Coalescence (physics)

Coalescence is the process by which two or more droplets, bubbles, or particles merge during contact to form a single daughter droplet, bubble, or particle. Coalescence manifests itself from a microscopic scale in meteorology to a macroscopic scale in astrophysics. For example, it is seen in the formation of raindrops as well as planetary and star formation.

For other uses, see Coalescence (disambiguation).
Representation of the coalescence of two droplets, bubbles, or particles to form a single entity.

In meteorology, its role is crucial in the formation of rain. As droplets are carried by the updrafts and downdrafts in a cloud, they collide and coalesce to form larger droplets. When the droplets become too large to be sustained on the air currents, they begin to fall as rain. Adding to this process, the cloud may be seeded with ice from higher altitudes, either via the cloud tops reaching −40 °C (−40 °F), or via the cloud being seeded by ice from cirrus clouds.

Contrast-enhanced ultrasound in medicine applies microscopic bubbles for imaging and therapy. Coalescence of ultrasound contrast agent microbubbles is studied to prevent embolies[1] or to block tumour vessels.[2] Microbubble coalescence has been studied with the aid of high-speed photography.[3]

In cloud physics the main mechanism of collision is the different terminal velocity between the droplets. The terminal velocity is a function of the droplet size. The other factors that determine the collision rate are the droplet concentration and turbulence.[4]

See also

References
  1. Postema M, Marmottant P, Lancée CT, Hilgenfeldt S, de Jong N (2004). "Ultrasound-induced microbubble coalescence". Ultrasound in Medicine & Biology. 30 (10): 1337–1344. doi:10.1016/j.ultrasmedbio.2004.08.008. PMID 15582233.
  2. Kotopoulis S, Postema M (2010). "Microfoam formation in a capillary". Ultrasonics. 50 (2): 260–268. doi:10.1016/j.ultras.2009.09.028. PMID 19875143.
  3. Poortinga AT, Postema M, Carlson CS, Anderton N, Yamasaku M, Otake N, Kudo N (2023). "Sonic cracking of calcium carbonate-encapsulated microbubbles observed at moderate acoustic amplitudes". Current Directions in Biomedical Engineering. 9 (1): 37–40. doi:10.1515/cdbme-2023-1010.
  4. Benmoshe N, Pinsky M, Pokrovsky A, Khain, A (2012). "Turbulent effects on the microphysics and initiation of warm rain in deep convective clouds: 2-D simulations by a spectral mixed-phase microphysics cloud model". Journal of Geophysical Research: Atmospheres. 117 (D06): D06220. Bibcode:2012JGRD..117.6220B. doi:10.1029/2011JD016603.
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