biology, soft-material design, and protocell development by harnessing controlled motility, shape transformation, and arrested coarsening.

A membraneless active droplet is a spatially confined, out-of-equilibrium liquid domain that exhibits dynamic behaviors—such as motility, shape transformation, non-classical diffusion, or regulated assembly—arising from the intrinsic dynamics of its components or from embedding active agents. Unlike classical emulsions bounded by material interfaces, these droplets lack a stabilizing membrane but nevertheless maintain persistent compartmentalization and dynamical function through mechanisms including liquid–liquid phase separation, chemical turnover, mechanical activity, and interface-mediated flows. Such droplets are fundamental to the compartmentalization of biological matter (e.g., biomolecular condensates) and underpin synthetic platforms for reconfigurable soft materials, protocells, and engineered nanoreactors.

1. Physicochemical Foundations and Classification

Membraneless active droplets can be classified by their driving mechanisms, internal composition, and emergent behaviors. The key distinctions lie between droplets whose activity is intrinsic to their scaffold (e.g., phase-separated IDP condensates with viscoelastic subdiffusion), those whose dynamical properties are conferred by embedded active agents (e.g., bacteria, motors, enzymes), and those in which activity emerges via environmental coupling (e.g., externally fueled microemulsions or colloidal inclusions).

Types of Active Droplets:

Type	Driving Activity	Representative Systems and Mechanisms
Scaffold-intrinsic	Phase separation + rheology	IDP condensates (viscoelastic subdiffusion)
Chemically-driven	Fuel input, reaction fluxes	Marangoni-emulsion swimmers, enzyme-driven
Embedded active matter	Motors, bacteria, colloids	Bacteria in LLPS droplets, light-driven
Environmental coupling	Feedback, boundary effects	Trail-avoiding and rheotactic microdroplets

Distinguishing features common to all cases include (i) a lack of impermeable bounding membrane, (ii) the persistence of phase boundaries realized by concentration, chemical, or field gradients, and (iii) activity-induced flows, fluxes, or nonequilibrium shape changes distinguishing them from passive, classical droplets.

2. Molecular Dynamics, Mesoscale Transport, and Viscoelasticity

At the nanoscale, membraneless droplets formed by phase separation of IDPs or synthetic analogs display complex rheological properties including subdiffusive transport and spatial heterogeneity. Coarse-grained molecular simulations show that, for droplets composed of 1,000 FUS-LCD proteins in a homogeneous periodic environment, the time-averaged mean squared displacement (TAMSD) of protein centers of mass exhibits transient subdiffusion ( $\overline{\delta_i^2(\Delta;t)} \propto \Delta^{\alpha}$ , $\alpha\approx0.85\pm0.05$ for lag times up to $1\,\mu$ s), with a crossover to confined diffusion at the droplet size scale. Instantaneous diffusivities fluctuate by an order of magnitude due to proteins sampling dense interior versus dilute, more mobile interface regions.

The non-Gaussian nature of protein step-size distributions ( $0.5\lesssim G(\Delta)\lesssim 1$ for short lag times) arises from this heterogeneity. Displacement autocorrelation functions reveal pronounced negative lobes, diagnosing viscoelastic memory and anti-correlation of forces. Such viscoelastic responses are consistent with a fractional-Brownian-motion kernel or a power-law generalized Langevin equation. The result is a dynamic regime in which the droplet interior acts as a soft viscoelastic matrix (transient anticorrelated motion, non-Gaussian steps, subdiffusive scaling) but relaxes to ergodic, nearly Gaussian transport on the time scale set by exchange between interior and interface (Watanabe et al., 19 Jan 2024).

3. Active Motility, Interfacial Dynamics, and Chemically-Driven Flows

Micron- to mesoscale active droplets driven by interfacial activity realize complex motile and collective behaviors. Canonical examples include oil droplets in surfactant-rich aqueous baths, where solubilization generates Marangoni stresses. The governing equations couple outward diffusion of interface-modifying agents (oil-laden micelles) to tangential Marangoni slip:

$U_{d} = \frac{\Delta\sigma}{3 (\eta_0 + \eta_i)} R$

where $\Delta\sigma$ is the surface-tension difference, and $R$ the droplet radius (Chen et al., 15 May 2024, Khan et al., 9 May 2024).

Advanced models (e.g., the Non-Markovian Droplet, or NMD, framework) incorporate chemical memory; each droplet propels itself while leaving a trail of filled micelles, leading to a non-Markovian, self-avoiding-walk–like motion with persistence decaying over the trail diffusion timescale. Trajectories exhibit ballistic regimes, memory-induced crossovers to diffusive/aging statistics, hovering behavior above substrates (due to vertical phoretic forces), and collective super-diffusive transport regimes when many droplets interact via overlapping trails. Effective propulsion parameters (drift velocity, diffusion constant, trajectory persistence) are tunable by surfactant concentration, micellar diffusivity, and inter-droplet trail coupling coefficients (Chen et al., 15 May 2024).

In confined or flow-driven geometries, droplets demonstrate multistable flow topologies and chemodynamic transitions. As the Péclet number ( $\mathrm{Pe} = U R / D$ ) is tuned by droplet size or chemical gradient, one observes transitions from pusher-type to puller-type to higher-multipolar flow fields, and eventually to dynamic multipolar and bistable states (Ramesh et al., 2022). Surface saturation by filled micelles leads to cessation of active flows over O(1) s timescales, further modulating long-term behavior.

