The substance behaved like a contradiction. Pour it from a beaker, and it flowed like sand. Yet optical microscopy revealed its true nature: water droplets, each coated in nanoparticles, forming an aggregate that somehow stayed dry to the touch.

This "dry liquid"—a powder comprising particle-stabilized water droplets—has found a new application. Researchers from the University of Leeds and Osaka Institute of Technology have demonstrated its transformation into non-spherical biodegradable microcapsules with unusual properties: they release small molecules gradually but trap larger ones until physically ruptured.

The work addresses two persistent challenges in encapsulation technology. Most microcapsules derive from spherical emulsion droplets, which offer minimal surface contact with target substrates—problematic for applications requiring adhesion. Additionally, growing environmental concerns demand biocompatible, degradable materials. The new approach delivers both: asymmetric morphology and green chemistry.

Beyond Spheres

Spherical capsules dominate commercial applications—pharmaceuticals, cosmetics, agrochemicals. Their geometry creates inherent limitations.

The mathematics are unforgiving. Spheres minimize surface area for a given volume. This efficiency, advantageous in some contexts, proves detrimental when capsules must adhere to surfaces or maximize interfacial contact. A sphere touching a flat surface contacts at essentially a point. Deform that sphere into an ellipsoid or irregular shape, and the contact area increases substantially.

Previous methods for creating non-spherical capsules exist—layer-by-layer assembly, nanolithography, inorganic templates, external fields during formation. These techniques share common drawbacks: time-intensive protocols, specialized equipment, difficult scalability. Industrial adoption requires simpler approaches.

The dry liquid route offers operational simplicity. The material itself—first reported in 2006—consists of liquid droplets individually coated with hydrophobic nanoparticles. Unlike liquid marbles, which are individual coated droplets, dry liquids are bulk powders with reduced cohesion between droplets. They flow freely, can be stored long-term, and disperse readily in oils.

Earlier work demonstrated dry liquids' utility in gas storage, heterogeneous catalysis, and sensor technologies. The current research extends this to capsule fabrication.

Powder to Emulsion

The fabrication begins with controlled chaos: 25 weight-percent polyvinyl alcohol (PVA) aqueous solution meets a powder bed of hydrophobic silica nanoparticles in a high-shear blender. Five seconds of blending suffices.

Before mixing, the PVA solution refuses to wet the nanoparticle powder—a visual confirmation of suitable surface chemistry. Post-blending, the result appears as free-flowing powder despite containing aqueous solution. No bulk liquid remains visible; the water exists entirely as encapsulated droplets.

Optical microscopy reveals the structure: irregularly shaped aggregates covered in silica nanoparticles, ranging from roughly 10 to 100 micrometers. The droplets deviate substantially from sphericity—a consequence of the formation mechanism.

During high-shear mixing, the PVA solution breaks into droplets that elongate under mechanical stress. Simultaneously, silica nanoparticles adsorb at the air-water interface. The high viscosity of PVA solution slows droplet relaxation toward spherical shapes. Meanwhile, particles crowd at the interface, becoming jammed in place.

This interfacial jamming locks the non-spherical morphology. Particles form a rigid network that prevents shape relaxation even after shear stress ceases. Another possible mechanism: partially coated droplets undergo arrested coalescence—when they merge, decreasing interfacial area, not all adsorbed particles can redistribute, creating interfacial networks that freeze mid-coalescence into deformed shapes.

Either way, the PVA enhancement proves critical. Dry liquid prepared from pure water produces predominantly spherical droplets. The 25% PVA formulation yields pronounced asymmetry.

Oil Dispersion and Polymerization

The researchers dispersed the dry liquid powder into various oils, evaluating stability and compatibility. Criteria included: interfacial tension with and without particles, solubility of the aqueous phase in oil, potential for unwanted polymerization reactions.

Alcohols and fatty acids showed problematic characteristics—rhodamine B dye (used as a tracer molecule) leaked into these polar oils. Pentane and octane caused oil evaporation and droplet aggregation. After systematic testing via pendant drop tensiometry and visual assessment, squalane emerged as optimal.

Squalane—a saturated hydrocarbon—is biologically relevant, appearing in human sebum and cosmetic formulations. It showed significant interfacial tension reduction in the presence of silica particles and PVA (compared to bare oil-water interfaces), minimal solubility of the aqueous phase, and no spontaneous polymerization of the capsule-forming monomer.

