Dynamic drying in the early-stage coalescence of droplets sitting on a plate

¹  ESEME, Ecole Supérieure de Physique et Chimie Industrielle, Laboratoire de Physique et Mécanique des Milieux Hétérogènes - 10, rue Vauquelin, 75231 Paris Cedex 05, France
²  ESEME, Service des Basses Températures, CEA-Grenoble - Grenoble, France
³  Laboratoire ASCI - Bat. 506, 91405 Orsay Cedex, France

Corresponding author: daniel.beysens@espci.fr

Received: 18 July 2007
Accepted: 13 December 2007

Abstract

The early-stage coalescence of two sessile drops is investigated theoretically and experimentally. The coalescence of small sessile drops of diethylene glycol on silicon wafer is induced either by condensation or syringe deposition. The bridge geometry in directions parallel and perpendicular to the substrate, the bridge contact angle and the direct fluid velocity are simultaneously analyzed. The process is characterized by the nucleation and growth of a bridge between the two drops. Three stages are identified. i) An initial stage of dynamic drying where the capillary number and the contact line does not move appreciably. The bridge does not wet the solid and its size grows as time t perpendicularly to the substrate and as parallel to the substrate. ii) A late stage where the contact line starts to move and where the bridge relaxes exponentially, making eventually the composite drop to be ellipsis like. This stage is followed by iii) a very slow evolution limited by the contact line motion where the drop relaxes to a circular shape with a dynamics that is 6 to 7 orders larger than bulk hydrodynamics predicts.