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Coalescence of immiscible sessile droplets on a partial wetting surface

Published online by Cambridge University Press:  21 September 2023

Huadan Xu ,
Xinjin Ge ,
Tianyou Wang  and
Zhizhao Che Open the ORCID record for Zhizhao Che [Opens in a new window]
Huadan Xu
Affiliation:
State Key Laboratory of Engines, Tianjin University, Tianjin 300350, PR China
Xinjin Ge
Affiliation:
State Key Laboratory of Engines, Tianjin University, Tianjin 300350, PR China
Tianyou Wang
Affiliation:
State Key Laboratory of Engines, Tianjin University, Tianjin 300350, PR China
Zhizhao Che*
Affiliation:
State Key Laboratory of Engines, Tianjin University, Tianjin 300350, PR China
*
†Email address for correspondence: chezhizhao@tju.edu.cn
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Abstract

Droplet coalescence is a common phenomenon and plays an important role in multidisciplinary applications. Previous studies mainly consider the coalescence of miscible liquids, even though the coalescence of immiscible droplets on a solid surface is a common process. In this study, we explore the coalescence of two immiscible droplets on a partial wetting surface experimentally and theoretically. We find that the coalescence process can be divided into three stages based on the time scales and force interactions involved, namely (I) the growth of a liquid bridge, (II) the oscillation of the coalescing sessile droplet and (III) the formation of a partially engulfed compound sessile droplet and the subsequent retraction. In stage I, the immiscible interface is found not to affect the scaling of the temporal evolution of the liquid bridge, which follows the same 2/3 power law as that of miscible droplets. In stage II, by developing a new capillary time scale considering both surface and interfacial tensions, we show that the interfacial tension between the two immiscible liquids functions as a non-negligible resistance to the oscillation which decreases the oscillation periods. In stage III, a modified Ohnesorge number is developed to characterize the visco-capillary and inertia-capillary time scales involved during the displacement of water by oil; a new model based on energy balance is proposed to analyse the maximum retraction velocity, highlighting that the viscous resistance is concentrated in a region close to the contact line.

JFM classification

Drops and Bubbles: Breakup/coalescence Drops and Bubbles: Drops
Type
JFM Papers
Information
Journal of Fluid Mechanics , Volume 971 , 25 September 2023 , A34
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Copyright
© The Author(s), 2023. Published by Cambridge University Press

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Xu et al. Supplementary Movie 1

Calescence process of two immiscible droplets, corresponding to figure 2. The two immiscible liquids are water (right) and n-dodecane (left).

Download Xu et al. Supplementary Movie 1(Video)
Video 6 MB

Xu et al. Supplementary Movie 2

Close-up view of the liquid bridge for the coalescence of a water droplet (right) and a low-viscosity oil droplet (bromocyclopentane, left), corresponding to figure 3c.

Download Xu et al. Supplementary Movie 2(Video)
Video 2.7 MB

Xu et al. Supplementary Movie 3

Close-up view of the liquid bridge for the coalescence of a water droplet (right) and a high-viscosity oil droplet (silicone oil 100 cSt, left) , corresponding to figure 3d.

Download Xu et al. Supplementary Movie 3(Video)
Video 8.1 MB

Xu et al. Supplementary Movie 4

Close-up view of the liquid bridge for the coalescence of a water droplet (right) and an ethanol droplet (left), corresponding to figure 3e.

Download Xu et al. Supplementary Movie 4(Video)
Video 371.6 KB