Once dispersed in squalane, the dry liquid forms a Pickering-type water-in-oil emulsion. Droplets maintain their particle coatings and non-spherical morphologies in the oil continuous phase.

Capsule formation proceeds through interfacial polymerization. Butyl 2-cyanoacrylate (BCA)—a biocompatible, biodegradable monomer related to medical adhesives—dissolves in the squalane and polymerizes via anionic mechanism exclusively at the water-oil interface. The aqueous phase provides hydroxide ions that initiate polymerization, while the oil phase supplies monomer.

The resulting poly(butyl 2-cyanoacrylate) film encases each droplet, creating a polymeric shell reinforced by embedded silica nanoparticles. Crucially, polymerization remains confined to interfaces; no bulk polymer forms in the squalane.

Quantifying Asymmetry

Flow cytometry provided detailed characterization. The technique measures individual particles as they pass through a detector, capturing two-dimensional projected images and calculating equivalent spherical diameters, aspect ratios, and circularity metrics.

Results confirmed substantial anisotropy. Capsules formed from PVA-based dry liquid exhibited:

• Number-average equivalent spherical diameter: 12 micrometers

• Volume-average diameter: 38 micrometers

• Average aspect ratio: 0.59 (major axis roughly 1.7× minor axis)

• Average circle fit: 0.62 (substantial deviation from perfect circles)

Only 7% of capsules appeared perfectly spherical; 37% had aspect ratios below 0.5, meaning their major axes exceeded twice their minor axes. By comparison, capsules from water-only dry liquid (no PVA) showed average aspect ratio of 0.75 and circle fit of 0.83—closer to spherical but still anisotropic.

Scanning electron microscopy of dried capsules revealed deflated but structurally intact polymeric shells retaining elliptical morphologies—evidence of genuine film formation rather than mere particle aggregation.

Size-Selective Release

Release studies distinguished between small and large molecular cargo.

Rhodamine B (molecular weight 479 Da) served as a small-molecule simulant. Capsules and unencapsulated dry liquid samples were placed in dialysis tubing submerged in 40 weight-percent isopropanol solution—a release medium that extracts the dye.

Initially, capsules showed markedly slower release than bare dry liquid, confirming the polymeric barrier's function. However, over 80 hours, capsules released 80% of rhodamine B; by 140 hours, essentially complete release occurred.

The small molecule could permeate through the porous poly(butyl 2-cyanoacrylate) structure and the gaps in the particle network. Extended exposure allows equilibration despite the barrier.

FITC-dextran (molecular weight 4.3 kilodaltons) told a different story. This fluorescent polysaccharide, roughly ten times larger than rhodamine B, remained completely trapped within capsules for 24 hours in the isopropanol release medium.

Fluorescence microscopy showed the difference clearly. Bare dry liquid released FITC-dextran immediately upon contact with isopropanol—the aqueous phase dissolves into the alcohol-water mixture, and the dextran spreads throughout. Capsules, by contrast, retained bright fluorescent cores with no leakage into the surrounding medium.

The polymeric shell and embedded nanoparticle network create a mesh too fine for the large polymer to traverse. Dextran stays imprisoned.

Release required force. Probe sonication—ultrasound delivered via immersed tip—fractured the capsules mechanically. Post-sonication microscopy revealed ruptured shells and fluorescence distributed throughout the medium.

Implications and Applications

The size selectivity suggests practical uses. Enzymes, typically 10-100 kilodaltons, fall into the "retained until rupture" category. Encapsulating enzymes for laundry detergents—a commercial application where mechanical agitation during washing could trigger release—becomes feasible.

The non-spherical morphology matters beyond academic interest. Asymmetric particles orient and pack differently than spheres. In coating applications, they provide better coverage. In adhesion contexts, they offer more contact area. In flow situations, they exhibit different rheological properties.

The biocompatibility and biodegradability of poly(butyl 2-cyanoacrylate) addresses environmental concerns. Alkyl cyanoacrylates degrade hydrolytically, breaking down in biological systems. Their long history in medical applications (sutures, tissue adhesives) provides safety data.

The dry liquid template introduces operational advantages. Once formed, the powder can be stored indefinitely without the stability concerns of liquid emulsions. It can be dosed precisely by weight. It can be transported without special handling.