4. Shape Transformations and Morphological Complexity

Membraneless active droplets manifest a rich spectrum of non-equilibrium morphologies driven by internal active stresses. In 3D phase-field and nematohydrodynamic models, extensile active nematic droplets produce finger-like protrusions where disclination lines intersect the interface; contractile droplets show surface invaginations, cup-shaped deformations, run-and-tumble motion, or active wrinkling, depending on the interplay between activity strength, elastic constants, surface tension, and droplet size. Critical activity thresholds, defined by the capillary number $\mathrm{Ca} \sim \zeta R / \sigma$ , set bifurcation boundaries for shape transitions (Ruske et al., 2020).

The dynamical pathways for morphogenesis in these "living droplets"—such as synchronized oscillations, interface wrinkling, or division-like events—provide mechanistic analogies to phenomena in tissue morphogenesis, cancer invasion (finger protrusions), and macropinocytosis (contractile cup formation).

5. Arrested Coarsening, Size Selection, and Non-Equilibrium Stabilization

Passive liquid–liquid phase separation typically yields droplets that grow coarsely via Ostwald ripening or coalescence. In active systems, multiple mechanisms can arrest coarsening and define robust droplet size distributions.

Enzyme-driven feedback: When conversion rates between condensed (droplet-forming) and soluble protein states are catalyzed by spatially diffusing enzymes, the mean droplet size saturates at a value set by the balance of production and encounter-driven destruction events: $N_*\sim c / (\rho_B (D_e + D_{dp}))$ , with $N_*$ the mean protein number per droplet, $c$ the local conversion rate, $\rho_B$ the enzyme density, and $D_e$ the enzyme diffusion constant (Fries et al., 18 Nov 2024).
Embedded active matter: Droplets containing motile bacteria exhibit rapid initial growth (cell-mediated aggregation), but subsequently arrest their coarsening due to anti-phase locking of long-wavelength interface fluctuations between neighboring droplets. Hydrodynamic coupling synchronizes low-frequency modes, preventing physical contact and thus further fusion. The resulting state is a stable population of actively fluctuating, motile droplets maintained far from equilibrium (Liu et al., 6 Nov 2025).
Chemical drive and surface fluxes: Nonequilibrium chemical reaction cycles localized inside droplets modulate partitioning and residence times of molecular species, finely tuning droplet permeability and encounter kinetics. There exists an optimum in reaction-driven fluxes that accelerates molecular encounters up to a maximum, with the system interpolating between trapping-dominated (slow encounters) and leaky (homogeneous) limits (Fries et al., 9 May 2025).

6. Collective Dynamics, Environmental Coupling, and Programmability

Active droplets are subject to a range of collective and environmentally-coupled phenomena:

Collective rotations and synchronization: Ensembles of self-propelled droplets may spontaneously self-organize into 2D hexagonal clusters that hover above boundaries and rotate coherently via hydrodynamic coupling, despite the absence of explicit chiral drivers (Hokmabad et al., 2021). The rotation rate $\Omega$ scales inversely with cluster size ( $\Omega \propto N^{-1}$ ), and formation is favored for moderate Pe and limited by the strength of Marangoni-induced coupling torque between nearest neighbors.
Rheotactic and boundary-guided navigation: Proximity to solid boundaries or imposed flow gradients induces rheotactic responses, with droplets migrating upstream within a well-defined window of shear rates. In the case of nematic LC droplets, elastic stresses and defect dynamics interplay with hydrodynamic torques to yield controlled oscillatory or trapped motions. Periodic flow modulation allows for programmable shuttling between locations or deliberate trapping (Dwivedi et al., 2021).
Magnetic and solid–fluid perturbations: Embedding anisotropic magnetic clusters within droplets offers external programmability of both propulsion orientation and emergent collective phenomena, such as controlled reorientation, chiral curling, or the breakup of attractive droplet chains under rotating perturbations. Such hybridization provides a route beyond purely chemical or boundary programming of active emulsions (Khan et al., 9 May 2024).

7. Functional Implications and Design Principles

The diversity and controllability of membraneless active droplet behaviors have significant implications for both biology and soft-matter engineering:

Nanoreactor optimization: Chemical activity within droplets can be tuned to optimize molecular encounter rates, suggesting strategies for cells to modulate reaction kinetics through ATP-driven cycles or for synthetic platforms to accelerate catalysis via droplet engineering (Fries et al., 9 May 2025).
Compartmentalized assembly lines: Spatially structured reaction–diffusion within droplets enables the organization of stepwise assembly processes, enforcing temporal order and high fidelity in multi-component biosynthetic pathways impossible in well-mixed reactors (Harmon et al., 2021).
Protocellular and microreactor design: Light-programmable, enzyme-responsive, or mechanically-coupled synthetic droplets realize reconfigurable reaction centers, minimal cell mimics, or adaptive soft materials with life-like capabilities for cargo selection, mixing, division, or environmental sensing (Grauer et al., 2021, Liu et al., 2021).
Stabilization via activity: Arrested coarsening and robust control of condensate size, number, and stability may be tuned via the interplay of active processes, regardless of equilibrium miscibility boundaries. Such feedback offers a physical principle for the persistence of cellular organelles and new smart-material design directions (Liu et al., 6 Nov 2025, Fries et al., 18 Nov 2024).

In conclusion, membraneless active droplets unify concepts across nonequilibrium statistical physics, soft-matter hydrodynamics, and cellular biochemistry. Their paper, enabled by increasingly sophisticated experimental, numerical, and analytic frameworks, illuminates the dynamic organization and functionality of biological matter and guides bottom-up design of functional, adaptive soft materials in the absence of a membrane.