Relaxation and contact angle dynamics during the coalescence of different sized vertically aligned water drops in different silicone oil viscosities

This paper reports an experimental investigation for the impact of the head drop size as well as the outer phase viscosity on the coalescence time of two water drops vertically aligned in silicone oil. Two types of silicone oil with different viscosities have been used in this work (47v350 and SilOil M40.165). A specific configuration of a sessile drop in direct contact with another drop placed above has been studied. The coalescence time is depending mainly, in this work, on the relaxation of the resulted drop together with its contact angle evolution during the merger process. For both liquid-liquid (LL) systems, the first stage of the coalescence process was dominated by the inertial forces induced by capillarity where the redefined Reynolds number Re is found to be much greater than the unity (Re >> 1). It's depicted that the smaller the head drop volume the faster the merger process. The viscosity of the outer phase has also found to have a main impact on the coalescence time and therefore on the relaxation of the resulted drop as well as its contact angle. It's found that increasing much more the outer viscosity provokes a long coalescence time accompanied with an elimination of the harmonic movement characterizing the merger drop relaxation as well as a damping of the contact angle dynamics. Focusing on the first instants of the merger process where a liquid bridge linking the drops is formed, it's shown that for the different head drop volumes used in the study that the variation of the dimensionless capillary pressure P* in function with the dimensionless time t* is the same for each LL system and it follows a power scaling law presented in the last section.