Kugeltropfen

Kugeltropfen, literally “ball droplets,” is a term used in colloid and interfacial science to describe droplets that adopt a nearly perfect spherical shape under the influence of surface tension. The expression is commonly applied to immiscible liquid droplets dispersed in another liquid (emulsions), as well as to droplets produced in sprays, atomization, or microfluidic devices. In many contexts it is used interchangeably with spherical droplets or microdroplets, though Kugeltropfen emphasizes a high degree of rounding.

Formation and physical basis: Kugeltropfen form when interfacial tension dominates over disruptive external forces such as gravity, viscous stresses, or flow. They arise during emulsification, atomization, jet breakup, or controlled droplet generation in microfluidic systems. The resulting shape remains close to a sphere as long as dimensionless numbers that compare forces are small, notably a low capillary number (surface tension to viscous forces). Deformations occur under strong shear, high flow, or rapid acceleration, and may lead to ellipsoidal or irregular droplets.

Properties and stability: Typical Kugeltropfen range from micrometers to millimeters in diameter. They usually contain one liquid surrounded by another immiscible liquid, with interfacial tension stabilizing the spherical form. Stabilization is achieved through surfactants or solid particles (Pickering stabilization) to prevent coalescence. Their internal composition, refractive index, and surfactant shell influence optical properties and stability against Ostwald ripening or coalescence.

Applications and measurement: Kugeltropfen are central to emulsions in food, cosmetics, and pharmaceuticals, as well as to inkjet printing, spray technologies, and materials synthesis. They are characterized by microscopy, dynamic light scattering, laser diffraction, and high-speed imaging to assess size, distribution, and sphericity.