Subdroplets

Subdroplets are droplets contained within a larger droplet, forming a hierarchical, multiphase structure in emulsions and aerosols. They occur in double emulsions (for example water-in-oil-in-water droplets), in microfluidic compartmentalization, and in condensational growth where small droplets nucleate inside a parent droplet.

Formation mechanisms include internal phase separation within a supersaturated droplet, partitioning of immiscible liquids during emulsification, and the creation of inner droplets during stabilization by surfactants or polymers. External conditions such as temperature, humidity, and interfacial tension influence the number and size of subdroplets.

Typical subdroplet sizes range from tens of nanometers to several micrometers, depending on materials and production method. They possess an internal interface with the surrounding phase and are stabilized by interfacial films or matrix materials. Stability depends on interfacial tension, viscosity contrasts, and diffusive exchange between compartments.

Measurement and imaging commonly use optical and confocal microscopy, fluorescence labeling, and scattering techniques. Electron microscopy can reveal internal structure in solid-like samples, while in situ observation remains challenging due to rapid dynamics and small scales.

Applications include microreactors for localized chemistry, models of cellular compartments, and components in drug delivery, cosmetics, and printing technologies. Subdroplets enable compartmentalized reactions and targeted transport within a single carrier.

Key challenges involve achieving uniform subdroplet size distributions, preventing coalescence between nested droplets, and developing consistent terminology and models for transport across multiple interfaces.