Coalescence of multiple sessile drops on a substrate

Wet coatings are of great interest in industrial processes as they allow for the protection or functionalization of surfaces by a thin film. Such films can be deposited in several ways including drop deposition techniques such as spraying or inkjet printing. In those cases, drops are deposited on a surface where they coalesce to form a continuous film. We present model experiments involving a small number of drops to study coalescence locally. For this purpose, an experimental set-up was designed to deposit glycerol drops on a glass plate according to specific patterns. During coalescence a liquid bridge grows between the two drops. Our experiments suggest that, in a viscous regime, the width of this bridge follows an exponential relaxation a up to a final state. The number of drops involved and the deposition pattern both have a strong influence on the dynamics and final geometry. The chronology of events can also significantly change the dynamics. Moreover we observe that contact angle hysteresis plays a major role in the geometry of the final state. It leads to contact line pinning and prevent liquid drawback after merging. As a consequence the coalesced droplet stays in an elongated geometry in the final state. Accounting for geometry and chronology will both bring a new point of view on multiple coalescence.