Most significantly, dry liquid allows independent control of dispersed phase composition and final capsule concentration. The powder can be loaded with active ingredients at any concentration, then dispersed into oil at any ratio. This decoupling simplifies formulation development.

Technical Considerations

Not all aspects are straightforward. Squalane proved suitable after extensive screening, but the selection process revealed constraints. Polar oils cause cargo leakage. Volatile oils evaporate. Some oils trigger unwanted bulk polymerization.

The interfacial polymerization requires careful execution. Monomer concentration, mixing speed, reaction time all influence film formation. Too little polymerization produces weak shells; excessive polymerization generates thick barriers that alter release kinetics.

The particle jamming mechanism that creates non-spherical morphology depends on precise formulation. Particle concentration, solution viscosity, shear intensity, and mixing duration interact. Reproducing the optimal balance may require process development.

Ultrasound-triggered release, while demonstrated, needs refinement for specific applications. Sonication parameters (power, frequency, duration) will determine fragmentation efficiency and fragment size distribution.

Broader Context

The work joins a growing body of research on anisotropic colloids and particles. Shape influences behavior in ways distinct from size or composition. Rods settle differently than spheres. Plates pack uniquely. Ellipsoids rotate in flow fields.

Controlled anisotropy in microcapsules opens design space. Degree of asphericity becomes a tunable parameter alongside size, shell thickness, material composition, and cargo type.

The dry liquid platform itself continues evolving. Recent reports describe dry liquids for clathrate hydrate formation (gas storage), heterogeneous catalysis supports, and sensing applications. The current capsule work suggests these powders offer broader utility than initially recognized.

Interfacial polymerization at Pickering emulsion interfaces represents another active area. The particle-stabilized interface provides a scaffold for polymer film formation. Different monomers, different particles, and different polymerization mechanisms could generate varied capsule properties.

Challenges Ahead

Scaling remains to be demonstrated. Laboratory blending of 25% PVA solution with silica powder works at small scale. Industrial production requires robust, reproducible processes. Continuous manufacturing would be preferable to batch processing.

The size distribution shows considerable polydispersity. Flow cytometry revealed number-average diameters around 12 micrometers but volume-average around 38 micrometers—indicating a tail of large droplets. Applications may demand narrower distributions.

Long-term storage stability of the final capsules needs assessment. The poly(butyl 2-cyanoacrylate) shell will eventually degrade, even without deliberate triggering. Understanding shelf life under various storage conditions becomes necessary for commercial viability.

The release kinetics for molecules of intermediate size—between rhodamine B and FITC-dextran—require mapping. Does a sharp cutoff exist, or does permeability decrease gradually with increasing molecular weight? How do molecular shape and charge influence transport through the capsule shell?

Future Directions

The researchers note potential extensions. Different electrode materials in the dry liquid could introduce functionality—magnetic nanoparticles for magnetic targeting, conductive particles for electrical triggering, photocatalytic particles for light-responsive release.

Alternative polymers for the shell—formed via different interfacial polymerization chemistries—could tune barrier properties, degradation rates, and mechanical strength.

Hybrid approaches combining the dry liquid template with other encapsulation strategies might access new property combinations. Layer-by-layer assembly onto the initial polymer shell could create multi-layered barriers with staged release profiles.

The fundamental insight—that non-spherical particle-stabilized droplets can template non-spherical capsules—applies beyond this specific system. Other dry liquid formulations, other oils, other interfacial reactions could generate diverse capsule libraries.

The powder sits in its container, appearing unremarkable. Yet each grain contains water, polymer, and cargo, individually packaged at the microscale. Add oil, trigger polymerization, and the grains become capsules—non-spherical containers that hold large molecules captive until mechanical force sets them free.

Progress in encapsulation technology increasingly recognizes that shape matters as much as size. The path from dry liquid to asymmetric capsule offers one route toward geometrically complex delivery systems, combining processing simplicity with functional sophistication.

Credit & Disclaimer: This article is a popular science summary written to make peer-reviewed research accessible to a broad audience. All scientific facts, findings, and conclusions presented here are drawn directly and accurately from the original research paper. Readers are strongly encouraged to consult the full research article for complete data, methodologies, and scientific detail. The article can be accessed through https://doi.org/10.1039/d5cc00209e