Droplets

A droplet is a small drop of liquid that forms a nearly spherical shape in air or another gas. Droplets cover a wide size range, from tens of micrometers to millimeters, and their behavior depends on size, surface tension, and ambient conditions.

Droplets form by condensation from vapor, atomization and spraying, splashing, dripping, or breakup of larger liquid bodies. In the atmosphere, droplets include cloud droplets, fog droplets, and raindrops. Cloud droplets are typically tens of micrometers across, fog droplets are similar, and raindrops range from about 0.5 to 5 millimeters. When droplets are very small, often below about 5 to 10 micrometers, they may behave as aerosols and remain suspended for extended periods.

Physical properties that govern droplets include surface tension, density, and evaporation rate, all of which depend on temperature and humidity. The motion and settling of droplets are driven by gravity and drag, with very small droplets also influenced by Brownian motion. Evaporation reduces size and mass, potentially transforming liquid droplets into aerosols or vapor, especially under dry or warm conditions.

Measurement and modeling of droplets employ optical imaging, laser-based techniques, phase Doppler analyzers, and computational fluid dynamics. Droplet science informs weather and climate studies, spray technologies, inhalation therapies, inkjet printing, and various industrial processes.

In public health, droplets are a key pathway for transmission of respiratory infections, leading to droplet precautions in clinical settings. In industry, control of droplet formation is important for spray cooling, agricultural applications, and pharmaceutical and cosmetic manufacturing.