Coalescence

Coalescence is the process by which two or more distinct entities merge to form a single one. The term is used across disciplines to describe merging phenomena driven by surface tension, gravitational or other attractive forces, or genealogical relationships.

In fluids, coalescence occurs when droplets or bubbles come into contact and form a neck that grows due to surface tension, leading to a single larger drop or bubble. The process is influenced by viscosity, ambient flow, and interfacial properties; theoretical descriptions use Navier–Stokes equations and scaling laws.

In atmospheric science, collision-coalescence is a dominant mechanism for rain in warm clouds, where larger droplets grow by colliding with smaller ones. Factors include droplet size distribution, turbulence, and updrafts; the process competes with breakup and evaporation.

In chemistry and materials science, coalescence refers to the merging of droplets of one phase into another in emulsions or polymer blends, often destabilizing the dispersion. Surfactants, emulsifiers, or crosslinking can impede coalescence by increasing interfacial film strength.

In population genetics, coalescent theory models the genealogical trees of alleles tracing back to a common ancestor. Kingman’s coalescent provides a mathematical framework used to infer demographic history from genetic data.

In astronomy, coalescence describes the merger of compact objects, such as neutron stars or black holes, producing gravitational waves. Observations by detectors like LIGO and Virgo have confirmed several coalescences, informing astrophysics and cosmology.