Droplet

A droplet is a small globule of liquid bounded by surface tension and surrounded by a different phase, typically air or another immiscible liquid. In the absence of strong external forces, surface tension tends to mold a droplet into a spherical shape; gravity, airflow, and interactions with surfaces can cause deformations.

Size ranges vary by context. In the atmosphere, cloud and fog droplets are usually tens of micrometers in diameter, while raindrops are larger, commonly from about 0.5 to a few millimeters. In sprays and atomization, droplets span from a few micrometers to tens of micrometers. In microfluidics and chemistry, droplets can be sub-microliter to nanoliter in size.

Formation mechanisms include atomization or spraying, where liquid is disintegrated into many droplets; condensation, where vapor forms droplets on nuclei; and emulsification, where immiscible liquids create dispersed droplets within another liquid.

Key properties include size distribution, which influences evaporation, transport, and optical scattering; surface or interfacial tension; wettability described by the contact angle when resting on surfaces; and evaporation dynamics, governed by vapor pressure, temperature, humidity, and air flow. Droplets may evaporate completely or leave residues.

Applications and contexts span atmospheric science (clouds, fog, and rain), inkjet printing and coatings, spray cooling and combustion, humidification and fogging, and microfluidics, where droplets act as discrete reaction vessels. Droplet-based methods enable high-throughput screening and digital assays in research and industry.