Propagation of capillary waves and ejection of small droplets in rapid droplet spreading
KAUST DepartmentPhysical Sciences and Engineering (PSE) Division
Clean Combustion Research Center
Mechanical Engineering Program
High-Speed Fluids Imaging Laboratory
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AbstractA new regime of droplet ejection following the slow deposition of drops onto a near-complete wetting solid substrate is identified in experiments and direct numerical simulations; a coalescence cascade subsequent to pinch-off is also observed for the first time. Results of numerical simulations indicate that the propagation of capillary waves that lead to pinch-off is closely related to the self-similar behaviour observed in the inviscid recoil of droplets, and that motions of the crests and troughs of capillary waves along the interface do not depend on the wettability and surface tension (or Ohnesorge number). The simulations also show that a self-similar theory for universal pinch-off can be used for the time evolution of the pinching neck. However, although good agreement is also found with the double-cone shape of the pinching neck for droplet ejection in drop deposition on a pool of the same liquid, substantial deviations are observed in such a comparison for droplet ejection in rapid drop spreading (including the newly identified regime). This deviation is shown to result from interference by the solid substrate, a rapid downwards acceleration of the top of the drop surface and the rapid spreading process. The experiments also confirm non-monotonic spreading behaviour observed previously only in numerical simulations, and suggest substantial inertial effects on the relation between an apparent contact angle and the dimensionless contact-line speed. © 2012 Cambridge University Press.
SponsorsH.D. was supported by 100 Talents Program of The Chinese Academy of Sciences and the National Natural Science Foundation of China (Grant No. 11172294). E.Q.L. and S.T.T. were partly supported by KAUST-BERKELEY AEA grant 7000000024. Y.S. and P.D.M.S. were supported from EPSRC under grant numbers EP/E046029/1.
PublisherCambridge University Press (CUP)
JournalJournal of Fluid Mechanics