The coalescence speed of a pendent and a sessile drop

S. T. Thoroddsen*, K. Takehara, T. G. Etoh

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

189 Scopus citations


When two liquid drops come into contact, they coalesce rapidly, owing to the large curvature and unbalanced surface-tension forces in the neck region. We use an ultrahigh-speed video camera to study the coalescence of a pendent and a sessile drop, over a range of drop sizes and liquid viscosities. For low viscosity, the outward motion of the liquid contact region is successfully described by a dynamic capillary-inertial model based on the local vertical spacing between the two drop surfaces. This model applies even when the drops are of different sizes. Increasing viscosity slows down the coalescence when the Reynolds number Rev = ρRaveσ/μ2 < 5000, where Rave is the average of the tip radii of the two similar size drops, ρ is the liquid density, σ is the surface tension and μ the dynamic viscosity. At Rev ≃ 50, the growth-rate of the neck radius has reduced by a half, which for water corresponds to a drop diameter of only 2 μm. For the largest viscosities, the neck region initially grows in size at a constant velocity. The neck curvature also becomes progressively sharper with increasing viscosity. The results are compared to previously predicted power laws, finding slight, but significant deviations from the predicted exponents. These deviations are most probably caused by the finite initial contact radius.

Original languageEnglish (US)
Pages (from-to)85-114
Number of pages30
JournalJournal of Fluid Mechanics
StatePublished - Mar 25 2005
Externally publishedYes

ASJC Scopus subject areas

  • Condensed Matter Physics
  • Mechanics of Materials
  • Mechanical Engineering


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